Importance of extracellular organic carbon production in the total primary production by tidal-flat diatoms in comparison to phytoplankton

Microphytobenthos, mainly benthic diatoms, produce mucilages containing extracellular polymeric substances (EPS) on the cell surface. The ratios of particulate organic carbon (C-14-POC) and total extracellular organic carbon (TEOC: the total of extracellular dissolved organic carbon [C-14-EDOC] and colloidal organic carbon [C-14-Colloidal-OC] extracted with EDTA) to total primary production (TPP) were measured using C-14-tracer to estimate the ratio for each fraction of organic carbon photosynthetically produced by microphytobenthos and phytoplankton. TEOC for microphytobenthos ranged from 42 to 73% of TPP and was made up mostly of C-14-Colloidal-OC. The TEOC/TPP for phytoplankton ranged from 1.5 to 22%. The ratio of C-14-EPS to C-14-Colloidal-OC for microphytobenthos was 41 to 53%, and for phytoplankton was 4.4 to 24%. Algal species in the transitional phase were used for experiments because microphytobenthos Nitzschia hybridaeformis most abundantly produced TEOC and EPS in the transitional phase during growth in experiments studying the effect of the growth phase. By using EDTA for the extraction of extracellular organic matter, it becomes clear that excretion of colloidal organic matter by microphytobenthos has always been underestimated. These results suggest that the large amount of TEOC containing EPS produced by microphytobenthos plays a more important role than secretion of phytoplankton as a source of organic carbon for heterotrophic organisms in the intertidal-flat ecosystem.