Bacteria-flagellate coupling in microcosm experiments in the Central Atlantic Ocean

The coupling between planktonic bacteria and bacterivorous protozoans was e xamined in microcosm experiments at several oligotrophic and ultra-oligotro phic sites in the subtropical and tropical Atlantic Ocean. Bacterial concen trations at these stations were in the range 2.2-8.1 X 10(5) cells ml(-1), heterotrophic nanoflagellates (HNF) in the range 100-800 cells ml(-1), bact erial doubling times (estimated from leucine incorporation) in the range 1- 100 days, and chlorophyll a levels in the range 0.03-0.36 mu g l(-1). The e xperimental uncoupling of the microbial loop by differential filtrations di d not result in an increased growth and grazing by nanoflagellates despite a stimulation and increase of bacterial abundance and mean cell volume due to the bottle incubations. A strong response of the grazer population occur red after increasing bacterial numbers about 10-fold by the addition of a c omplex substrate source (yeast extract). Bacteria responded immediately to the substrate enrichment with an increase in mean cell size and abundance, and reached stationary phase already after about 24 h. In contrast, HNF dev elopment showed a pronounced lag phase, and it needed between 3 and 7 days until grazers reduced bacterial numbers to about the initial values. The gr azing impact on the bacterial assemblage in the bottles resulted in feed-ba ck effects that resembled those known from other, more productive systems: protozoan size-selective grazing removed preferentially larger sized bacter ia and shifted the size-distribution towards the initial, natural situation with a dominance of small cocci. Grazing-resistant morphotypes consisted o f bacterial aggregates embedded in a polysaccharide matrix whereas filament ous forms did not develop. These experiments provide evidence that bacteria l assemblages have the capacity to respond to enhanced substrate availabili ty (for example in micropatches) and to utilise these substrates without si gnificant grazer control. (C) 2000 Elsevier Science B.V. All rights reserve d.