Fluctuations in heterotrophic bacterial community structure, activity and production in response to development and decay of phytoplankton in a microcosm

Abstract

Recently upwelled water from the southern Benguela upwelling system was incubated in a 60 l microcosom for 43 d under simulated in situ conditions, to follow the development and activity of the heterotrophic microplanktonic community associated with phytoplankton growth and decay. The initial bacterial population (40 .mu.g C l-1), dominated by small rods (.hivin.v = 0.198 mm3) and large cocci (.hivin.v = 0.142 .mu.m3), with Vibrionaceae as the dominant plateable strain, exhibited slow turnover times for added 14C-labeled substrates .hivin.x = 5.7 h 106 cells-1). Net bacterial growth was exponential (0.016 h-1) during phytoplankton growth (12 .mu.g C l-1 h-1, Days 0 to 4). At maximum phytoplankton and bacterial biomass (1330 and 136 .mu.g C 1-1 respectively, Day 4) Pseudomonadaceae dominated by the plateable isolates; bacterial turnover times for 14C-substrates were rapid (glucose: 1.5 h 106 cells-1, alanine: 0.49 h 106 cells-1, glutamate: 0.29 h 106 cells-1), suggesting a close coupling between phytoplankton growth and the ability of bacteria to utilise dissoved organic carbon (PDOC) substrates. Bacterial biomass was reduced to < 15 .mu.g C 1-1 by Day 9, due to diminished availability of PDOC during phytoplankton senescence and predation by microflagellates which developed inthemicrosom (< 5 .mu.g C l-1 up to Day 4, 96 .mu.g C 1-1 on Day 8). After phytoplankton senescence (Day 10) detrital carbon stimulated exponential growth (0.021 h-1) of a second bacterial community (max. biomass: 231 .mu.g C 1-1 on Day 25) dominated by small cocci .hivin.v = 0.009 .mu.m3) and large rods (.hivin.v = 0.672 .mu.m3), with Flavobacteriaceae as the dominant plateable bacteria. As this community exhibited no uptake of added 14C-labelled substrates we surmise that it was exploiting POC which dominated carbon resources at this time. Estimates of bacterial production calculated from net growth rates were ca 50 to 97% higher than values based on [methyl-3H] thymidine incorporation (TTI). These differences may be due to inadequate DNA extraction procedures, large numbers of bacteria without thymidine transport systems, or isotope dilution. Empirically determined conversion factors to correct for these differences fell within a range of 1.6 to 46 .times. 106 cells mol-1 TTI.