GRAZING AND UTILIZATION OF CHROOCOCCOID CYANOBACTERIA AND HETEROTROPHIC BACTERIA BY PROTOZOA IN LABORATORY CULTURES AND A COASTAL PLANKTON COMMUNITY

Field and laboratory experiments were conducted to compare the rates of ingestion of planktonic protozoa for chroococcoid cyanobacteria and heterotrophic bacteria, and the fate of this ingested biomass. Laboratory experiments tested the ability of cyanobacteria and bacteria to support the growth of 3 species of bacterivorous protozoa. Two species of heterotrophic bacteria supported faster growth rates and higher cell yields of the protozoa than 3 strains of cyanobacteria. When mixtures of bacteria and cyanobacteria were offered, however, all protozoa grew as rapidly as when bacteria were offered alone. One protozoan showed a marked feeding selectivity against 1 strain of cyanobacteria when offered mixtures of bacteria and cyanobacteria. Grazing rate measurements performed in Vineyard Sound, Massachusetts, USA, revealed removal rates as high as 54% of the cyanobacterial assemblage d-1 and 75% of the heterotrophic bacterial assemblage d-1. Nanoplanktonic protists (organisms < 20-mu-m) were the major consumers of both cyanobacterial and bacterial biomass in this environment on 5 sampling dates. Based on measurements of the ingestion rates of nanoplanktonic consumers and the carbon content of cyanobacteria and heterotrophic bacteria. we conclude that cyanobacterial biomass in this coastal environment at times reaches 30 % of the total prokaryote biomass consumed by the nanoplankton. During times of peak abundance of chroococcoid cyanobacteria, this biomass is an important source of organic carbon for planktonic protozoa feeding on bacteriasized particles.