Uncoupling the microbial loop: growth and grazing loss rates of bacteria and heterotrophic nanoflagellates in the North Atlantic

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Abstract
Growth and grazing loss rates of bacteria and heterotrophic nanoflagellates were investigated in the northeast Atlantic in spring 1989. Experiments were conducted in 2 different oligotrophic areas at 18-degrees-N, 30-degrees-W and at 33-degrees-N, 20-degrees-W. Numbers of bacteria and protozoa, namely heterotrophic nanoflagellates (HNF), heterotrophic dinoflagellates and ciliates, were low in both areas, and the average abundance of microorganisms in the euphotic zone varied little. The microbial loop was uncoupled experimentally by differential filtration. Microplankton samples were size-fractionated by gravity filtration and changes in numbers of bacteria and their potential predators were measured in microcosms over several days. Maximum potential growth rates were 0.087 h-1 for bacteria and 0.123 h-1 for HNF. Ingestion rates ranged from 9 to 36 bacteria HNF-1 h-1, clearance rates ranged from 0.3 to 3.0 X 10(-5) ml HNF-1 h-1. Based on the experimental results, 3 modes of control of growth rates and abundances were identified - (1) substrate control: both bacteria and HNF did not grow unless substrates were added; (2) substrate and grazing control both effective: substrate supply via recycled dissolved organic matter enhanced bacterial growth moderately, followed by increasing HNF numbers which reduced the bacterial peak; (3) grazing control: bacterial production was balanced immediately by grazing protozoans, namely HNF, and removal of predators led to a pronounced bacterial peak. Results confirmed the hypothesis of a linear food chain consisting of bacteria - HNF - larger protozoa which is tightly coupled in the natural environment.

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