ALGAL ORGANIC-CARBON EXCRETION IN LAKE-MICHIGAN

Algal external metabolites have been claimed to be an important carbon source for aquatic heterotrophic bacteria. We measured the release by algae of recently-fixed carbon by following the accumulation of 14C-organics in filtrates of Lake Michigan samples incubated under natural light for 8 to 26 hours. Pretreatment of samples with an antibiotic and a suite of nonradioactive amino acids, to prevent microbial uptake of excreted products, did not affect the apparent release rates which ranged between 2% and 21% (X .+-. SD = 11.34 .+-. 9.32 mg-C m-3 day-1) of short-term autotrophic production. Comparison of our release rates of 0.42 to 1.54 mg-C m-3 d-1 (X .+-. SD = 0.84 .+-. .40 mg-C m-3 day-1) to estimates of bacterial demand made during a simultaneous study suggests that organic carbon released from recently-fixed internal pools may not alone support bacterial production.