Interactions among dissolved organic carbon, microbial processes, and community structure in the mesopelagic zone of the northwestern Sargasso Sea

At the Bermuda Atlantic Time-Series Study (BATS) site, the field observations of dissolved organic carbon (DOC) dynamics indicate that seasonally produced "semilabile" DOC is resistant to rapid microbial degradation in the surface waters but available for microbial remineralization once it is delivered into the mesopelagic zone after convective overtum. In this study, we employed an experimental simulation of convective overtum events to determine whether the remineralization of semilabile DOC would occur in a controlled laboratory setting. Seawater culture experiments were conducted in which surface ('10 m) and mesopelagic (250 m) 0.2-pLm filtrates were inoculated with unfiltered water from •10 and 250 m in an assortment of combinations to simulate various mixtures of nutrients, DOC quantity and quality, and microbial assemblages. Results indicate that (I) microbial inocula from the upper euphotic zone were incapable of remineralizing the seasonally accumulated semilabile DOC (j.mol C L-' resolution) on the timescales of the incubations; (2) the utilization of semilabile DOC was greatest when the inoculum source was from 250 m and the filtrate source was from the upper 10 m; and (3) the decrease in bacterioplankton diversity, estimated with the Shannon-Wiener diversity index, was greater in treatments in which inoculum from 250 m was mixed with filtrate from 10 m than in treatments in which the surface inoculum was mixed with the surface filtrate. Our findings are that a portion of the surface semilabile DOC can be metabolized by microorganisms in a laboratory setting and that mesopelagic nutrients alone are insufficient to stimulate DOC drawdown > 1.3 p.mol L-'. Transformations of microbial community structure were associated with the drawdown of surface DOC in simulated mixing events and suggest that microbial community structure is a factor in surface-layer DOC dynamics. The production of oceanic dissolved organic carbon (DOC), regardless of mechanism (see Carlson 2002), is ulI To whom correspondence should be addressed. Present address: Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 93106-9610 (carlson@ lifescii.ucsb.edu). 2 Present address: Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, California 931069610. Acknowledgments We thank the officers and crew of the RV Weatherbird 11 for their valuable assistance and support. We thank the BATS chief scientists S. Bell, J. Mitchell, P. Lethaby, and R. Johnson for assisting in water collection and accommodating wire time requests. This manuscript benefited from constructive comments provided by two anonymous reviewers. This work was supported by National Science Foundation's timately constrained by the magnitude of primary production. DOC uptake by heterotrophic prokaryotes is the dominant DOC removal mechanism in the sea (Azam 1998). However, it is estimated that 17% of global new production escapes rapid microbial degradation and accumulates in the surface waters as DOC (Hansell and Carlson 1998). DOC that resists rapid microbial remineralization, with lifetimes of weeks to years, is referred to as semilabile DOC (Kirchman et al. 1993; Carlson 2002). If the DOC turnover time is greater than the intervals between major vertical mixing events, then the DOC can be exported into the ocean interior (Copin-Montegut and Avril 1993; Carlson et al. 1994; Hansell 2002). The majority of exported DOC is remineralized Oceanography and Microbial Observatory programs and grants MCB-9977918 and MCB-0237728 to C.A.C; MCB-9977930 to S.J.G.; and OCE-9726091 to D.A.H. This is BBSR contribution 1648 and JGOFS contribution 1039.