AME 09:127-136 (1995)  -  DOI: https://doi.org/10.3354/ame009127

The fate of phytoplankton nitrogen through a simplified microbial food web was investigated. In a 2-stage culture system, I separated production and degradation processes of a diatom, Phaeodactylum tricornutum. In the first stage, the algae grew axenically and continuously. In the second stage, maintained in the dark, a 20 μm ciliate grazed on the diatoms and constituted the last trophic step. A microbial loop, based on bacterial utilization of partially degraded P. tricornutum cells, egested with the ciliate digestive vacuoles, developed rapidly. Ciliate production was maintained only because the P. tricornutum grazer was able to feed simultaneously on bacterivorous nanoflagellates, themselves preying upon bacteria using egested material of the ciliate. Dissolved organic nitrogen rapidly constituted more than 50% of the identified nitrogen forms (dissolved + particulate; mineral + organic). The nitrogen budget compiled during the succession of the different prey-predator relationships revealed the important role of dissolved organic matter adsorption on particles and aggregate formation including unstable organic colloids. The temporary importance and turnover of colloidal nitrogen was thus demonstrated.

Dissolved organic nitrogen . Microbial loop . Phaeodactylum tricornutum . Ciliate