Production of transparent exopolymer particles (TEP) in cultures of Chaetoceros calcitrans under nitrogen limitation

Transparent exopolymer particles (TEP) are considered to be generated abiot ically from dissolved extracellular polysaccharides released mainly by phyt oplankton. TEP may affect the aggregation rate of particles and therefore i nfluence the flux of organic carbon to the deep ocean. The role of NO3- lim itation in the production of TEP by the marine diatom Chaetoceros calcitran s was investigated. C. calcitrans was grown in batch cultures with differen t initial nitrate concentrations (25, 75, 150, 250 and 450 mu mol l(-1)). N itrate affected the production of TEP in 2 distinct ways. The initial speci fic growth rate and maximum concentration of biomass las estimated by chlor ophyll a, cell number, total particulate carbon or total particulate nitrog en) reached in every culture was directly dependent on the initial NO3- con centration. Maximum TEP concentration followed this trend, and was signific antly linearly correlated with several biomass-related variables and with t he initial NO3- concentration. However, despite the general trend of direct covariation between TEP concentration and phytoplankton biomass, NO3- has a more specific effect. A close examination of the exponential phase shows that the net production of TEP per biomass was higher in N-limited cultures . In the N-sufficient cultures, during Day 2, there was even a decrease in the concentration of TEP per unit of biomass with respect to the inoculum. Our results support the hypothesis that, under N-limitation, a large propor tion of the photosynthetically fixed carbon is channeled to TEP. The concen tration of carbohydrate in the particulate fraction (PCH) was larger in N-l imited cultures. The effect was clearer during exponential growth. In this phase there was a good agreement between the response of TEP and PCH to N l imitation. Unlike TEP, PCH decreased sharply during the stationary and sene scent phases. The decrease of PCH was associated with an increase in dissol ved organic carbon and an increase in the number of bacteria in the culture s.