This year-long study in the Pacific ocean, through monthly sampling of the water column (surface to 4,750 m), shows that the ocean's interior, earth's largest biome, is mostly an archaea dominated ecosystem.
Using single-cell, fluorescent in situ hybridization (FISH) with specific rRNA-targeted oligonucleotide probes for either pelagic archaea (crenarchaeota and euryarchaeota) or bacteria, the authors calculated that world’s ocean contains approximately 1.3x1028 archaeal cells and 3.1x1028 bacterial cells and that both archaeal groups as well as bacteria collectively, lack a clear seasonal trends in cellular abundance.
Bacteria generally tend to dominate the microbial population in the upper 150m of the water column, representing up to 90% of all picoplanktonic cells. Then they clearly decrease in relative abundance with increasing depth, and below 1,000m they represent only 35±40% of total cells.
Pelagic archaea instead, show different patterns of abundance in the open sea. Pelagic euryarchaeota occasionally appear with low numbers in the near surface layer, but they generally remains at a few per cent of the total count over the entire water column. By comparison, a large fraction of total marine picoplankton appear to be represented by one specific clade, pelagic crenarchaeota. These archaea, are a consistent and significant component of deep sea microbiota and they represent one of the ocean's single most abundant cell types. In surface layers, pelagic crenarchaeota are only present sporadically, and never abundant numerically. The major relative increase in their abundace occurs below the euphotic zone (>150m). Then, below 1,000m and throughout the entire mesopelagic and bathypelagic zones, the fraction of crenarchaeota constantly increases with depth, even to the extent that it can equal bacterial population in cell numbers.
On the other hand, the authors found that the total number of microbial cells, sharply decreases with depth (an order of magnitude between 150 and 1,000 m), so they came to the conclusion that with increasing depth, there may be a population shift to microbial groups better adapted to deep sea conditions, such as pelagic crenarchaeota. As a matter of fact, bacteria and crenarchaeota standing stocks are inversely proportional and crenarchaeota can often rival bacterial abundances in the meso and bathypelagic zones. Moreover, most of pelagic deep-sea microbes (archaea and bacteria) were found to contain significant amounts of rRNA, this means that they are metabolically active contributors to the ecosystem. Thus, archaea are an important component of world's deep oceans and their large distribution throughout the entire oceanic water column suggest that they are really far from being merely confined just to extreme habitats as we thought in the past.
Reference:
Karner, M.B., delong, E.F. and Karl, D.M. (2001) Archaeal dominance in the mesopelagic zone of the Pacific Ocean. Nature 409: 507–510.
1 comment:
Giuseppe - it's hard to realize how significant the papers on marine Archaea were. Of course, they were already known in extreme habitats but 1n 1992 the first environmental genomic studies showed that they are present in pelagic waters. Nine years later, this paper showed that one group of the Archaea (Crenarchaeota) completely dominated deep waters but Karner at al. did not know what their function or activity was. Since 2005 we have known that they are metabolically active oxidizing ammonia. There seems to be both autotrophic and heterotrophic activities going on.
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