Production of colloidal organic carbon and trace metals by phytoplankton decomposition

Abstract

Colloids are important intermediates and can considerably influence the biogeochemical cycling of organic carbon and trace metals in aquatic systems. Previous studies have determined the release of carbon and trace elements from decomposing phytoplankton debris, but the degradation of biogenic particles to colloids and their significance in metal and carbon cycling remain unknown. In this study, we measured the release of carbon and trace elements (Cd, Cr, Se, and Zn) from the debris of two phytoplankton (diatom Thalassiosira pseudonana, and dinoflagellate Prorocentrum minimum) into a colloidal phase (operationally defined as 5 kDa–0.2 mm). In general, Cd, Se, and Zn were released at a faster rate and Cr was released at a slower rate than the release of C from the decomposing debris into the dissolved phase (<0.2 mm). Release of trace metals appeared to closely follow C release. The release rate coefficients of Cd, Cr, Se, Zn, and C in the two phytoplankton debris with microbial addition were 1.67–1.68 d−1 for Cd, 0.147–0.239 d−1 for Cr, 0.712–0.845 d−1 for Se, 0.765–1.14 d−1 for Zn, and 0.443–0.512 d−1 for C. The percentages of trace elements (Cr, Se, Zn) and C associated with the colloidal phase were relatively high within the first 5 d ofdecomposition. After 5 d, the fraction of colloidal trace elements in the <0.2–µm dissolved phase was 5–10% for Cd, 13–52% for Cr, 13–28% for Se, 14–30% for Zn, and 16–41% for C in the diatom decomposing experiment; and 4–9% for Cd, 15–62% for Cr, 15–31% for Se, 3–27% for Zn, and 22–38% for C in the dinoflagellate decomposing experiment, respectively. The partitioning of metals in the colloidal phase was not greatly affected by the microbial activity. Although the overall percentage of organic carbon release was related to that of metals from decomposing debris, no significant correlation between the percentages of colloidal metals and colloidal organic carbon was observed, implying that organic C and metals have different partitioning behavior during their release from decomposing phytoplankton debris. Our study demonstrated that the decomposition of biogenic particles may contribute considerably to the production of colloids in marine environments.