RELEASE OF DISSOLVED ORGANIC-MATTER IN A TROPICAL STRAIN OF SYNURA-PETERSENII (CHRYSOPHYCEAE) GROWN UNDER HIGH IRRADIANCES

Carbon (C-14) assimilation and the release of labeled dissolved organi c carbon ((DOC)-C-14) by Synura petersenii Korshikov (Chrysophyceae) w ere investigated at a range of irradiances (100-1260 mu mol photons m( -2) s(-1)) in axenic batch cultures. At up to 1260 mu mol m(-2) s(-1), carbon assimilation was not saturated, although a 25% chlorophyll-a r eduction in relation to the initial value at 1000 mu mol m(-2) s(-1) a nd 28% at 1260 mu mol m(-2) s(-1) was observed. The (DOC)-C-14 release was low at all irradiances investigated and dependent on carbon fixat ion; the highest release (4.8% of the total C-14 assimilated) was obse rved at 1260 mu mol m(-2) s(-1). In a second experiment, C-14 assimila tion by S. petersenii was measured at 130 mu mol m(-2) s(-1). The subs equent loss of labeled particulate organic carbon ((POC)-C-14) and rel ease of (DOC)-C-14 were determined during a 5-h period in which cells were incubated in nonlabeled medium at different irradiances. At 2000 mu mol m(-2) s(-1) there was a 20.6% (POC)-C-14 loss. The values of (D OC)-C-14 release were low for both 2000 mu mol m(-2) s(-1) and 1000 mu mol m(-2) s(-1) (4.9 and 4.6%, respectively). At lower irradiances, ( DOC)-C-14 release was close to 2.0%. Preliminary analysis showed that the high molecular weight fraction released was an extracellular heter opolysaccharide.