Remineralization and recycling of iron, thorium and organic carbon by heterotrophic marine protists in culture

To characterize trace metal cycling in marine systems as mediated by hetero trophic protists, we conducted a series of laboratory experiments in 2-orga nism model systems consisting of bacteria and protistan grazers. Trace meta l isotopes (Fe-59 and Th-234),C-14, and bulk organic carbon measurements we re used to follow the chemical transformation of bacterial carbon and assoc iated trace metals by several different grazer species. Results indicate th at grazers were able to cause repartitioning of Th and regeneration of Fe f rom bacterial prey into the dissolved phase (<0.2 <mu>m), even in particle- rich laboratory cultures. For both Th and Fe, protist grazing led to the fo rmation of relatively stable dissolved and colloidal metal-organic species. Metal/carbon ratios of the particle pool in some model systems with grazer s were significantly altered, indicating a decoupling of trace metal and or ganic carbon cycling through the grazing process. Different protist species exhibited substantial variation (up to a factor of 10) in their ability to quantitatively remobilize trace metals from bacterial prey. The implicatio ns of these findings for trace metal cycling in marine systems are discusse d.