Effects of Marinobacter adhaerens HP15 on polymer exudation by Thalassiosira weissflogii at different N:P ratios

Abstract

In the ocean, exopolymer accumulation and the resulting aggregation of marine phytoplankton drive the flux of sinking organic matter. Little is known about the contribution of bacteria to the release and build-up of such exudates. We describe an in-depth investigation of exopolymer production under differing nutrient conditions using a diatom- bacteria model system. Responses of the marine diatom Thalassiosira weissflogii to different nutrient re gimes and the impact of the co-incubated bacterium Marinobacter adhaerens on algal exudation were evaluated by analyzing quantity and quality of exudation products. Cultures of T. weissflogii were grown at nutrient-balanced conditions (Redfield ratio N:P = 16) as well as at nitrogen- or phosphorus-depleted conditions (N:P = 1 and N:P = 95, respectively) in the presence or absence of M. adhaerens. The impact of M. adhaerens on the concentration of dissolved organic carbon and transparent exopolymer particles (TEP), as well as on the composition of amino acids and carbohydrates, depended on the nutrient regime. Under nutrient-balanced conditions, M. adhaerens stimulated both T. weissflogii growth and TEP production. Under nutrient-depleted conditions, TEP production was enhanced regardless of whether bacteria were present. Phosphorus limitation resulted in an appreciable change in the %mol composition of dissolved amino acids in xenic and axenic treatments. Differential lec tin staining revealed that the presence of M. adhae rens enhanced and modified the pro duction of specific extracellular substances. A better understan ding of the effects of diatom-bacteria interactions on the accumulation of exudation products is crucial in modelling and predicting consequences of environmental changes on oceanic organic matter and nutrient cycling. © Inter-Research 2012 · www.int-res.com.