Microbial Synthesis of Detritus-Like Particulates from Dissolved Organic Carbon Released by Tropical Seagrasses

Abstract

Dried leaves of Thalassia testudinum and Syringodiurn filiforrne released 12.6 % and 19.4 %, respectively, of their organic carbon as dissolved organic carbon (DOC) dunng 3 d of axenic leaching. When inoculated with microbes, the DOC was rapidly converted to bacterial aggregates of a size that could be ingested by macroconsumers. Large populations of ciliates and flagellates also developed, presumably feeding on the unaggregated bacteria. In treatments containing the residual macroparticulate organic carbon (MPOC), 75-95 % of the bacteria present were attached to the leaves, and suspended aggregates were not observed. The protozoan community was dominated by freeswimming flagellates that grazed on the suspended bacteria and were thus responsible for the absence of those forms. Total microbial populations in the DOC treatments were 10-12 times greater per unit of seagrass carbon originally added than in the MPOC containing flasks. These results show that seagrass DOC may rapidly be made available to higher consumers through processes that produce suitable sized food particles-viz., growth of protozoans on bacteria and formation of bacterial aggregates.