Microbial Life of Deep-Sea Anoxic Brine Pools

The taxa associated with this page are among those microbes found in or just above deep-sea hypoxic brine pools.

Deep-sea hypoxic brine pools are pools of hypersaline, dense, oxygen-depleted water found in depressions in the ocean floor. They are rare, found predominantly in the eastern Mediterranean Sea, the Red Sea, and the southern Gulf of Mexico. These brines can be formed in various ways but are associated with tectonic activity and the presence of salty evaporite deposits on or just below the seabed; extremely salty brine (an order of magnitude or more saltier than surrounding seawater) is created by the dissolution of these deposits and is too dense to mix with the overlying ocean water, coming to rest in depressions on the seafloor. In addition to high ion concentrations, brine pools are often characterized by elevated temperature, sulfide concentration, and methane content. All of these differences from the overlying seawater are sharply delineated, within a vertical distance of just one to five meters. This boundary area is known as the chemocline. Even beyond these extreme conditions, brine pools are also generally located at great depth — 1000m or deeper, well beyond the reach of sunlight. Despite their unique physiochemical conditions, brine pools show strong evidence of supporting microbial life (Gill et al 2005; Sass et al 2008; Antunes et al 2011).

Indirect evidence of life within the chemocline and even the brine itself has been gathered from many locations (Eder et al 2001; Eder et al 2002; Borin et al 2009). This evidence includes microbial rRNA sequences (Sass et al 2008; Eder et al 2001; Eder et al 2002), stable-isotope signatures of carbon and oxygen (Gill et al 2005; Sass et al 2008), and direct observation of cells (Eder et al 2001; Borin et al 2009). None of these results are definitive proof of an active microbial community in the brine or chemocline — rRNA and other cell contents can and do appear in brine due to the lysing of planktonic bacteria which sink out of their true range, stable-isotope analyses are highly tangential evidence, and even direct observation of cells in brine samples does not necessarily demonstrate that these cells are potentially active in a brine environment (Sass et al 2008; Eder et al 2002). The key piece of evidence, only occasionally successfully gathered, is a cultivation experiment — successfully culturing bacterial cells from brine samples in conditions similar to those of their sample environment (Antunes et al 2011). Taken together, however, there is a compelling body of evidence to suggest that a specialist microbiota adapted to the extreme environment of deep-sea brine pools exists, especially as certain types of rRNA sequence and cell morphotype are uniquely associated with brine pool habitats (van der Wielen et al 2005)

The chemocline in particular seems to be a local hotspot of biological activity, with observed cell frequencies up to 100 times greater than the adjacent water column (Borin et al 2009; Sass et al 2001). This may be due to its complex, variegated environment — the chemocline contains overlapping gradients of several variables. Additionally, the rapid change of density changes in the chemocline act as a ‘particle trap,’ collecting debris and microbes filtering down from the water column. It is even possible that some dormant falling bacteria reactivate and propagate when they reach the more nutrient-rich conditions of the chemocline (Borin et al 2009). 

No hypothesis explaining the continued existence and dispersal of brine pool specialist species has yet been supported. It is possible that the strains found in the chemocline are present in the water column in much lower densities and successfully colonize chemocline environments, though they have never been observed at any distance from brine pools. A more outlandish hypothesis is that brine-pool colonists may have lain dormant in endospore form for thousands or millions of years in the evaporite deposits underlying their brine pools. Some microbes are known in theory to have this ability, but this is strict conjecture at the present time (Antunes et al 2011).  

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© Peter Everill

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