One of the main topics of this cruise is to better understand how the organic matter is consumed by organisms. We focus our attention on bacteria with a special consideration to their environmental condition of life. We are considering two kinds of pressure effects on organic mineralization:
1) Bacteria attached to particles sinking through the water column. These bacteria coming from the surface waters are submitted to an increase of pressure (and a decrease of temperature) that limits their metabolism and then their degradation of organic particle.
2) Free-living bacteria in deep-sea environments. These kinds of bacteria are adapted to their environment and then they are more active under pressure than after decompression of the sample.
To study these microscopic organisms, we have developed original (and heavy) high pressure devices together using high pressure bottles (HPBs). These HPBs are used in the first case to simulate the increase of pressure that bacteria attached-to-particles experiences (PASS: PArticles Sinking Simulator) and in the second case to maintain their in situ conditions all the time during our experiments (HPSS: High Pressure Serial Sampler and not French CTD as it is call here!!). On board the RSS Discovery, we are occupying a whole container (and many more…).
During this cruise, we have used particles recovered from PELAGRA (see blog from 31 July) in order to estimate their degradation by bacteria simulating a sinking velocity of 200m/day.
Also, we are measuring deep-sea microbial activity under in situ pressure conditions. Joined with the IODA6000 data (see blog of the 3rd August), we will expect to better estimate the bacterial carbon demand of bacteria living in the dark ocean.
Christian Tamborini and Mehdi Boutrif