The magnitude of spring bacterial production in the North Atlantic Ocean

Abstract

Dissolved organic carbon (DOC), a major reservoir in the ocean carbon cycle, is produced by a profusion of plankton sources and processes but is consumed mainly by bacterioplankton. Thus bacterial metabolism regulates the entry of DOC into the longer scale global carbon cycle. Bacterial production (BP) is the routinely measured quantity for evaluating the roles of bacteria in carbon cycling. However BP cannot be measured directly and instead is estimated from related metabolic processes requiring the use of poorly constrained conversion factors. BP, and thus the total carbon utilization, are potentially uncertain by a factor of two or more. In the North Atlantic Bloom Experiment (NABE), BP was estimated to be about 30% of the simultaneous particulate primary production (PP), with some daily estimates exceeding 50%. Here we reassess these estimates, synthesizing knowledge and understanding of plankton dynamics gained since the 1989 NABE study. Daily BP derived from six different conversion factors averaged 20% of PP but ranged from 3 to 68%. The coupling of BP to PP was not consistent with either short-term cycling of labile DOC (hours) nor with much longer term cycling of semilabile DOC (seasons). Trophodynamic processes, including release of DOC from phytoplankton, by themselves could have maintained BP at about 15% of PP. Use of decomposing POC or previously accumulated semilabile DOC could each have supported some additional increment of BP for brief periods. Both reconsideration of observations and model results indicated that higher estimates of BP exceeding 20% of PP could not be supported without extraordinary and prolonged inputs of allochthonous carbon. Recent assertions of high BP in the tropics and other oceanic regimes should be considered carefully, especially if external subsidies are not obvious.

Prokaryotic respiration and production in the meso- and bathypelagic realm of the eastern and western North Atlantic basin

“…Furthermore, it has been recognized that prokaryotes, notably bacteria, play a key role in the decomposition of nonliving organic particles in the surface layers (Ducklow and Carlson 1992) and in mesopelagic waters (Cho and Azam 1988;Karl et al 1988). The latter two studies presented evidence that POC concentrations decrease with depth more rapidly than one would deduce from bacterial activity measurements.…”

“…Bacterial carbon demand and remineralization rates are often deduced from bacterial production measurements, calculating respiration from an average bacterial growth yield of 20-30% (Nagata et al 2000;Ducklow et al 2002). Thus, the major fraction of the carbon flow mediated by heterotrophic bacteria, i.e., respiration, is deduced from measurements of a minor fraction, i.e., organic carbon production.…”

The Effects of System-Level Nutrient Enrichment on Bacterioplankton Production in a Tidally-Influenced Estuary**

“…Estimates of BP were typically reported in carbon units and converted to a standard rate of gC L Ϫ1 h Ϫ1 . Rates of leucine and thymidine incorporation were converted to carbon units using 3.1 kg C mol Leu Ϫ1 (SIMON and AZAM, 1989) and 1.89 kg C mol TdR Ϫ1 (DUCKLOW, KIRCHMAN, and ANDERSON, 2002), respectively. Bacterial production reported as change in cell abundance was converted to carbon units using 20 fg C cell Ϫ1 (LEE and FUHRMAN, 1987).…”

Carbon cycling by microbes influenced by light in the Northeast Atlantic Ocean

“…The change was precipitated by new assessments of bacterial growth efficiency (BGE), which had been assumed to be ∼50%, 3-fold higher than the value of 15% suggested by new data (del Giorgio & Cole 1998, Carlson et al 1999. When the lower estimates of BGE were taken into consideration, it appeared that BP should be lower relative to PP, and that the BP:PP ratio cannot be higher than 0.15 (Ducklow et al 2002).…”

“…The initial data indicated that heterotrophic BP was 30% of PP (BP:PP = 0.3) , Li et al 1993, Lochte et al 1993, consistent with other information available at that time from a range of freshwater and coastal marine systems (Cole et al 1988) as well as modeling studies (Williams 1981). However, the emergence of new data and knowledge necessitated a reformulation of these initial conclusions (Ducklow et al 2002). The change was precipitated by new assessments of bacterial growth efficiency (BGE), which had been assumed to be ∼50%, 3-fold higher than the value of 15% suggested by new data (del Giorgio & Cole 1998, Carlson et al 1999.…”