Role of deep-sea bacteria in organic matter mineralization and adaptation to hydrostatic pressure conditions in the NW Mediterranean Sea

Abstract

Ectoaminopeptidase activity (EAA), 14C-glutamic acid assimilation (GA) and respiration (GR) rates were measured over spring and fall, whilst maintaining in situ hydrostatic pressure condition through a 2000 m water column in the NW Mediterranean. Depth-integrated EAA was maximal in the deep-sea layer due to the lack of ready-to-use compounds. Conversely, depth-integrated fluxes for GA and GR in the bathypelagic zone, though far from negligible, were much lower than in the photic layer, the exception being in the 1000 to 2000 m layer during fall when respiration fluxes and particle fluxes were optimal. Metabolic rates obtained from samples recovered and incubated under in situ pressure conditions were 4.53 ± 4.45 (mean ± SD; n = 19) times higher than their decompressed counterparts, proving that deep-sea bacteria were adapted to high pressure. 14C-glutamic acid uptake rates (GU = GA + GR) measured under in situ pressure conditions were used to calculate the bacterial 14C-glutamic acid assimilation yield (GYG = GA/GU). In decompressed samples this yield was always lower by around 20% than in those measured under in situ pressure conditions. Because experiments currently used to estimate bacterial growth efficiency (BGE) are not carried out under in situ pressure conditions, we hypothesize that pressure affects BGE in the same way that it affects GYG. Thus, the stress caused by decompression induces an increase in energy cost, and so underestimates BGE. Growth of deep-sea bacteria is highly dependent on the quantity and quality of sinking particles reaching 1000 m. During mesotrophic conditions (spring), high fluxes of relatively fresh particulate organic carbon generated relatively small depth-integrated EAA rates (20.6 mg C m-2 h-1) and high GYG (65%). Meanwhile, during oligotrophic conditions (fall), a minimal flux of old organic matter generated maximal depth-integrated EAA rates (62.2 mg C m-2 h-1) and lower GYG (12%). From these observations and from the literature, we explore the relationship between the nutritive sources available to deep-sea bacteria, their role in the mineralization of peptide compounds and consequently the implications for the global carbon cycle.