Deep-sea sediments - An open door for homeless microbes

Primary productivity in the upper layers of the ocean influence the structure of heterotrophic macro-fauna and micro-organism communities. Events such as phytoplankton blooms in particular, are the main food source for pelagic organism and can shift entire community structures. The amount of organic matter that escapes the remineralisation of the biomass and is transported to the seafloor is less then 2%. Life in the abyss is dependent on this input of photosynthesis- derived particulate organic carbon (POC) and are thought to influence microbial benthic community structures. Most previous studies, focused on macro-faunal benthic communities, found similar benthic communities in high and low productivity areas. To investigate the effects of phytoplankton blooms on the less studied benthic microbial communities, Ruff et al. (2014) undertook a survey in the Southern Atlantic Ocean, where there is a large, but sporadic export of organic matter to the deep sea. Samples were collected from four abyssal sediments and one shallow reference site and analysed using a comparative 16S rRNA sequencing and CARD-FISH method. 

Fresh algal pigments found on sites suggest a quick transport of the bloom derived POC to the deep sea. The transport may not take longer then a few days or weeks, which suggests a strong benthic-pelagic coupling. Using 16S rRNA  extraction is a useful indicator for viability, as once an organism is dead, RNA molecules are degraded quickly. The microbial communities generally showed a high diversity and evenness across all sampling sites. Even though, spread out over a very large spatial distance (4000 km), these sites experience different surface primary productivity. There was no shift found of the most important clades, alphoproteobacterial Rhodospirillaceae and Rhodobacteraceae as well as gammaproteobacterial ´Marine Benthic Group`(Sinobacteraceae) and NOR5/OM60. Although some organisms responded to the input of fresh POC, the total cell number in the sediment seemed to stay stable. Comparison with results from previous incubation experiments however, leads to discrepancy and it seems like the fate of POC in the seafloor still remains largely unknown.

NOR5/OM60, normally found in surface waters and coastal sediments with high productivity, were found in high cell numbers as part of the micro-organism community. As these organisms were found in the deeper sediment layers, they are thought to be indigenous to the benthic habitat. It seems like benthic organisms occupy the same ecological niche as their pelagic relatives. NOR5/IM60 only being an example, it may well be that organisms originated from the surface, accumulated, proliferate and diversify in the benthic sediment, establishing new variable communities. 

Viable Flavobacteriacea found in a thick layers of diatom ooze, in form of broken diatom shells and fecal pellets, suggest that these populations are transported to the seafloor being attached to fecal pellets or diatoms.  Fast settling salp fecal pellets especially might be a possible way of a quick way of organic matter being exported down to the deep sea. Events such as mass mortality and aggregate formation of Chaetoceros spp., as it was recorded at one sampling site, could also cause massive pulses of POC to deep sea areas. 

Even though this study is of a high standard, it has to cope with a few problems. The authors mentioned that a reference sample, which had not been influenced by phytoplankton blooms, would be necessary for a better understanding of microbial benthic communities in the deep sea. However, such a reference site could only be found in the winter, when harsh environmental condition make them unaccessible. I also think that the currents, eddies and meandering of frontal systems which may cause lateral transportation of the settling material should be taken more into account.

Overall, phytoplankton derived organic matter and possibly viable pelagic microorganisms are quickly enough transported to the deep sea, causing a community shift towards bacterial clades that seem to be specialised in the degradation of algal detritus. The polar front deep-sea sediment may represent a niche for these organisms that was so far overlooked.

Ruff S. E., Probandt D., Zinkann A., Iversen M. H., Klaas C., Würzberg L., Krombholz N., Wolf-Gladrow D., Amann R. & Knittel K. (2014) Indications for algae-degrading benthic microbial communities in deep-sea sediments along the Antarctic Polar Front. Deep-Sea Research II. 108: 16-16.