PHOTOSYNTHETIC CARBON METABOLISM AND BIOCHEMICAL-COMPOSITION OF SPRING PHYTOPLANKTON ASSEMBLAGES ENCLOSED IN MICROCOSMS - THE DIATOM - PHAEOCYSTIS SP SUCCESSION

Samples of natural phytoplankton assemblages were taken on 4 occasions between 28 March and 19 April 1990 at 2 stations located to the southwest of Plymouth (UK) and incubated in experimental microcosms under constant light and temperature. C-14 incorporation into cell constituents [proteins, polysaccharides, lipids and low molecular weight metabolites (LMWM)] and the biochemical composition of the phytoplankton were monitored during the experiments. Different phytoplankton populations developed in the microcosms. Microflagellates which dominated in the first experiments were followed in later experiments by a multispecific bloom of chain-forming diatoms, and these in turn by a bloom of Phaeocystis sp. Each assemblage was characterized by a different metabolic behaviour. High C-14 incorporation into LMWM and lipids was observed during non-growth periods when microflagellates dominated. High C-14 incorporation into proteins and relatively high specific rates of protein synthesis (0.45 +/- 0.1 d-1) were measured during the late growth phase of the diatom bloom. Synthesis of acid-soluble carbohydrates (ASC) also occurred in this period. As Phaeocystis sp. increased, C-14 incorporation into LMWM and polysaccharides showed a significant rise. These metabolic patterns were consistent with earlier models of the development of Phaeocystis sp. blooms. Polysaccharide-carbon concentration, both acid-soluble and acid-insoluble, showed dramatic increases during the early stages of the bloom. Enhanced polysaccharide synthesis was related to the formation of the mucilaginous colonial matrix and is suggested to be a physiological strategy tending to increase colony buoyancy. During the decline of the Phaeocystis sp. bloom, the proportion of C-14 incorporated into LMWM decreased, possibly as a result of disruption of the colonies, whereas incorporation into lipids exhibited a consistent increase. The accumulation of high-energy storage products is interpreted as a metabolic adaptation for overwintering under very low light levels. As a consequence of the distinct metabolic patterns of each successional stage, the protein-C/acid-insoluble polysaccharide-C ratio showed a marked decrease, whereas the relative contribution of ASC-C to the total measured carbon displayed a significant increase. In addition, carbon incorporation into LMWM also showed a slight but continuous increase. It is suggested that the metabolic changes associated with the diatom - Phaeocystis sp. succession are in accord with a marked shift in the trophic structure of the planktonic ecosystem from the classical food chain in the earlier stages, and mainly during the diatom bloom, to a preponderance of the microbial loop at the end of the Phaeocystis sp. bloom.