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).
|NOAA/Lisa Levin.Photo courtesy of|
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].