Mismatch between community respiration and the contribution of heterotrophic bacteria in the NE Atlantic open ocean: What causes high respiration in oligotrophic waters?

Abstract

Heterotrophic bacteria have long been considered as the major respirers of the open ocean. During a survey in the eastern N Atlantic Subtropical Gyre we measured H-leucine heterotrophic bacterial production (BP), C particulate primary production (PPP) and the O2 community metabolism rates: gross primary production (GPP), net community production (NCP) and community respiration (CR). We estimated heterotrophic bacterial respiration (BR) from BP and literature models of bacterial growth efficiency [BGE BP/(BP BR)]. BP was an order of magnitude lower than PPP (integrated means of 17 2 and 207 29 mg C m d, respectively). Although volumetric PPP and GPP were significantly correlated (r 0.51; p 0.009; n 25), CR (1120 122 mg C m d) bore no significant relationship with either primary or heterotrophic bacterial production and exceeded GPP (578 117 mg C m d) at 5 out of 6 sampling stations. Integrated values of CR were only significantly correlated, but negatively, with chlorophyll a values (r 0.95; p 0.001; n 6). Integrated BR tended to decrease with increasing CR and, with a mean estimated BGE of 6%, it accounted on average for a much lower fraction of CR (33 7%) than currently assumed. By estimating the contributions of other trophic groups a large amount (48 10%) of the measured CR remained unaccounted for. This gap in respiration estimates casts some doubt on the magnitude of the net heterotrophic balance frequently observed in oligotrophic waters from changes of O2 over 24 h dark in vitro incubations.