Stepping Stones in the Deep

Stepping Stones in the Deep

Deep sea habitats such as seeps and vents are ephemeral. Organisms which inhabit these temporally restricted niches are thought to rely on larval dispersal over great distances to ensure the success of the next generation. It has been hypothesised that large food falls such as whale carcasses, fallen logs or kelp falls could provide stepping stones from one habitat to the next. Previous studies using experimental whale sinks have documented the colonisation of carcasses by highly adapted chemosynthetic organisms found at vents and seeps. Findings such as these support the stepping stone hypothesis. However little is known about how chemosynthetic organisms are attracted organic falls and the biogeochemical and microbial processors which create sulfidic niches from wood falls. In particular there is some mystery surrounding cellulose and lignin degradation as it is normally very slow in anoxic conditions. In the reviewed study Bienhold et al., (2013) aimed to investigate the processors at large food falls which lead to the production of methane and sulphur compounds, providing an energy source for chemosynthetic vent and seep organisms on their journey through the desert of the deep sea.

Bienhold et al., (2013) set up a clever field experiment in which they sunk logs at various distances from an active methane seep in the Mediterranean (1700 metres deep). Log sites were sampled using a ROV after 1 year, samples were taken from the surrounding sediment as well as the logs. The study combined a comprehensive set of in-situ and ex-situ measurements in order to collect biogeochemical data such as oxygen uptake, sulphide and oxygen concentration and pH. In order to characterise the associated microbial communities, methods such as direct cell counts and in depth 454 sequencing were used.

Concurring with previous work, Bienhold et al., (2013) found that wood-boring bivalves transform carbon from the wood into energy which can be digested by other organisms, they can therefore be considered keystone species. Biogeochemical data showed that oxygen consumption increased at the logs compared to control sediment samples, they also found sulphidic compounds increased at the log sites. The authors used an interesting calculation based on carbon content and organism biomass to estimate it would take 35years for all the wood-derived carbon to be used up. With regards to microbial community, there was a two fold increase in microbe cell count number after a year of submersion. 454 sequencing revealed a community shift from Gammaproteobacteria dominance when the logs were first sunk to a dominance of Actinobacteria after one year. More specifically, after one year, the dominant genus was Demequina, of which isolates were shown to be closely related to Cellulomonas fermentans (cellulolytic bacteria), indicating that the Demequina genus could be of major importance for wood degradation, and hence carbon cycling, in the deep sea. Taxa containing sulphate reducing bacteria were also found on the logs.

Transcriptomics will provide the next step forward in this type of work and will aid a deeper, mechanistic understanding of the processors involved in the production of sulphidic niches at fallen food sites. More information is required on the metabolically active members of the microbial consortium as well as information on specific functions and pathways carried out by the microbes. Furthermore it would be nice to see temporal sequence experiments to provide more detailed data on succession. I think the evolutionary significance of this work has been overlooked by the authors and I would be interested to discuss this further…

Bienhold, C., Pop Ristova, P., Wenzhöfer, F., Dittmar, T., & Boetius, A. (2013). How Deep-Sea Wood Falls Sustain Chemosynthetic Life. (D. L. Kirchman, Ed.)PLoS ONE, 8(1), e53590.

 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0053590