Marine phytoplankton: how many species in the world ocean?

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Abstract
The breakdown of organic carbon of dead Synechococcus cell walls, added to selectively filtered Lake Vechten water, was not accelerated by protozoa. During 4 weeks of incubation at 15°C no significant decrease of total organic carbon was observed. However, heterotrophic nanoflagel-lates (HNAN) and ciliates strongly increased the remineralization of N and especially P, from both cell walls and cell extract. Bacterioplankton growth did not result in net P mineralization but in P uptake. P was remineralized only in the presence of protozoan grazers. Both HNAN and ciliates grazed on bacteria, with ingestion rates estimated at 27–96 bad HNAN−1 h−1 and 129 bact ciliate−1 h−1 respectively. Grazers increased N mineralization too, although N was also mineralized in the absence of protozoa. The phytoplankton cell walls yielded less P but more N remineralization than the cell extract. Thus, protozoa can strongly accelerate cycling of specific nutrients through plankton. Nuclepore filters were found to cause artificial DOC release during selective filtration.

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