Microbial Decomposition of Organic Matter Derived from Phytoplankton Cellular Components in Seawater

Abstract

To test the hypothesis that different phytoplankton cellular components undergo distinct decomposition processes, we examined bacterial decomposition of extracted dissolved organic matter (EX-DOM) and cell debris from the diatom (Skeletonema costatum) using long-term incubation experiments. The decay rates indicated that EX-DOM is extremely bioreactive (1.2-1.7 d-1), and that particulate organic carbon (POC) from cell debris is resistant to rapid microbial degradation (0.047-0.055 d-1). In the experiment with grazers, the percentage of accumulated dissolved organic carbon (DOC) in the EX-DOM and cell debris experiments was 5% and 64%, respectively, of the initial DOC concentrations derived from phytoplankton cells. The initial C/N ratio of DOM in the cell debris experiments was 19.2-19.7, higher than those in the EX-DOM experiments (5.5-6.0). The percentage of accumulated DOC in the cell debris experiment with grazers was 35% at day 13. Thereafter DOC accumulated until it reached 64%. Our data indicate that these two distinct phytoplankton-derived organic materials (EX-DOM and cell debris) decay and accumulate differently. These results suggest that structural components of phytoplankton cells may persist as semi-labile POC, and that DOC produced by solubilization of structural components of cell and phytoplankton-derived DOM with high C/N ratios may accumulate as semi-labile DOC in seawater.