Phosphorus limitation affects the molecular composition of Thalassiosira weissflogii leading to increased biogenic silica dissolution and high degradation rates of cellular carbohydrates

Diatoms in general, and Thalassiosira weissflogii (T. weissflogii) in particular, are among the most ubiquitous phytoplanktonic species, while phosphorus (P) is an essential nutrient that limits productivity in many oceanic regimes. To investigate how T. weissflogii cultures grown under different P regimes are chemically altered before and during their prokaryotic degradation, T. weissflogii cells were cultivated under two contrasting P conditions; P-stress and P-replete. Biodegradation experiments were conducted in natural seawater comprising a natural prokaryotic community. The particulate fraction was monitored for 3 weeks for organic carbon (POC), nitrogen (PON), biogenic silica (bSiO(2)), total carbohydrates (PCHO) and individual monosaccharides, including prokaryotic counting. Results indicated that P-stress induced changes in the chemical composition of the T. weissflogii cells, causing a decrease in the Si/N (1.1-0.46) and Si/C (0.17-0.08) ratios. The P-stress T. weissflogii cells were characterized by high amounts of galactose (23% of PCHO), xylose (21%) and glucose (19%), compared to the P-replete T. weissflogii cells. The latter were dominated by ribose (20% of PCHO), further indicating the exhaustion of ribose-rich molecules (e.g., ATP) in T. weissflogii under P-stress conditions. The degradation experiments showed that bSiO(2) produced under P-stress conditions dissolved more rapidly than bSiO(2) formed under P-replete conditions, whereas POC and PON exhibited higher degradation rate constants in the P-replete T. weissflogii than in the P-stress T. weissflogii experiment. Overall, these observations show that submission of T. weissflogii to P-limitation results in changes in its initial biochemical composition, increased frustule dissolution rates, and decreases the degradation of T. weissflogii-organic matter by marine prokaryotes. (C) 2020 Elsevier Ltd. All rights reserved.