GROWTH-KINETICS OF THE OMNIVOROUS OLIGOTRICH CILIATE STROMBIDIUM SP

The marine ciliate Strombidium sp. can be raised through multiple generations on a monoxenic diet of the bacterium Vibrio natriegens, but Strombidium specific growth rates and yields are 3-4 times higher when a heterotrophic microflagellate is included as prey. In contrast to earlier studies we found that Strombidium grew inefficiently when feeding as a bacterivore, with gross growth efficiencies (K1) of 11-20% (determined on a nitrogen-, carbon-, and protein-specific basis). On the same bacterial diet K1 of the scuticociliate Uronema sp. was 49-70%. Bacterivorous Uronema attained a 9-fold higher specific growth rate and 30-fold higher cell yield than Strombidium. Thus, the species composition of a ciliate assemblage can markedly influence the fate of microbial loop production. Cell volume of live Strombidium sp. varied > 3-fold during-a growth cycle. Cell organic composition also varied: the C: N (mass) ratio decreased from 7.7 in stationary phase cells to 4.5 in exponentially growing cells, reflecting the ingestion of N-rich prey. In addition to these natural variations in cell volume, preservatives can shrink cells to half of their live volume, resulting in spurious values of ciliate growth efficiency if computed on a volume-specific basis.