OMG in the OMZ: massive marine microbes

Today, I'm drawing inspiration from the Census of Marine Life, a decade-long project which has produced a huge inventory of marine life - a baseline catalogue to be used for further research and to inform the management and conservation of marine life. The Census looked at all scales from microbes to whales, at all latitudes and at all depths. The Census has produced a range of books, both popular and technical - one of the most straightforward and non-specialist, 'Citizens of the Sea' (Knowlton 2010) provided a couple of snippets that induced me to delve into the detail rather more...

First up, megabacteria - not the disease of budgies (which is actually a yeast), but very large true bacteria discovered off the coast of Chile and Peru in the 1960s. Placed in the genus Thioploca, the bacteria are filamentous and 2 to 7 cm (yes, cm) long. Secreting mucus, they form vast mats (the largest covering 130,000 km-sq) in/under the 'oxygen minimum zone' (OMZ), an area at 40-280 m with very little dissolved oxygen; instead they have to rely on hydrogen sulphide in the sediments. They oxidise this using nitrates (from sea water) which they can concentrate up to 500 mM in the liquid vacuole that occupies over 80% of their cell volume, even though the concentration of nitrates in sediment is only around 25 μM. Mucus-sheathed transport filaments send this nitrate 5–10 cm down into the sediment and reduce it, thus oxidising the hydrogen sulphide and creating a coupling of the nitrogen and sulphur cycles in the sediment (Fossing et al. 1994), producing pyrite and elemental sulphur as a result (Ferdelman et al. 1997). Thus, organic matter (in the form of anaerobic dissolved organic carbon) can be oxidised at low oxygen concentrations. The mats also provide food and shelter for a range of animals including squat lobsters (Pleuroncodes monodon), amphipods, and ophiuroids (Grupe 2011). As the OMZ shares features with conditions during the Proterozoic period (2.5 bya to 650 mya), and similar microfossils have been found, such bacteria may provide an insight into ancient life forms and ecology as well as performing a still little-known but key function in nutrient cycling. Research is ongoing with one recent example investigating Thioploca found in Danish waters where (in the species T. ingrica) nitrate accumulation was lower at around 3 mM, with bicarbonates and acetates used as carbon sources, and no mat being formed (Høgslund et al. 2010). 

A core from a Thioploca bacterial mat. The core is about 8 cm across and the mat about 1 cm thick. The mat is made up of many bacterial filaments with individual cells visible to the naked eye as white threads. Huge for bacteria! Photo courtesy of NOAA/Lisa Levin.

Now, ocean acidification due to carbon emission from fossil fuels may affect marine microbes - with microbial ecosystems responsible for between 50 and 90% of all marine biomass and over 95% of marine respiration, they maintain Earth's habitability though their influences on climate (they can sequester atmospheric carbon dioxide), nutrient cycling and the decomposition of pollutants (Leahy 2012). So, this could be very serious indeed and current research is looking at the  sensitivity of marine microbes to acidification. If I find links to results from this research, I'll post an update, plus I have some more bacterial and marine posts (among others) in the pipeline.

References

Ferdelman, T.G., Lee, C., Pantoja, S., Harder, J., Bebout, B.M. & Fossing, H. (1997). Sulfate reduction and methanogenesis in a Thioploca-dominated sediment off the coast of Chile. Geochimica et Cosmochimica Acta 61(5): 3065-3079. Fossing, H, Gallardo, V.A., Jørgensen, B.B., Hüttel, M., Nielsen, L.P., Schulz, H., Canfield, D.E., Forster, S., Glud, R.N., Gundersen, J.K., Küver, J., Ramsing, N.B., Teske, A., Thamdrup, B. & Ulloa, O. (1994). Concentration and transport of nitrate by the mat-forming sulphur bacterium Thioploca. Nature 374: 713-715.
Grupe, B. (2011). Sea Floor Habitats of the Chile Margin. NOAA Ocean Explorer [accessed 15/02/2012].
Høgslund, S., Nielsen, J.L. & Nielsen, L.P. (2010). Distribution, ecology and molecular identification of Thioploca from Danish brackish water sediments. FEMS Microbiology Ecology 73(1): 110-120.
Knowlton, N. (2010). Citizens of the Sea: Wondrous Creatures from the Census of Marine Life. National Geographic, Washington DC.
Leahy, S. (2012). Giant Bacteria Colonize the Oceans. Tierramérica. [accessed 15/02/2012].