Oribatids - Higher

200-145 mya Jurassic

Palaeosols Plants Ectomycorrhiza Earthworms Insects Macro-aggregation

Denizens of the Dirt

Oribatid mites can reach impressive numbers—up to 500,000 individuals per m2 in forest soils This is equivalent to 4000 mites in a handful of soil! What are they all doing - these half million legged armoured creatures doing? It must be vital.

Oribatid mites are one of the three most common animals found in soil. Do the oribatid mites - so numerous round the world - show us something about soil evolution? It seems that lower oribatids running around as far back as the Carboniferous period were decomposers - but not of lignin. That needed white rot fungi.

This continues the story of oribatid evolution

400-360 mya (Late Devon)
330-300mya (Late Carb)
300 -250 mya (Permian)
Oribatids - Higher (200-145 mya Jurassic)
Mites (145-66 mya (Cretaceous)

It was only when higher oribatids emerged around 200 mya who could chew wood debris that the complete decomposition process arrived. The mites - along with worms - were able to turn the debris into humus, the process called humification. This is the piece in the puzzle that explains the structure of soil today. In this period they change their size (getting smaller) and shape - to fit the newly emerging pore sizes. That is why I call them the denizens of the dirt.

Higher Oribatids 200-100mya
1 Hydrozetidae-Limnozetidae, 196–189 mya;
2 Achipteriidae, 145–140 mya;
3 Carabodidae, 122–99 mya;
5. Trhypochthoniidae 122–99 mya;
4 Camisiidae, 85–83 mya;
details see Supplementary information

Oribatid Phylogeny

A hypothetical oribatid mite phylogeny, as extracted and modified from the works of Grandjean, Haumann and Weigmann is offered" Maraun 2004 following the hypothesis of Grandjean (1969)
(1) the basal and most ‘primitive’ Palaeosomata,
(2) the species rich Enarthronota including, for example, the Brachychthonioidea and Hypochthonioidea,
(3) the small group Parhyposomata,
(4) the ‘Mixonomata’, which includes groups like the Lohmannioidea, Eulohmannioidea and the box mites (Phthiracaroidea and Euphthiracaroidea),
(5) the ‘Desmonomata’, a species rich group, including, for example, Nothridae, Hermanniidae, Camisiidae, Trhypochthoniidae and Malaconothridae

(6) the species-rich Circumdehiscentiae (=Brachypylina, ‘Higher Oribatida’) includes five groups,
(a) Opsiopheredermata,
(b) Eupheredermata,
(c) dorsodeficient Apheredermata,
(d) pycnonotic Apheredermata, and
e) Poronota (Trave´ et al. 1996).

Oribatid evolution through fossils

Malaconothridae

Nothridae

I originally thought that higher orbatids evolved with the 'lower' ones in the Carboniferous period. But the more I looked, the less sense it made. It is now clear that the big jump in their evolution started in this Jurassic period and carried on into the next period. The present day distribution helps explain their evolution

Oribatids provided evidence of tectonic plate shifts - when originally the theory was not fully endorsed. (Lu et al 2024)

Brown columns = no. of oribatid branching events

Orange & black arrows = "Shifts in body size and body form correlate with the advance of vascular plants" (Schaefer & Caruso 2019)

Phthiracaroidea

As Pangea broke up, the emerging 'higher' oribatids were confined to the Palearctic, the largest of 8 biogeographic realms

"Within Eurasia, oribatid mite diversity was highest in Southeast Asia likely reflecting the long tropical history of this region - 200my" (Lu et al 2024)

Is Lohmannia more 'middle' orib and evolved as Pangea broke up? It does not have more characteristic higher oribatid body - the brown one.

Click box for more. I lived on the other side of the road from Westfield College (Westfield Road NW5), not realising this character was working there at the same time!

MS Ghilarov crops again - as as the lead on this Russian investigation: 'Oribatids as indicators of soil type' by Krivolutsky - where they talk about humus! This fits with my hypothesis about the denizens of the dirt (above).

"Ideas are developed further in a global context, and as a first approach we have modified the distribution data given by Balogh (1972) and Ghilarov & Krivolutsky (1975) in the light of the most recent records available." (Hammer & Wallwork 1979)

What extraction tells us about their resilience

When I was extracting higher oribatids, with Tullgren funnels, I had to be very careful. They live much further down than springtails, in more confined conditions. So they are harder to persuade to emerge, as they have further to emerge from the soil samples. Their defence is to roll up when conditions warm up.

My 60-funnel extraction unit

They wrap themselves up - as Ghilarov drew.

The UK Countryside Survey examined oribatids but found that there were many less oribatids, from one survey to the next. They tried to put it down to changes in agricultural practices over the ten years. However bad those practices maybe they couldn't be bad enough to reduce their numbers to half. The differences were almost certainly due to a slight change in the extraction technique.

This ability to roll up and retract appendages is called 'ptychoidy ' (Schmelzle & Bluttgen 2019) They usually live 1-2 years ( along time in soil) and even 8 years in polar regions. They recover from fire damage (Won & Isni 2013). They "colonise young soils within a few years" Lemitz et al 2012. It seems that parthenogenesis - no-sex reproduction - played a part in the wide distribution of these higher oribatids.

Reproductive Mode

Eupelops hirtus

Partheneogenesis

Highly derived oribatid mites (Brachypylina) seem to fit most predictions of evolutionary theory regarding the ecological, geographical and taxonomic distribution of parthenogenesis. Earlier derivative groups generally do not. We suggest that the ancestors of large, completely parthenogenetic families (for example, Brachychthoniidae, Lohmanniidae, Camisiidae, Trhypoch-thoniidae, Malaconothridae, Nanhermanniidae) were themselves parthenogenetic, and that’ speciation’ and radiation occurred in the absence of sexual reproduction" It would seem that not having sex saves valuable energy and resources.

Nothrus silvestris

Sex

We saw earlier that the mites in the habitats like mosses & lichens rather than soil were usually sexual. But it is not that simple to say lower oribatids are sexual whereas higher are parthenogenic. It is - as you may have guessed - more complex.

Damaeus - 'higher' beetle mite - with spermatophore. From Ghilarov

Ghilarov's description

"Without going into an analysis of possible phylogenetic relationships of various systematic groups of ticks, it is interesting to trace some features of fertilization in representatives of different groups that differ in their ecology. In ticks that live in soil, litter and similar substrates, i.e. in an environment in which the air is saturated with water vapor, external-internal fertilization has been recorded, which is also characteristic of other groups of chelicerates living in similar conditions. Thus, external-internal fertilization is characteristic of Oribatei (Pauly, 1952, 1956; Shaller, 1954, etc.) and Trombiculidae (Lipovsky et al., 1957; Wen Tin-whan, 1958, 1959, 1960, etc.). In oribatids of the genus Belba (=Damaeus) (left) males lay club-shaped spermatophores on the substrate (soil, etc.), resembling the fruiting bodies of mold fungi, for which they were apparently previously taken, since the insemination of these mites has been studied recently. The deposition of spermatophores occurs even in the absence of females, as happens with external fertilization in common species of marine invertebrates, but since oribatids are often found in large clusters, a real "turf" of spermatophores is often formed. When laying a spermatophore, the male touches the substrate with the genital plate and then raises the posterior end of the body. When in contact with the substrate, a sticky substance emerges from the genital opening, adhering to the hard surface, and as the body rises above the substrate, the thread of the spermatophore stalk is pulled out, and when the abdomen is raised to its maximum, the spherically expanded head of the spermatophore and a drop of sperm are released. Females, running past a fresh spermatophore, are attracted by it, examine it and, passing over it so that the spermatophore is under the spread genital plates, capture the sperm, quickly pressing themselves to the substrate".

(MS Ghilarov Regularities in Adaptations of arthropods to the terrestrial life NAUKA Moscow 1970) p170-1

Structured Resource Theory of Sex (SRTS)

"Parthenogenetic species (of oribatid mites) flourish in habitats where resources are plentiful and/or easily accessible supporting large populations, and this is consistent with predictions of the SRTS..Generally, the observed patterns suggest that the mode of reproduction is related to bottom-up rather than top-down regulation of populations ". (Maraun et all 2019) They found that the reproductive mode (ie whether sexual or parthenogenic) does not correlate with species numbers or diversity, altitude or latitude. They do not seem to have checked the mode with lower/higher oribatids. The results prefer SRTS rather than Red Queen theory we came across earlier.

Oribatids & Soil Processes

Let us look in more detail at how oribatids contribute(d) to the soil processes that make soil as we know it today. They not only play a key role in decomposition and soil building via humus formation, they may also help horizon development - but I have no evidence to provide. But the way they can move through small passages, survive all sorts of conditions, and mix materials, they must be strong candidates - especially as the horizons are now appearing in Podzols in this period. The more we understand about their comings and goings the more we will understand the soil processes.

Oribatids & Horizons

The formation of distinct soil horizons, or layers, is a complex process driven by a combination of biological, chemical, and physical factors. Oribatid mites contribute to this process in several ways, although they are just one part of a broader system. Let’s break down how their activities can contribute to the formation of these distinct layers:

1. Selective Feeding and Organic Matter Breakdown

Feeding Preferences: Oribatid mites tend to prefer certain types of organic matter, such as decaying leaves, wood, and fungi. Their selective feeding leads to the differential breakdown of organic material. Over time, this selective decomposition contributes to the accumulation of specific types of organic matter in the upper layers of soil, which characterizes the organic horizon (O horizon).
Organic Matter Stratification: As oribatid mites and other organisms break down organic material, the byproducts, like humus, tend to accumulate in the uppermost layers. This accumulation of well-decomposed organic matter forms a distinct layer that is different from the underlying mineral layers, helping to create the O horizon.

2. Humification and Horizon Formation

Humus Formation: The breakdown of organic material by oribatid mites contributes to humus formation. Humus is a stable, dark organic material that binds with mineral particles to form aggregates. The formation of these aggregates in the upper soil layers differentiates the A horizon (topsoil) from lower layers. The A horizon is rich in humified organic material, which gives it a darker color and a distinct texture compared to the layers below.
Impact on Soil Texture: The activity of oribatid mites, along with other soil fauna, affects soil texture by contributing to the mix of organic and mineral particles. This mixing creates a gradient in organic content and texture, which is a key factor in the development of distinct horizons.

3. Nutrient Cycling and Chemical Differentiation

Nutrient Redistribution: Oribatid mites contribute to the cycling of nutrients like nitrogen and phosphorus. As they process organic matter, they release nutrients that can be leached down into lower soil layers, contributing to the differentiation between nutrient-rich upper horizons and nutrient-poor lower horizons. This process can enhance the formation of a distinct A horizon where organic matter and nutrients are concentrated.
Formation of the E Horizon: In some soils, the activity of soil organisms, including oribatid mites, helps form an E horizon (an eluviation or leaching horizon) where organic acids from decomposing matter leach minerals and organic compounds downward. The E horizon becomes distinct due to the loss of organic matter and minerals, which are carried into the horizons below.

4. Bioturbation and Physical Separation

Limited Bioturbation: While oribatid mites are not major bioturbators like earthworms, their activity still contributes to the gradual mixing of soil materials. However, their small size and the scale of their activity tend to lead to finer-scale mixing, which doesn’t completely homogenize the soil. This limited mixing helps maintain the integrity of distinct horizons by contributing to the vertical stratification of organic and mineral components.
Creation of Microenvironments: The microhabitats created by oribatid mites and their contribution to soil structure support the development of microenvironments within the soil. These microenvironments can lead to localized differences in organic matter content, moisture, and texture, which contribute to the development of distinct soil layers over time.

Conclusion

While oribatid mites are not the primary drivers of horizon formation, their contributions to organic matter decomposition, humus formation, nutrient cycling, and soil structure play crucial roles in the development of distinct soil horizons. Their activities help to create gradients in organic content, texture, and nutrient distribution, which are essential for the differentiation of soil layers over time.

Oribatids & Humification

Why are higher oribatids so brown?

The answer to this simple question throws a light on the evolution of one the the main soil processes - humiifcation.

Oribatid mites are perhaps the largest group of soil animals that contribute to humification. They chew ( they are in the "chewing Acarifromms" clade Sarcoptiformes ) decomposed organic matter, and the processes in their digestive systems help further break down complex organic compounds into humic substances. Their activity helps in the formation of stable humus, improving soil structure and nutrient retention.

"The changes consist in an increase of Humus Index and the density of mites, especially oribatids" Salmon 2018 Humification is one of the great functions of soil. It gets energy into soil and the humus glues the soil together. It was probably in soils for a long time, although not very effective in Carboniferous times - the period defined as laying down carbon. There is no evidence that decomposition extended dramatically then, until this period. That may have been due to animals like higher oribatids and earthworms incubating the anaerobic bacteria responsible in their guts and then running round distributing the results - humus

I am still trying to work out the role of higher oribatids with the process of humification. It looks like earthworms play a major part. Both these animals need air but provide anaerobic conditions for microorganisms to break own plant materials. Just about all commentators talk about how oribatids are involved with decomposition, without saying whether it is mineralisation or humification or both. While many consider humifaction as having been around for all time, it wasn't extensive till this period. Perhaps that dates its evolution from 150-200mya., when higher oribatids evolved and earthworms appeared. Now humus formation is seen as key characteristic to many soil, I think the evolution of humification is relatively late - and has a lot to do with oribatids and earthworms.

We were taught, and I was questioned in PhD interview, that humification is the the most important soil process. Yet it is not everywhere. It needs water. What do the higher oribatids have to do with it? 200my previously, lower oribatids would have fed off aerobic decomposition. To tap the energy in anaerobic conditions provides major evolutionary advantage. Think of all that energy which didn't escape with coal - no decomposition. That may well explain the extent and depth of oribatids in our soils.

Scheloribatida

Scheloribates poo

While looking at S. moestus on litter decomposition dynamics and chemical transformations, it was found that "Mites stimulated extracellular enzyme activities, enhanced microbial respiration rates by 19% in corn litter and 17% in oak litter over 62 days, and increased water-extractable organic C and N. Mites decreased the relative abundance of polysaccharides in decomposing corn litter but had no effect on oak litter chemistry, suggesting that the effects of S. moestus on litter chemistry are constrained by initial litter quality. We also compared the chemistry of mite feces to unprocessed corn litter and found that feces had a higher relative abundance of polysaccharides and phenols and a lower relative abundance of lignin. Our study establishes that S. moestus substantially changes litter chemistry during decomposition, but specific effects vary with initial litter quality. These chemical transformations, coupled with other observed changes in decomposition rates and nutrient cycling, indicate that S. moestus could play a key role in soil C cycling dynamics. "

Oribatids & litter

While oribatid are present in the litter layer, they also able to descend.

I propose that while oribatid are present in the litter layer, they are better able to descend, dig deeper and feed on dead matter that others could not.

Oribatids alter litter. Corn and oak litter from habitats with large populations of Scheloribatesmoestus and in microcosms with and without mites measured respiration rates, nitrogen availability, enzyme activities, and molecular-scale changes in litter chemistry. Mites stimulated extracellular enzyme activities, enhanced microbial respiration rates by around a fifth over a couple of months and increased water-extractable organic C and N. Mites decreased the relative abundance of polysaccharides in decomposing corn litter but had no effect on oak litter chemistry, suggesting that the effects of S. moestus on litter chemistry are constrained by initial litter quality.

Oribatids are found in high concentration 50 – 500,000 per square meter, particular in coniferous soils, slightly less under deciduous trees, and less under grass, even less under arable and few in tundra. This reflects the evolution of plants – conifers first emerging about this time, then deciduous later and grasses not for another two hundred million years or more

Steganacarus magnus

Going brown

Is lignin or humus linked to the melanin in their cuticle?

What is the significance? The colour derives from melanin. Is that related to lignin or humus in any way? As if it is it may be telling us that higher oribatids became able to digest lignin. It seems they are not far apart.

"Humic acid-type polymers (melanins) synthesized in culture media by the fungi Aspergillus glaucus, Eurotium echinulatum, Hendersonula toruloidea, Stachybotrys atra and Aspergillus sydowi were analysed for elemental composition, functional group content, infrared (IR) and electron spin resonance (ESR) properties. Results were discussed in comparison with range values referred for soil humic acids. The fungal polymers showed significant differences in carboxyl and nitrogen content and C/H atomic ratios, reflecting a different degree of condensation (aromaticity) among the various samples. IR analysis gave evidence of: (a) the predominant aromatic character of melanins from A. glaucus, E. echinulatum and H. toruloidea; (b) the high content of aliphatic and olefinic components of S. atra melanin; (c) the typical presence of amide bonds in the nitrogen-richest melanins from A. sydowi and H. toruloidea; and (d) the generally low amount of free carboxyl groups, which often appeared involved in hydrogen bonds. ESR spectra showed that all the melanins studied contained appreciable concentrations of organic free radicals of prevailing semiquinonic nature and of the same order of magnitude commonly measured in humic acids from soil and other sources. The free electron concentration was shown to be directly related to the C/H atomic ratio and to the degree of aromaticity shown by IR analysis. This indicated that the highest free radical content in the melanins from E. echinulatum and A. glaucus was associated with the highest presence of condensed aromatic structures. Humic acid-type polymers synthesized by soil fungi may, therefore, contribute to the total free radical content of soil humic substances and play important roles in all reactions involving free radicals in soils and related environments."

Oribatids & Bacteria

This when the third important wave of bacteria and soil made an appearance – in the guts of mites.

They differ from earlier bacteria in springtail guts, which are predominantly aerobic – ie only capable of living with oxygen. Many of these in mite guts cannot live with oxygen.

One study found in the oribatid guts; Bacillus, Pseudomonas, Coryneforms, Actinomycetes, Mycobacterium, Alcaligenes,Flavobacterium, Acinetobacter, and Citrobacter (Wolf & Rocket 2009)

If ever there was a job I’m glad somebody else does then it is counting bacteria in mite guts. It is a big step forward in isolating bacteria to particular environments, rather than lumping them all into a microbial mush. One study found the following (Wolf & Rockett 2009)...

There were some ‘strict’ Anaerobes like Citrobacter species, motile by means of flagellae. They are commonly found in water, soil and intestinal tracts of animals, including humans. There were also ‘possible’ anaerobes like Bacillus subtilis, ‘generally regarded as an aerobe, but grows under strict anaerobic conditions using nitrate as an electron acceptor and should be designated as a facultative anaerobe’. (Hoffman et al 1995) Alcaligenes are aerobic bacteria although some strains are capable of anaerobic respiration in the presence of nitrite or nitrate. Again, this shows adaptability to respond to anaerobic conditions – and using nitrates to replace oxygen. In so doing they too will be using nitrates that could be feeding plants.

Pseudomonas aeruginosa had been considered as an obligately aerobic bacterium previously, but it is now recognized to be highly adapted to anaerobic conditions (Arai 2011). P. aeruginosa inhabits soil and water as well as animal, human, and plant-host-associated environments. The ubiquity would be attributed to its very versatile energy metabolism.

There were also a number of aerobes like Mycobacterium, usually bacillary in form, Flavobacterium, strictly aerobic, Acinetobacter, strictly aerobic & non-motile (Howard et al 2012), Coryneform which are aerobic or facultatively anaerobic, and common in soil water and plants. (Yassin et al 2003) and finally Actinomycetes which are a group of aerobic and anaerobic bacteria (Sullivan & Chapman 2009)

Gut biome

What about the gut biome of Higher Oribatids?

The knowledge about gut biome of of oribatids is increasing, now with molecular methods is on the increase. But we are probably not yet at the stage to analse difference between lower and higher oribatids to determine feeding habits. This is because

Oribatids - like most soil animals - are generalist feeders. They may favour some foods, but survive because they eat virtually anything
Bacteria are know to switch quite rapidly
Bacteria in oribatids guts bears more relation with their hosts than their food - they have phylogenetic affinities. While there is general agreement on this, the actual relations of which bacteri with what oribatids is still being worked out.

This field of study is being explored - as part of a wider explration of the role of gut biomes in animals. We shall see..in the meantime here are some ramblings..

Oribatid Gut bacteria

Bacterial communities in the gut of oribatid mites are shaped predominantly by physiological attributes of the host.

"In detritivores, microorganisms form part of the diet and function as food resource, thereby gut microbiota of detritivores may reflect the diet and trophic niches of the consumer. However, the relative contribution of consumer species attributes, represented by their phylogenetic affinities and food resources, to gut microbial community assembly has not yet been explored. In this study, we used oribatid mites (Oribatida, Acari), a ubiquitous and diverse soil microarthropod taxon that feeds on a variety of food resources, to investigate the driving factors of gut microbiota and to uncover the contribution of host phylogenetic relatedness and trophic niches to the assemblages of gut microbiota...

The species include primary decomposers using detritus and root exudates as resources, secondary decomposers consuming microbes and/or microbial residues, and predators feeding on other soil animals. ..Abundance of bacteria in the gut of oribatid mites generally was tenfold greater than that of fungi ..Gut bacterial communities varied more with phylogenetic affinity than with the trophic niches of consumers, suggesting that bacterial communities in the gut of oribatid mites are shaped predominantly by physiological attributes of the host, being similar in closely related species (Chen et al., 2017), rather than by their trophic niches

The results indicate that both bacterial and fungal communities in the gut of oribatid mites are related to host/consumer phylogenetic and trophic distances, consistent with earlier findings showing that phylogeny and/or diet of hosts impact gut microbiota in other animals.

Gut bacterial communities varied more with phylogenetic affinity than with the trophic niches of consumers, suggesting that bacterial communities in the gut of oribatid mites are shaped predominantly by physiological attributes of the host, being similar in closely related species (Chen et al., 2017), rather than by their trophic niches. Phylogenetic clustering of bacterial communities in the gut of the oribatid mite species E. hirtus, N. silvestris, P. peltifer, S. magnus and T. bisulcatus underlines the predominant role of deterministic processes in community assembly suggesting that oribatid mites select for phylogenetically related bacteria, which is consistent with findings of gut microbiota in nematodes (Berg et al., 2016). Higher NRI than NTI in most of the studied oribatid mite species (except for P. peltifer) suggests coexistence of bacterial species of high taxonomic level, such as Proteobacteria, Cyanobacteria and Firmicutes. Presumably, these ubiquitous bacteria are associated with the diet of oribatid mites but may also function as commensals or mutualists facilitating chitin digestion and thereby the digestion of fungi in the gut of oribatid mites.

Coevolutionary processes may have facilitated close association of gut bacteria with the oribatid mite hosts...Two oribatid mite species in our study, A. coleoptrata and P. peltifer, had lower abundance of the assumed bacterial chitinase (EC: 3.2.1.14) gene than the other oribatid mite species. These two species, characterized by low δ15N values, may feed little on fungi, but rather directly on plant litter resources, thereby functioning as primary decomposers, whereas the other oribatid mite species likely function as secondary decomposers as indicated by δ15N signatures (Maraun et al., 2011). " Gong et al 2018

They use these 2 oribatids

Platynothrus peltifer;

Achipteria coleoptrata;

Coevolution of oribatids & gut microbes

How this co-evolution might occur

Here is an outline of how this co-evolution might occur:

Initial Association: The association between oribatid mites and gut bacteria likely began with random encounters between mites and environmental bacteria. Some bacteria that entered the gut of oribatid mites may have found favorable conditions for growth and colonization.
Beneficial Interactions: Over time, certain bacteria within the gut of oribatid mites may have provided benefits to their hosts. These benefits could include aiding in digestion, synthesizing essential nutrients, or protecting against pathogens.
Selective Pressures: As oribatid mites and their gut bacteria interacted, there would have been selective pressures acting on both parties. Mites with beneficial gut bacteria may have had a survival advantage, leading to the proliferation of these bacteria within the mite population. Similarly, bacteria that provided greater benefits to their hosts would have been favored through natural selection.
Co-evolutionary Dynamics: The relationship between oribatid mites and their gut bacteria is likely characterized by reciprocal adaptations. As mites evolved physiological features to accommodate and support specific bacterial communities, the bacteria may have co-evolved traits that optimize their survival and function within the mite gut.
Specificity and Diversity: Over time, the association between oribatid mites and gut bacteria may have become more specific, with certain bacterial species or strains being tightly associated with particular mite species or lineages. Additionally, the gut microbiota of oribatid mites may be diverse, with different bacterial taxa occupying distinct ecological niches within the gut.
Stability and Maintenance: Once established, the co-evolved relationship between oribatid mites and their gut bacteria would likely be stable and maintained across generations through mechanisms such as vertical transmission (from parent to offspring) and environmental acquisition (from the surrounding habitat).

This fascinating example of symbiosis highlights the intricate ecological and evolutionary dynamics that shape microbial associations in soil ecosystems.

These illustrate the diversity and potential complexity of these relationships:

Acari: Oribatida
- Oribatula tibialis: This species of oribatid mite is known to host diverse bacterial communities in its gut, including members of the phyla Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria.
- Oribatid mite species from the genus Oppiella: Studies have identified specific bacterial taxa associated with the gut of Oppiella mites, including genera such as Bacillus, Pseudomonas, and Enterobacter.
Bacterial Species
- Bacillus: Some species of Bacillus bacteria have been found in the guts of oribatid mites. These bacteria are known for their ability to produce enzymes that may aid in the digestion of organic matter.
- Pseudomonas: Certain Pseudomonas species have also been detected in the gut microbiota of oribatid mites. These bacteria are known for their metabolic versatility and may contribute to nutrient cycling processes within the mite gut.

As they say: further research is needed to elucidate the specific co-evolutionary dynamics between mite species and their associated bacterial communities across different species and geographic regions. in the meantime, here are some examples:

Oppiella nova
- Gut Bacteria: Members of the genera Bacillus, Pseudomonas, and Enterobacter have been detected in the gut microbiota of Oppiella nova.
Oribatula tibialis
- Gut Bacteria: This species of oribatid mite has been found to host diverse bacterial communities in its gut, including members of the phyla Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria.
Archegozetes longisetosus
- Gut Bacteria: Specific bacterial taxa associated with the gut microbiota of Archegozetes longisetosus include members of the genera Bacillus, Pseudomonas, and Enterobacter.
Hypochthonius rufulus
- Gut Bacteria: Studies have identified various bacterial taxa in the gut microbiota of Hypochthonius rufulus, although specific associations at the species level may not be well-documented.
Ceratozetes constrictus
- Gut Bacteria: Limited information is available regarding the gut microbiota of Ceratozetes constrictus and its associated bacterial taxa.

The presence of Bacillus, Pseudomonas, and Enterobacter bacteria in the guts of oribatid mites can provide insights into the type of food these mites consume and the potential roles of these bacteria in their digestive processes. However, it's important to note that the specific functions of these bacteria within the mite gut and their relationship with the mite's diet may vary depending on factors such as the mite species, environmental conditions, and microbial interactions. Here are some general implications:

Bacillus: Bacillus species are known for their ability to produce various enzymes that can degrade complex organic compounds, including cellulose, hemicellulose, and chitin. Therefore, the presence of Bacillus bacteria in oribatid mite guts suggests that these mites may consume a diet rich in plant material, fungi, or other organic matter that requires enzymatic breakdown for digestion.
Pseudomonas: Pseudomonas species are known for their metabolic versatility and the production of various enzymes and metabolites. In the context of oribatid mites, the presence of Pseudomonas bacteria in their guts may indicate a diet that includes a wide range of organic substrates, including plant material, fungal hyphae, and bacteria. Pseudomonas species may contribute to the degradation of complex organic compounds and the recycling of nutrients within the mite gut.
Enterobacter: Enterobacter species are facultative anaerobic bacteria that are commonly associated with the digestive tracts of animals. While their specific roles in the guts of oribatid mites are less well-characterized, the presence of Enterobacter bacteria may suggest that these mites consume a diet that includes organic matter rich in carbohydrates and proteins. Enterobacter species may contribute to the fermentation of complex carbohydrates and the metabolism of nitrogen-containing compounds in the mite gut.

This suggests that these mites are likely omnivorous or detritivorous, consuming a diet that includes a variety of organic substrates such as plant material, fungi, bacteria, and other detritus. The diverse microbial community in their guts probaly enable them to eat all manner of food, contributing to the nutrient cycling processes in soil and leaf litter ecosystems.

Tritegeus bisulcatus

"Microbiome plays an important role in digesting preferred fungal prey due to the molecular specificities of the fungal cell wall, it is likely that Zygoribatula and Scheloribates share mutualistic microbiota capable of degrading similar hyphae components." (Sanchez-Chavez et al 2023)

Different guts different bugs

Microbes in the oribatid gut changed with the oribatid species and habitats.

The microorganisms in the oribatid gut in one study significantly changed with the oribatid species and with the oribatids' habitats. Rhysotritia and Pergalumna taken directly from the natural habitat (leaf litter) and dissected, contained significantly lower frequencies of Bacillus and Pseudomonas isolations than those found in the moss-soil samples. (Wolf & Rockett 2009)

Bacteria in the alimentary canals of two oribatid mites, Rhysotritia sp. and Pergalumna sp., were examined from three different habitats. The three habitats included the mites'natural habitat and two artificial laboratory habitats, one with food and one without food. Gut microorganisms were obtained by dissecting paraffin-embedded, surface sterilized mites. Each alimentary canal and its contents, removed during dissection, were inoculated into 10 microbiological culture media. Bacteria, isolated from the oribatid gut, included: Bacillus, Pseudomonas, coryneforms, actinomycetes, Mycobacterium, Alcaligenes, Flavobacterium, Acinetobacter, and Citrobacter. Microorganisms from moss-soil samples of the oribatids natural habitat and laboratory foods were obtained and identified for comparison.

The microorganisms in the oribatid gut significantly (P < 0.05) changed with the oribatid species and with the oribatids' habitats. Rhysotritia and Pergalumna taken directly from the natural habitat and dissected, contained significantly lower frequencies of Bacillus and Pseudomonas isolations than those found in the moss-soil samples. After laboratory maintenance with food, dramatic shifts in the frequency of Bacillus and Pseudomonas were noted from both mites. A variety of bacteria were obtained from both oribatid species which had been starved for a period of 13 days.

Wolbachia

Most abundant symbiont

We determined the distribution of bacteria Wolbachia sp., Rickettsia sp., Cardinium sp., Spiroplasma sp., Arsenophonus sp., Hamiltonella sp., and Flavobacterium in oribatid mites (Acari: Oribatida).

Wolbachia is the most abundant intracellular symbiont among terrestrial Arthropoda. This bacterium together with other microorganisms, i.e., Cardinium, gained fame mainly as the causative agent of host sex-ratio distortion. Across the impressive diversity of oribatid mites (Acari: Oribatida), the microbes have been found in both parthenogenetic (Oppiella nova, Ceratozetes thienemanni, Hypochthonius rufulus) as well as sexually-reproducing (Gustavia microcephala, Achipteria coleoptrata, Microzetorchestes emeryi, Damaeus onustus) species. Wolbachia found in Oribatida represents supergroup E and is related to bacterial endosymbionts of springtails (Hexapoda: Collembola).

Aerobic & Anaerobic

Has to be both sort of bacteria, to deal with the changes soil conditions.

At first, I thought it odd that there were both aerobic and anaerobic bacteria. But what we do not know is whether some are confined to particular mites, or they are in all. We do not know, but it would be interesting to dig down a bit and find out whether some mites had certain sorts of bacteria, and if so where they live. At present we have only indicative signs rather than any ‘proof’.

It is also obvious that there has to be both sort of bacteria, to deal with the changes conditions in the soil – the Redox potential, we explored earlier. There is great scope for further research – especially comparing mite and springtail gut bacteria, but also many other small soil creatures. Another avenue yet to be explored is how and whether Bacillus and Alcaligenes are anaerobic in mite guts as a result of some relation with redox causing them to change to nitrate substrates. This would make some sort of sense, because when reductive breakdown processes start, the first substrate to be used are nitrates.

Digestive tract in Scheloribatida1 Parasagittal section of the digestive tract The oesophagus penetrates the centre of the synganglion (SG). The food bolus (FB) is in the ventricular part of mesenteron (M). 2 Mucoid particle (MD) in the caecum. 3 The ventricular part of mesenteron (M), deep parts of the caecum (CA) and the rectal sac (RE)
4, The digestive tract is shown at the same angle as in Fig. 3. CA, caecum; FB, food bolus; M, ventricular part of mesenteron; RE, rectal sac. (Shimano & Matsuo 2002)

Human gut bacteria

Actinomycetes – also found in oribatid guts - decline in elderly people.

We are hearing more about human gut bacteria. When we do, we often talk about fermentation – which is an anaerobic process, like we discussed earlier. So perhaps our bacteria have to be both aerobic and anaerobic. There is much debate and lots more research is needed, but it seems that having a diverse range of gut bacteria is generally beneficial to healthy. (Si et al 2021)

In human guts, these were identified as the top 5 bacteria phyla - Bacteroidetes, Firmicutes, Proteobacteria, Actinobacteria (same as Actinomycetes) and Verrucomicrobia (Si et al 2021). They found that Actinomycetes – also found in oribatid guts, declined in elderly people. The vast majority of Actinobacteria are important saprophytes capable of breaking down a wide range of plant and animal debris in the process of decomposition.I wonder where we got ours from.

In soil microbiology these days, there is a bit of a microbial mush. All the microorganisms are in some sort of DNA mix, with no distinction about where they may be. Up to half may be dead, with no way of telling. Some are free-living and have been for millions of years but others are more confined to guts, but we cannot tell. By identifying the role of oribatids and gut bacteria we can get a better picture of one of the multitude of living environments in soil. Anaerobic bacteria live in mite guts, but mites themselves need oxygen. The mite bacteria are a mix of both, better able to respond to changing conditions.

First Oribatids
Lower Oribatids (Carboniferous) Higher Oribatids (Cretaceous)

The 'higher' oribatids have evolved to transform soil, by spreading the humiifcation process much more widely. They can get into small pore spaces and so help not only produce humus, but help stick it to mineral particles. This makes larger aggregates improving the resilience of the soil structure. We will see, as a result to their wider distribution, how they change their eating habits in the next period - Cretaceous mites.

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