Nitrogen-fixing bacteria

There are several unrelated lines of bacteria that are able to use gaseous nitrogen to convert it into a solid form, such as nitrate or nitrite. The most ancient of these groups is the cyanobacteria (aka blue-green algae). This is a globally and evolutionarily important group of photosynthetic bacteria.

Other groups, such as the rhizobia found in the root nodules of legumes, the actinomycete bacteria in the alders (Alnus), and non-cyanobacterial diazotrophs (NCDs), are also able to convert nitrogen, but they are unrelated to these cyanobacteria. This webpage focuses primarily on cyanobacteria.

Given that 78% of the atmosphere is nitrogen gas, and this molecule is a crucial building block of chlorophyll, essential for all photosynthesis, these bacteria play a vital role in the Earth's nitrogen cycle. Until the advent of the Haber-Bosch reaction, this was the only method of obtaining a significant amount of solid nitrogen.

Cyanobacteria, also known as the blue-green algae, played a significant role in oxygenating the atmosphere of early Earth, billions of years ago. Today, water pollution from fertilizers can lead to blooms of cyanobacteria. As this overabundance of algae dies, the decomposition process removes oxygen from the water, which kills animal life. This process is known as eutrophication.

Features

Photosynthetic

Phycocyanin gives bluish color ("blue-green algae")
Chlorophyll a, which has peak absorption in the red and violet spectra
The chloroplasts in plants, which contain chlorophyll a, comes from endosymbiosis with a cyanobacterium

Nitrogen-fixation

Many "fix" nitrogen, converting gaseous N2 into ammonia (NH3), nitrate (NO3), or nitrite (NO2)
These bacteria use a structure called the heterocyst to convert the atmospheric nitrogen to a solid (usable) form
These bacteria have been known to form root relationships with cycads and Azolla and a single angiosperm (Gunnera), as well as non-root-forming organisms, like hornworts, some green algae (e.g. Chlorella), and lichens that form symbioses with cyanobacteria
The conversion of nitrogen, from the gaseous phase to solid phase, is difficult because of the bonds formed by N2
- Cyanobacteria are the only organisms able to naturally make this conversion
- Some nitrogen gas is converted to solid forms through lightning strikes, but this is limited
- In the early 20th Century, an artificial form of nitrogen fixation was created, called the Haber-Bosch process

Above: A cyanobacterium, Anabaena, displaying many vegetative (photosynthetic) cells and a single heterocyst: the site of nitrogen fixation

Red / pink pigmentation

Some forms are red or pink from the pigment phycoerythrin
These bacteria are often found growing on greenhouse glass, or around sinks and drains.
The Red Sea gets its name from occasional blooms of a reddish species of cyanobacteria called Trichodesmium erythraeum
- Trichodesmium is thought to fix nitrogen on such a scale that it accounts for almost half of the nitrogen fixation in marine systems globally (Bergman et al. 2013)
African flamingos get their pink color from eating Spirulina, which contains a natural pink dye called canthaxanthin

Above: Blooms of cyanobacteria in the Red Sea

Above: Oscillatoria strands

Above: Spirulina strands

Geologic Age

Paleoarchean (3.43 billion years ago) - present
Cyanobacteria as a whole branched off from other bacteria around 3.4 billion years ago (Fournier et al. 2021)
This group may be as old as 3.8 bya (Eoarchean)

Stromatolites

Stromatolites are large bacterial communities that cement sand as they grow, building up layers gradually over time.
- The cyanobacterial layer coats the top portion of the stromatolite, conducting photosynthesis
Possible evidence of stromatolites from Greenland sediments as early as 3.7 Ga (Nutman et al. 2016)
Maximum diversity of stromatolites during Mesoproterozoic (between 1.0 and 1.6 billion years ago)
Stromatolites begin to precipitously decline at about 1.0 Ga (Bernard et al. 2013)
Living stromatolites still survive on the western coast of Australia

Nitroplast & Diazoplasts

Nitroplasts are nitrogen-fixing organelles found in certain algae (e.g., haptophytes)
Diazoplasts are nitrogen-fixing organelles found in certain species of diatoms
Both organelles have their own DNA, similar to chloroplasts and mitochondria. This suggests an endosymbiotic origin from a nitrogen-fixing bacterium or archaea (Schwartz et al., 2024).

Above: Close-up of a living stromatolite

Below: Stromatolites at Shark's Bay, Australia

Additional Resources

Long-term analysis yields clearer picture of toxin-producing blue-green algae blooms (Phys.org 11Nov2025)

└Two dueling Dolichospermum strains in an Oregon lake determine occurrence of cyanotoxic blooms (Dreher et al., 2025)

A genetic switch lets plants accept nitrogen-fixing bacteria (Phys.org 6Nov2025)

└Two residues reprogram immunity receptors for nitrogen-fixing symbiosis (Tsitsikli et al., 2025)

Nitrogen fixation phenomenon discovered in the Arctic could boost marine life (Phys.org 20Oct2025)

└Nitrogen fixation under declining Arctic sea ice (von Friesen et al., 2025)

Tiny ocean partnership between diatoms and N2-fixing bacteria reveals secrets of evolution (Phys.org 29Aug2025)

└Stepwise genome evolution from a facultative symbiont to an endosymbiont in the N2-fixing diatom-Richelia symbioses (Grujcic et al., 2025)

New method detects toxic blue-green algae in lakes before blooms form (Phys.org 28Jul2025)

└Sound et al. (2025) A Protein-Centric Mass Spectrometry Approach for Species Identification within Harmful Algal Blooms

Researchers look to microbial interactions for early indicators of harmful algal blooms (24Jul2025)

└Siddiquee et al. (2025) Uncovering microbial interactions in a persistent Planktothrix bloom: Towards early biomarker identification in hypereutrophic lakes

Scientists predict Lake Erie's algae bloom will be smaller than last year (Phys.org 27Jun2025)
Living Near Toxic Algae Blooms Cuts ALS Survival by Year (ScienceBlog 17Jun2025)

└Batterman et al. (2025) Life Course Exposure to Cyanobacteria and Amyotrophic Lateral Sclerosis Survival

Keeping time in cyanobacteria: Scientists discover 'ticking' mechanism driving nature's simplest circadian clock (Phys.org 13Jun2025)

└Furuike et al. (2025) The priming phosphorylation of KaiC is activated by the release of its autokinase autoinhibition

Microbiologists document algal toxins in Lake Erie beach (Phys.org 12Jun2025)

└Moots et al. (2025) Microcystin persistence in Lake Erie foreshore sands

What's really 'fueling' harmful algae in Florida's lake Okeechobee? (Phys.org 10Sep2024)

└Lapointe et al. (2024) Nutrient availability in a freshwater-to-marine continuum: Cyanobacterial blooms along the Lake Okeechobee Waterway

Scientists discover first nitrogen-fixing organelle (Phys.org 11Apr2024)

└Coale et al. (2024) Nitrogen-fixing organelle in algae

└Massana (2024) The nitroplast: a nitrogen-fixing organelle

Scientists use blue-green algae as a surrogate mother for 'meat-like' proteins (Phys.org 27Feb2024)

└ Zedler et al. (2023) Self-Assembly of Nanofilaments in Cyanobacteria for Protein Co-localization

Scientists Just Came Up With a Wild Idea For Making Oxygen on Mars (ScienceAlert 21Oct2023)
Cyanobacteria reveal a blueprint for photosynthesis (Phys.org 1Sep2022; Dominguez-Martin et al. 2022)
A natural CO2-sink thanks to symbiotic bacteria (Max Planck Institute for Marine Microbiology 3Nov2021)
Bacteria could be speeding up the darkening of Greenland's ice (The Guardian 2016)
Cyanobacteria could offer hope for hard-to-treat cancers (Experimental Biology April 2016)
Toxic cyanobacteria proliferating in European and North American Lakes (SciNews 27Feb2015)
Foraminifera may have led to decline of stromatolites (Woods Hole Oceanographic Institution 28May2013)

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