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

Abstract

Transparent exopolymer particles (TEP) are considered to be generated abiotically from dissolved extracellular polysaccharides released mainly by phytoplankton. TEP may affect the aggregation rate of particles and therefore influence the flux of organic carbon to the deep ocean. The role of NO3- limitation in the production of TEP by the marine diatom Chaetoceros calcitrans was investigated. C. calcitrans was grown in batch cultures with different initial nitrate concentrations (25, 75, 150, 250 and 450 μmol l-1). Nitrate affected the production of TEP in 2 distinct ways. The initial specific growth rate and maximum concentration of biomass (as estimated by chlorophyll a, cell number, total particulate carbon or total particulate nitrogen) reached in every culture was directly dependent on the initial NO3- concentration. Maximum TEP concentration followed this trend, and was significantly linearly correlated with several biomass-related variables and with the 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 proportion of the photosynthetically fixed carbon is channeled to TEP. The concentration of carbohydrate in the particulate fraction (PCH) was larger in N-limited 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 limitation. Unlike TEP, PCH decreased sharply during the stationary and senescent phases. The decrease of PCH was associated with an increase in dissolved organic carbon and an increase in the number of bacteria in the cultures.