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

Abstract

Microphytobenthos, mainly benthic diatoms, produce rnucilages containing extracellular polyrneric substances (EPS) on the cell surface. The ratios of particulate organic carbon (I4C-POC) and total extracellular organic carbon (TEOC. the total of extracellular dissolved organic carbon [i4C-EDOC] and colloidal organic carbon ["C:-Colloidal-OC] extracted with EDTA) to total primary production (TPP) were rneasured using "C-tracer to estirnate the ratio for each fraction of organic carbon photosynthetically produced by microphytobenthos and phytoplankton. TEOC for rn.icrophytobenthos ranged frorn 4 2 to 73% of TPP and was rnade up mostly of 14C-Colloidal-OC The TEOC/TPP for phytoplankton ranged frorn 1.5 to 220,). The ratio of I4C-EPS to I4C-CoUoidal-OC for rnicrophytobenthos 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 experirnents studying the effect of the growth phase. By using EDTA for the extraction of extraceUular organic matter, it be-Comes clear that excretion of colloidal organic matter by rnicrophytobenthos has always been underestimated. These results suggest that the large arnount of TEOC containing EPS produced by microphytobenthos plays a more mportant role than secretion of phytoplankton as a source of organic carbon for heterotrophic organisms in the intertidal-flat ecosystem.