Introduction

The seagrass Halodule wrightii is infected by the protistan parasite Plasmodiophora diplantherae in the Gulf of Mexico (Braselton and Short 1985, Walker and Campbell 2009), but how it may contribute to current seagrass losses is unknown. This parasite causes plant stunting and seagrass uprooting, which can be detrimental to seagrass ecosystem health and coastal seagrass restoration projects. It was first reported from the north-central Gulf of Mexico (LA and MS) in 2009 (Walker and Campbell); however, its wider distribution and abundance in this economically import region are currently unknown and may have expanded since the Deepwater Horizon incident in 2010.

Seagrass ecosystems, including those in Gulf of Mexico region, are currently under a variety of pressures and suffering unprecedented losses (Orth et al. 2006). Anthropogenic impacts such as water quality decline, dredging and boating, and shoreline alteration contribute to stressful growing conditions for seagrass plants. Stressed plants are prone to disease, therefore mapping the incidence of P. diplantherae infection will help to better understand H. wrightii health. Seagrass restoration projects often are initiated in stressful environments where plants have died-off previously and disease may inhibit successful re-establishment. Loss of seagrass and restoration failures can have negative consequences on economic fisheries activities, working waterfronts, and coastal communities.

Examples of sudden large-scale declines in seagrass populations (mass mortality) include the eelgrass wasting disease epidemic, which killed up to 90% of Zostera marina plants along the Atlantic coasts of North America and Europe in the 1930s (Muehlstein 1989). This event had a significant negative impact on the fisheries industry and waterfowl populations in the region (Orth et al. 2006).  Smaller scale events have been reported along the US east coast in the 1980s (Short et al., 1987), as well as other parts of the world (Armiger 1964, Vergeer and den Hartog 1971, den Hartog et al. 1996).   In the Gulf of Mexico an estimated loss of up to 90% of Thalassia testudinum plants in parts of Florida Bay occurred between 1987 and 1990 (Robblee et al. 1991) with Labyrinthula sp. implicated as a potential causative agent (Blakesley et al. 1999).  There is evidence that increased temperature and salinity favor increased incidence of the disease, as both favor activity of the pathogen and contribute to stress in the plants (reviewed in Sullivan et al. 2013).  Whether P. diplantherae is capable of contributing to similar catastrophic seagrass loss is currently unknown.

Additionally, protistan parasites such as P. diplantherae, play an underappreciated role in carbon dynamics in coastal marine ecosystems. The amount of organic carbon stored in healthy living seagrass biomass is estimated at 2.52 Mg C ha-1 on average, much of which is buried in the soil as rhizomes and roots (Fourqurean et al., 2012). Loss of seagrass populations can release this sequestered carbon back into the ocean and the atmosphere. Poor root development in H. wrightii plants infected with P. diplantherae causes them to be easily uprooted by the motion of currents, meaning carbon from their roots cannot be added to sediment carbon pools (Gleason et al. 2013). Experts in marine protistan parasite ecology suspect this parasite may be expanding its range and serve as an indicator of climate change in this region (F. Gleason, pers. comm.). How climate change may interact with this parasite-host plant system is currently unknown, but the northern Gulf of Mexico is likely to become warmer under predicted climate change regimes (Turner 2003) with unknown consequences for living natural resources.

Plasmodiophorids (Phytomyxea) form an enigmatic group of obligate biotrophic parasites, with little known about their ecology and pathology in marine ecosystems. Most of the 41 known plasmodiophorid species are associated with terrestrial and freshwater ecosystems. Their potential to influence marine ecosystems, either directly by causing host disease or indirectly as vectors of viruses, is enormous, although still unexplored (Neuhauser et al 2011a). In all, 20% of the currently described phytomyxean species are parasites of some of the key marine primary producers, such as seagrasses, brown algae and diatoms; however, information on their distribution, abundance and biodiversity is lacking. The majority of the currently known marine single-stranded RNA viruses structurally resemble the viruses transmitted by phytomyxean species to crops in agricultural environments. Qualitative and quantitative data on distribution and interaction with primary and alternative host plants are needed for plasmodiophorid marine parasites (Neuhauser et al 2011a).

Infections with Plasmodiophora spp. can decrease host fitness by reducing the growth of the host plant (Johnson and Sparrow 1961; den Hartog 1989; Walker and Campbell 2009), reducing the formation of inflorescences (den Hartog 1989), and by altering host metabolism and/or reproductive success (Neuhauser et al 2011a). Changes in host morphology are illustrated in Figures 1 and 2. Plasmodiophorid parasites cause increased uprooting of their angiosperm hosts as a consequence of reduced root growth. In areas where seagrass-restoration projects are ongoing, this uprooting can subsequently damage seagrass beds (Walker and Campbell 2009) and there is also a considerable risk of floating plants spreading this pathogen to adjacent populations of seagrasses (Neuhauser et al 2011a). As well, resting spores of these parasites can persist in marine sediments for an unknown period of time (Figure 2b).

Recent 18S rDNA surveys have revealed a diversity of small flagellate eukaryotes such as Plasmodiophorids in many different aquatic habitats (Moreira & López-García, 2002; Gleason et al., 2008; Lefèvre et al., 2008; Lopez-Garcia & Moreira, 2008; Sime- Ngando et al., 2011). In all studies an unexpectedly high diversity of parasitic organisms was found, but Plasmodiophorids still remain under-sampled, with multiple undescribed species thought to exist in most habitats (Neuhauser et al. 2011b). Studies with primers specific for Plasmodiophorids could provide information on biodiversity, distribution and abundance of these parasites in a wide range of aquatic habitats. This information is needed to accurately and rapidly assess their ecological and economic significance. Recent studies highlight the importance, diversity and understudied nature of eukaryotic marine microbes in the North-Central Gulf of Mexico region (Salamone and Walker, in review; Mata and Cebrián 2013; Walker and Campbell 2009, 2010).

The objectives of this study are to map the incidence and relative abundance of the seagrass parasite P. diplantherae in the North-Central Gulf of Mexico (LA, MS, AL, FL) and to obtain genetic material (DNA sequences) to place this parasite phylogenetically. Successful DNA extraction and amplification will aid in future development of rapid molecular diagnostic tools for the detection of this parasite in seagrass plants and coastal sediments.

__________________________________________________________________________________________________________

Literature Cited

Armiger, LC. 1964. An occurrence of Labyrinthula in New Zealand Zostera. New Zealand Journal Botany 2, 3–9.

Blakesley, B.A., Landsberg, J.H., Hall, M.O., Lukas, S.E., Ackerman, B.B., White, M.W., Hyniova, J., Reichert, P.J., 1999. Seagrass disease and mortality in Florida Bay: understanding the role of Labyrinthula. In: Florida Bay and Adjacent Marine Systems Science Conference, Nov. 1-5, Key Largo, Fl., pp. 12-14.

Braselton JP and FT Short 1985. Karyotypic analysis of Plasmodiophora diplantherae. Mycologia 77: 940-945.

den Hartog, C. 1965. Some notes on the distribution of Plasmodiophora diplantherae, a parasitic fungus on species of Halodule. Persoonia 4: 15-18.

den Hartog, C, L Vergeer, and AF Rismondo. 1996. Occurrence of Labyrinthula zosterae in Zostera marina from Venice Lagoon. Botanica Marina 39, 23–26.

Fourqurean, JW, Duarte, CM, Kennedy, H, Marbà, N, Holmer, M., Mateo, MA, Apostolaki ET, Kendrick GA, Krause-Jensen D, McGlathery KJ & O Serrano. 2012. Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience 5(7): 505-509.

Gleason FH, Kagami M, Lefèvre E, and T Sime-Ngando. 2008. The ecology of chytrids in aquatic ecosystems: roles in food web dynamics. Fungal Biology Reviews 22:17–25.

Gleason FH, van Ogtrop F, Lilje O, and AWD Larkum. 2013. Ecological roles of zoosporic parasites in blue carbon ecosystems. Fungal Ecology 6(5): 319-327.

Guindon S and O Gascuel. 2003. A simple, fast, and accurate algorithm to estimate large phylogenies by maximum likelihood. Syst Biol 52:696-704.

Lefèvre EB, Amblard RC and T Sime-Ngando. 2008. The molecular diversity of freshwater picoeukaryotes reveals high occurrence of putative parasitoids in the plankton. PloS One 3: e2324.

Lopez-Garcia P and D Moreira. 2008. Tracking microbial biodiversity through molecular and genomic ecology. Research in Microbiology 159: 67–73.

Mata JL and J Cebrián. 2013. Fungal endophytes of the seagrasses Halodule wrightii and Thalassia testudinum in the north-central Gulf of Mexico. Botanica Marina 56(5-6): 541–545.

Moreira D and P López-García. 2002. The molecular ecology of microbial eukaryotes unveils a hidden world. Trends in Microbiology 10: 31–38.

Muehlstein LK 1989. Perspectives on the wasting disease of eelgrass Zostera marina. Diseases of Aquatic Organisms 7: 211-221.

Neuhauser S, Kirchmair M and FH Gleason. 2011a. Ecological roles of the parasitic phytomyxids (Plasmodiophorids) in marine ecosystems – a review. Marine and Freshwater Research 62(4): 365–371.

Neuhauser S, Kirchmair M and FH Gleason. 2011b. The ecological potentials of Phytomyxea (‘Plasmodiophorids’). Hydrobiologia 659:23–35.

Neuhauser S, Bulman S and M Kirchmair. Plasmodiophorids: the challenge to understand soil-borne, obligate biotrophs with a multiphasic life cycle. In: Gherbawy, Y.; Voigt, K., editors. Current Advances in Molecular Identification of Fungi. Springer; Heidelberg, Germany: 2010. p. 51-78.

Orth RJ, Carruthers TJB, Dennison WC, Duarte CM, Fourqurean JW, Heck KL, Hughes AR, Kendrick GA, Kenworthy WJ, Olyarnik S, Short FT, Waycott M, and SLWilliams . 2006. A global crisis for seagrass ecosystems. Bioscience 56: 987–996.

Robblee MB, Barber TR, Carlson Jr. PR, Durako MJ, Fourqurean JW, Muehlstein LK, Porter D, Yarbro LA, Zieman RT, and JC Zieman. 1991. Mass mortality of the tropical seagrass Thalassia testudinum in Florida Bay (USA). Marine Ecology Progress Series 71: 297-299.

Salamone A and AK Walker. Fungal diversity and succession in artificial reef marine biofilm in the north-central Gulf of Mexico. Microbial Ecology (In review)

Schmitt I, Barker FK (2009) Phylogenetic methods in natural product research. Nat Prod Rep 26:1585-1602.

Short, FT, Muehlstein, LK, and D Porter. 1987. Eelgrass wasting disease: cause and recurrence of a marine epidemic. Biological Bulletin 173, 557–562.

Sime-Ngando T, Lefevre E and FH Gleason. 2011. Hidden diversity among aquatic heterotrophic flagellates: ecological potentials of zoosporic fungi. In: SIME-NGANDO T, NIQUIL N (Editors) : Disregarded microbial diversity and ecological potentials in aquatic systems. Hydrobiologia, Special Issue 659 (1): 5-22.

Sullivan, BK, Sherman, TD, Damare, VS, Lilje, O, and FH Gleason. 2013. Potential roles of Labyrinthula spp. in global seagrass population declines. Fungal Ecology 6, 328–338.

Turner, R.E. 2003. Coastal Ecosystems of the Gulf of Mexico and Climate Change. Pages 85-103 in Z. H. Ning, R. E. Turner, T. Doyle, and K. K. Abdollahi, editors. Integrated assessment of the climate change impacts on the Gulf Coast region. Gulf Coast Climate Change Assessment Council and Louisiana State University Graphic Services, Washington, D.C., USA.

Vergeer, L, C. Hartog, Den.1991. Occurrence of wasting disease in Zostera noltii. Aquatic Botany 40, 155–163.

Walker AK and J Campbell. 2009. First records of the seagrass parasite Plasmodiophora diplantherae from the Northcentral Gulf of Mexico. Gulf and Caribbean Research 21: 63-65.

Walker AK and J Campbell. 2010. Marine fungal diversity: a comparison of natural and created salt marshes of the north-central Gulf of Mexico. Mycologia 102:513-521.