Biodegradation of photosynthetically produced extracellular organic carbonfrom intertidal benthic algae

C-14-labeled extracellular products of a natural microphytobenthic communit y and two species of benthic diatoms (Nitzschia hybridaeformis and Amphora coffeaeformis) were fractionated into extracellular dissolved organic carbo n (C-14-EDOC), organic carbon extracted with EDTA (C-14- EDTA-extractable O C) and extracellular polymeric substances (C-14-EPS). The biodegradation of this labeled extracellular organic carbon by bacteria in sediments was exa mined to determine the processes of enzymatic degradation of photosynthetic ally-produced extracellular organic carbon from microphytobenthos in an int ertidal flat ecosystem. In addition, primary production as well as extracel lular enzyme activities (beta- and alpha -glucosidase) were measured to eva luate the possible relationship between organic carbon production and micro biological degradation at the Isshiki intertidal flat in Mikawa Bay, Japan. With all three C-14-fractions extracted from a natural microphytobenthic a ssemblage and two species of benthic diatoms, more than 50% of the added su bstrates were mineralized within 24 h by the bacterial community in sedimen ts. At that time, the percentage of high-molecular-weight compounds (>5 K M W) to total MW compounds of C-14-EDTA-extractable OC and C-14-EPS fractions decreased within 24 h from 50.9 to 6.6% and 74.5 to 11.1%, respectively. I n situ, beta- and alpha -glucosidase activity in sediment was higher than i n the seawater column (at a depth of 1 m), though the photosynthetic produc tion of microphytobenthos was equal to that of phytoplankton. Based on our previous studies that microphytobenthos produced much more extracellular pr oducts than phytoplankton, it is assumed from these results that carbon flo wing into the microbial loop through the mediation of enzymatic degradation of extracellular products in a benthic system exceeds that in the overlyin g water column. (C) 2001 Elsevier Science B.V. All rights reserved.