Estimation of bacterial respiration and growth efficiency in the Ross Sea, Antarctica

Seawater cultures were conducted in large volume (36 l) gas impermeable tri-laminate bags for the purpose of empirically deriving bacterial growth efficiency (BGE) and carbon conversion factors (CCF) in the south central Ross Sea. This experimental design allowed for concomitant measurements of metabolic reactants (loss of total and dissolved organic carbon [TOC and DOG]) and products (gain of total carbon dioxide [TCO2] and bacterial biomass) to be made from a single incubation vessel. Some previous studies have relied on proxy measurements (e.g. O-2, H-3-thymidine incorporation and cell abundance) to determine BGE and CCF rather than direct carbon measurements. Our experimental design enabled a complete carbon budget to be constructed and eliminated variability associated with normally employed parallel bottle incubations. Utilization of TOC was well balanced by the production of TCO2, in 7 of 8 experiments, validating the use of tri-laminate bags for measuring microbial respiration. In 3 experiments, where TOC, DOC, TCO2 and bacterial biovolume were directly measured, carbon mass balance yielded BGE estimates of 12, 32 and 38 % and bacterial CCF of 77, 95 and 134 fg C mu m(-3) In experiments where independent DOC measurements were not made we used our empirically derived CCF values to determine bacterial carbon production and calculated DOC concentrations and BGE for these remaining experiments. The BGE derived from all the bag experiments conducted throughout the austral spring and summer 1995-1997 ranged from 9 to 38 %. Our experimental design and carbon mass balance approach could be applied to other aquatic systems to empirically derive the BGE and CCF, factors essential for determining carbon flux through bacterioplankton.