Do Microbes Run With The Cartel? - Eukaryotic Microbes Dominate Bathypelagic Marine Snow

Marine snow is composed of a variety of different organic and inorganic materials. These include fecal matter, dead phytoplankton and sand. The majority of marine snow is held together by polysaccharides excreted as waste by marine phytoplankton and bacteria, TEPs. The varying components which make up marine snow influence the buoyancy of each individual aggregate particle. This variety of buoyancy combined with the small size of marine snow particles means it can take weeks before particles reach the sea floor if they are negatively buoyant enough to do so.

A recent study by Bochdansky et al collected bathypelagic marine snow to investigate the relative distribution of microbial eukaryotes. Samples were collected during a research expedition to the North Atlantic and Arctic Basin using Niskin bottles. Depths for collection were selected to specifically target bathypelagic water columns. The water collected was then gravity fed through 30μm filters. This method was used to collect samples which can be neutrally buoyant so are unaffected by settling velocity. CARD-FISH was then performed on the 30μm filter. This was done to examine the composition of eukaryotic microbes and their abundance in relation to 'prokaryotic' microbes. Counts were then taken of the eukaryotes and bacterial microbes to obtain information on the spatial heterogeneity. This was done to make sure the organisms present on the filter membrane was due to marine snow particulate and not random intercepts.

The authors found that 'prokaryotes' were 5 times more over dispersed than expected from a random distribution. The standard deviation for 'prokaryotes' was also 83% of the mean. This shows that microbes which landed on the filter were not random intercepts but part of a marine snow aggregate. It was shown that fungi and labyrinthulomycetes accounted for approximately 1/5 of all eukaryotes found on the filter membrane. Being very resistant these organisms have the potential to contribute significantly to the breakdown of deep sea organic matter. The ratio of eukaryotic to 'prokaryotic' microbes was similar to that of surface environments where bacterivores control 'prokaryotic' microbial populations. The authors therefore suggest that bacteriovores have the same capacity to control 'prokaryotes' on marine snow in the bathypelgic as at the surface. The authors also found that Fungal cells are present 1000 times less often than 'prokaryotic' organisms, making their biomass approximately equal. The authors then proceed to discuss the possibility of fungal hyphae stabilising marine snow similar to that with sediments and discussing the different types of organic matter fungi degrade and how that relates to the competition with bacteria. Noting that fungi degrade Particulate organic matter with a higher carbon to nitrogen ratio so may not compete with bacteria for resources but work together in a synergistic fashion.  This is interesting as it raises the question of what role fungi play in the breakdown of organic matter in the bathypelagic. Perhaps more study into this field could really broaden our understanding of where eukaryotic organisms fit into the deep sea organic carbon flow.

Labyrinthulomycetes which are found to be of higher numbers than fungal cells in the water column were found to be of lower numbers on marine snow. This is an interesting discovery as labyrinthulomycetes have previously been suggested as an important degrader of sedimentary deep sea organic matter. I wonder what could be causing this difference in marine snow particulate but their large biomass, accounting for 1/5 with fungi, makes it clear that they still have a large role in the degradation of organic matter on marine snow. 

It is clear from the paper that when doing carbon flow budgets of the bathypelagic, neutrally buoyant or very slow sinking marine snow particulate need to be considered. The added buoyancy of the nutrient rich, high concentration of TEP in marine snow allows sufficient time for eukaryotic microbes to grow. It is clear from this paper that there are key similarities between TEPs from the surface waters and in bathypelagic marine snow. I would be interested to see more studies of this in the future. Linking together the similarities and differences between the microbial community of marine snow in the bathypelagic and the TEPs present in the sea surface interface in more detail. Further research into why there is such a diverse ecological niche separation between marine snow and ambient water could also be of use in better understanding the deep water carbon flow.

The ISME Journal advance online publication, 20 September 2016; doi:10.1038/ismej.Bochdansky, A.B., Clouse, M.A. and Herndl, G.J., 2016. Eukaryotic microbes, principally fungi and labyrinthulomycetes, dominate biomass on bathypelagic marine snow. The ISME Journal.