Effects of nitrogen to phosphorus ratios on algal growth and arsenate metabolism by Microcystis aeruginosa with dissolved organic phosphorus and nitrate as nutrients

Little information is available on how the nitrogen to phosphorus (N/P) ratios influencing algal growth originated from different nutrient status and arsenate (As(V)) metabolism with nitrate and dissolved organic phosphorus (DOP) (d-glucose-6-phosphate (GP)) as the sole nitrogen (N) and phosphorus (P) sources. We then investigated the growth and arsenic (As) metabolisms of nutrient-starved and enriched M. aeruginosa cultured in different N/P ratios media with and without As(V) using nitrate and GP as N and P sources. Results showed that compared to As(V) free, starved cells changed to grow better in media with As(V), while enriched cells got worse, especially with the lower N/P ratio at 2.0:0.40. As(V) metabolic amounts for starved cells showed an inverted U shape with increasing N/P ratios, yet it was opposite to present U shape for enriched cells. The increasing N level promoted As(V) biotransformation to arsenite (As(III)), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) for starved algal cells, and to As(III) and MMA but not to DMA for enriched cells, especially under low DOP conditions. The decreasing DOP accelerated As(V) biotransformation to As(III) and MMA for enriched cells, whereas inhibited As(V) metabolism and accumulation as well as the subsequent transformation to As(III) for starved cells. Furthermore, As metabolisms in algae developed along with generating stable humic substances in GP media, which was supported by the observation of excitation-emission matrix (EEM) fluorescence. Additionally, FTIR spectroscopy analysis evidenced that the functional groups of C-O-C and CO bonds in carbohydrates and CN in aliphatic amine were the character matters of reflecting algal growth and of acting as the inhibitor on As(III) generation. Accordingly, our results highlighted that As(V) metabolism fates and the effects on M. aeruginosa growth were affected not only by the N/P ratio but also by their initial nutrient status in the GP environment.