Every surface in the marine environment has a layer of adsorbed organic molecules that shapes the initial microbial colonization of substrates. Once attached, many microbes will build a biofilm using extracellular polymeric substances (EPS) as their cement. This matrix facilitates survival and colonization in conditions where solitary or planktonic life could not persist. Surface and environmental properties influence the initial attachment and EPS production, with a domino effect on biofilm architecture and community composition.
However our understanding of biofilm formation is based on laboratory studies, which exclude the influence of complex environmental interactions. There is a growing interest in examining the ecological dynamics of biofilms and comparing the relative roles of substrate and environmental conditions in biofilm community development. Deep sea biofilms have been especially neglected, which is why this study focused on a cold seep deep sea environment.
When pressure and tectonic activity force fluid expulsions over the seabed, a cold seep is born. Hollows in the ocean floor can retain these fluids, creating a hypersaline environment called a brine pool, with a seep water environment moving across the top of it. The banquet of hydrogen sulphide, methane and other hydrocarbons supports a diverse party of sulfur/sulphide oxidising, sulfur reducing and methane oxidising Bacteria and Archaea.
The taxa described so far in cold seeps have all been from symbioses, sediments and microbial mats. Though microbial mats are essentially very large and complex biofilms, they initiate in nutrient rich sediments rather than on the adsorbed organic layers of surfaces like biofilms. Therefore they have distinct routes of initiation and development. This study probed the hypothesis that biofilm and free living community composition on artificial substrates would differ between and within the brine pool and the overlying seep water. The microbiological tools used included 16S rDNA tag pyrosequencing, clone libraries and scanning electron microscopy.
Sulfate concentrations were high in the brine pool, so sulfate must have been being transported there from the seep water. This may be simply because sulfate sinks, so microbial or viral processes may be driving sulfate input into the brine pool. Regardless, the result was the selection of sulfate reducer biofilms; this type of metabolism is favoured by the increased sulfate uptake and survival provided by biofilms in these extreme conditions.
The planktonic community of the seep water was more diverse than the biofilms there, suggesting that substrate had a strong selective effect on what community compositions developed on it. Between the brine pool and the seep water, biofilm communities were highly distinct, the former being dominated by Deltaproteobacteria sulfate reducers (Desulfobacula) and the latter by a Gammaproteobacteria genus (Thiomicrospira) known for its sulfide, thiosulfate and sulfur oxidising abilities. Thiomicrospira are notable because they use an alternative sulfur oxidation pathway from the soxB gene one used by most other sulfur oxidisers, which may allow them to thrive in deep sea vent systems. Hydrogen sulfide was abundant only in the seep water, which probably favoured Thiomicrospira, subsequently creating high sulfate levels in the brine pool due to this bacteria’s oxidation of hydrogen sulfide. Sulfate becomes the main oxygen donor in suboxic conditions, favouring the sulfate reducers. Clearly there is a unique sulfur cycle in cold seeps which drives microbial separation between brine pool and seep water biofilm communities. It also dwarfs the influence of substrate, showing that environmental conditions are stronger determinants of biofilm composition in cold seeps.
Though the authors highlight the implications for understanding biocorrosion and fouling of deep sea facilities, I think this is actually an important description of the role of cold seep microbial systems in ocean processes. The coupling of these complementary communities between two adjacent but distinct environments warrants further study, to understand its roles in climate change and ecology.
Lee, O. O., Wang, Y., Tian, R., Zhang, W., Shek, C. S., Bougouffa, S., ... & Qian, P. Y. (2014). In situ environment rather than substrate type dictates microbial community structure of biofilms in a cold seep system. Scientific reports, 4.