Robic Micro-Laboratory to Perform In Situ Measurement of Rates of Chemoautotrophic Carbon Production at Deep-Sea Hydrothermal Vents
Stefan M. Sievert (Participant), Craig D. Taylor (WHOI), and Jeremy J. Rich (Brown University)
Knowledge of the in situ metabolism of microorganisms carrying out CO2-fixation at deep-sea hydrothermal vents is very limited. Particularly lacking are studies measuring rates of autotrophic carbon fixation in situ, which is a measurement ultimately needed to constrain production in these ecosystems. In addition, although recent data suggests that nitrate reduction either to N2 (denitrification) or to NH4+ (dissimilatory reduction of nitrate to ammonium, DNRA) might be responsible for a significant fraction of chemoautotrophic production, nitrate-reduction rates have never been measured in situ at hydrothermal vents. With funding from the NSF Ocean Technology and Interdisciplinary Coordination (OTIC) program received in 2011, we have developed a robotic micro-laboratory, the Vent-Submersible Incubation Device (Vent-SID), for routine application by the oceanographic community for measuring rates of relevant metabolic processes at hydrothermal vents at both in situ pressures and vent temperatures.
We successfully tested the instrument on a recent cruise to the deep-sea vents at 9ºN EPR, allowing us for the first time to determine rates of carbon fixation at both in situ pressures and temperatures at diffuse flow deep-sea hydrothermal vents, a parameter that is of critical importance to assess the productivity of these systems. In addition, we also simultaneously measured rates of dissimilatory nitrate transformations and took samples to link these process rate measurements to the expression of functional genes to evaluate the importance of nitrate reduction in autotrophic carbon fixation.
We view this instrument as an important component of future deep-sea hydrothermal vent research, including cabled deep-sea vent observatories, by making it possible to routinely obtain a quantitative measure of microbial rate processes and to directly correlate the measurement of these processes with community composition and the gene expression patterns within a given vent over time or between different vents. This represents a major step forward in the way we study these ecosystems, by allowing us to get more realistic estimates of the overall productivity occurring in them and to assess the ecosystems’ roles in cycling carbon, nitrogen, and other chemicals throughout the ocean.