Extracellular carbohydrates released by the marine diatoms Cylindrotheca closterium, Thalassiosira pseudonana and Skeletonema costatum: effect of P-depletion and growth status.

A laboratory study was performed on the extracellular production of carbohydrates by the marine diatoms Cylindrotheca closterium, Thalassiosira pseudonana and Skeletonema costatum. The investigation was aimed at elucidating the role of P-starvation and growth status on abundance and chemical characteristics of the released non-attached polysaccharides. Inorganic phosphorus depletion determined an increase of total polysaccharides in all species examined compared to nutrient-replete (complete f/2) conditions. The highest abundance of polysaccharides per unit cell was found in T. pseudonana (28.4 micromol C 10(-6) cells), followed by C. closterium (2.56 micromol C 10(-6) cells) and S. costatum (1.18 micromol C 10(-6) cells). Maximum production rates were found at the transition between exponential and stationary growth phase. Gas-chromatographic analysis of the dissolved fraction showed glucose to be the most abundant monomer in exponentially growing, P-replete cultures (81.6%, 90% and 32% as molar percentage of total aldoses in C. closterium, T. pseudonana and S. costatum, respectively). A strong reduction in glucose was found in C. closterium, but not in T. pseudonana and S. costatum, under P-depleted conditions. Species-specific variations in the amount and aldose signatures of the released polysaccharides according to nutrient status and growth conditions can provide useful insights on the production and persistence of these organic compounds in the water column.