In the context of nowadays increasing atmospheric CO2 concentration, a large effort is being made to understand how the ocean life accelerates its sequestration in the deep ocean. A large fraction of the CO2 transformed to organic molecules via photosynthesis performed by marine plankton, returns to CO2 through the activity of heterotrophic microbes in the shallow part of the water column. Yet, a non-negligible part of this organic matter aggregates and falls down to deeper ocean layers, which induces carbon sequestration. When associated with dense minerals like opal (produced by silicifying plankton) and calcite (produced by calcifying organisms) or dust brought by the winds and falling into the sea, this organic matter may fall quicker to the ocean depths, increasing atmospheric CO2 sequestration.
Understanding the rate of transformation of marine organic carbon back to CO2 by heterotrophic microbes is crucial if we want to evaluate the magnitude and duration of atmospheric CO2 sequestration to the deep ocean. Along their journey through the deep ocean, organic particles and microbes experience temperature decrease, along with an increase in hydrostatic pressure.
In order to assess the effect of increasing hydrostatic pressure, high-pressure systems have been developed at the Mediterranean Institute of Oceanography (MIO, Marseille, France), which allow collecting water at the in situ pressure, maintaining this pressure while bringing the bottles back to the surface, and transferring fractions of the deep seawater sample aboard the ship, to measure different parameters. Among those, the incorporation of radioactive organic matter by the microbes allows measuring the Prokaryotic Heterotrophic Production (PHP). Since the microbes are diluted in the ocean and their activity is very slow, it is detected with more precision using very sensitive methods involving radioactive tracers. In association with the measurement of oxygen consumption, PHP allows estimating the intensity of the rate of “re-mineralization” to CO2 by the microbes.
During PAP cruise DY032, a container has been brought on board by the French team (Christian Tamburini, Marc Garel, Nagib Bhairy, Sophie Guasco and Virginie Riou, MIO), fitted with all the hyperbaric equipment, and fulfilling the security requirements for the use of radioactive tracers. In situ prokaryotic activities were measured in seawater samples and aggregates collected at different depths (using conventional bottles as well as marine snow catchers) and compared to the same activity measured at atmospheric pressure. These data will be related with parallel measurements of microbe diversity, and of the chemical composition of the particles in order to understand if opal, calcite and/or dust influence the rate of natural organic carbon re-mineralization to CO2 at the PAP site.
In addition, controlled experiments were performed in the container using the hyperbaric equipment, which consisted in simulating particle sinking through the water column by progressively increasing the hydrostatic pressure and decreasing the temperature allowing mimicking the in situ conditions during the fall of organic carbon-containing particles to the deep ocean. A pneumatic system inducing regular inversions of the hyperbaric bottles maintains particles in suspension.
Collaboration with Manon Duret, PhD student at the University of Southampton, NOCS, specialized in the nitrogen cycle, allowed looking at the effect of hydrostatic pressure on the link between organic matter re-mineralisation and dark CO2 fixation, and the nitrogen cycle. The hyperbaric system was also used in collaboration with Rob Young (NOC), to evaluate the pressure effect on microbial activity and diversity at 4000m depth.