Cold, Dark but Not Alone: A Distinct Cold Coral Community

Deep-water cold corals spend their lives in dark, high pressure environments and are found on the productive continental margins. They contain no photosynthetic symbionts but their tissues are still home to thriving microbial communities. The water column above reefs are also home to abundant bacterioplankton, but whether these are any different to those in the surrounding water was not known. To right this a study of the diversity of microbes across the three domains from the Røst reef, in the Norwegian Sea, was carried out.

Seawater samples were collected by ROV from a depth of 300-370m from three sites on the reef a meter above the seafloor and adjacent to coral colonies. Water samples were filtered through 0.02µm filters and frozen. As a comparison, samples were taken 80km away, at 300-350m in mid-water above a methane seep (the reef also cyclically releases ethane and methane). Nucleic acids were extracted and PCR targeting rRNA sequences carried out. DGGE was then carried out to assess the genetic diversity at a community level. PCR product was used to produce a clone library using E. coli. Appropriate sequences from the clones and DGGE bands were sequenced. Phylogenetic analysis was carried out on the rRNA gene sequences alongside additional sequences from GenBank.

It was revealed that the Eukarya showed the highest diversity, followed by the Bacteria and finally Archaea. With the reef possessing the highest diversity, the authors presumed this was driven by turbulence caused by close proximity to the seafloor. For the Eukarya the reef site contained phylotypes unique to the reef, with a distinct microeukaryote community. At the reef site the Alveolata held the largest number of sequences. Within this, the poorly studied Group 1 containing parasitic members had the most sequences. Could these organisms play a role in cold water coral disease? Strangely the mid-water site was dominated by sequences from the Jellyfish Nanomia bijuga. Presumably fragments of the animals got through the filter. But it strikes me as a poor comparison, perhaps metazoan sequences should have been analysed separately?

Bacterial communities were also distinct with little overlap. But both reef and mid-water were largely dominated by the Proteobacteria, both Alpha- and Gamma-. No host associated Vibrio strains were detected in the water column. Surprisingly Synechococcus cyanobacteria were detected at the reef, likely imported from surface waters and operating heterotrophically. In the Archaea many of the mid-water phylotypes were present on the reef, but not the reverse with many sequences being exclusive to the reef. The Archaea was largely dominated by Marine Group 1 of the Thaumarchaeota, with similar sequences to those found in deep water and poplar regions. In terms of bacteria involved in biogeochemical processes the reef possessed more sequences associated with hydrocarbon users such as Oceanosprillales. Various sulphur oxidising bacteria were detected and it was assumed that the Archaea play a leading role in ammonia oxidation. However functional genes were not quantified which would have shed more light on these questions.

In conclusion, deep water corals may be cold and dark but they are not alone, with a distinct pelagic microbial community. The influence of this on the ecology of the system is currently only guess work. Future research should focus on investigating the functional genomics of the microbiota to further unpick the dynamics of these fascinating, economically important and endangered communities.

Ref: Jensen, S., Bourne, D.G., Hovland, M. & Murrell, J.C., (2012). High diversity of microplankton surrounds deep-water coral reef in the Norwegian Sea. FEMS Microbial Ecology, 82, 72-89.