Effect of N/P ratio on ecological risk of arsenic metabolism of microalgae in the presence of organic phosphorus

Abstract:

[Objective] To explore the effect nitrogen/phosphorus (N/P) ratio on growth, arsenic (As) metabolism, and microcystins (MCs) release of Microcystis aeruginosa in arsenate (As⁵⁺)-polluted water when d-glucose-6-disodium phosphate (GP) was the only phosphorus source. [Methods] The experiment was carried out on N- and P-starved algal cells in As-containing water with different N/P ratios. The algal cell density (OD₆₈₀), chlorophyll a (Chla), actual photosynthetic yield (Yield), superoxide dismutase (SOD) activity, and the content of As species and MCs were measured. Thereby, the physiological response of M. aeruginosa to As stress and the metabolic pathways of As were analyzed. [Results] The N- and P-starved algal cells can well adapt to the low N/P ratio in the case of high GP level (0.1 mg/L), and high N/P ratio in the instance of low GP level (0.02 mg/L) can significant improve the OD₆₈₀, Chla, and Yield of the algal cells in the early stage of culture. The effect of N/P ratio on SOD activity was more significant at the beginning and end of culture. After 8 days of culture, arsenite (As³⁺) changed into the dominant As species in the medium with N/P ratio of 10:0.1, which made up 78.8% of the total As (TAs) in the water, but As⁵⁺ was still the main As species in the media with other N/P ratios. Meanwhile, As⁵⁺ was the dominant As species in algal cells for different N/P ratios, and in the case of N/P ratio at 1:0.1, the proportion of organic As in TAs in algal cells was the largest. As⁵⁺ metabolism in M. aeruginosa cells was significantly affected by GP level, and the high amount of metabolized As per cell was found in the instance of high GP level (0.1 mg/L). In the case of N/P ratio of 10:0.1, As metabolism was dominated by the reduction of As⁵⁺ and the release of As³⁺, and methylation level of As was elevated in the case of low GP level (0.02 mg/L). The concentration of MCs in the medium was related to GP level and the concentration was the lowest in the case of low N/P ratio and high GP level. [Conclusion] The results are of great significance for comprehensive understanding of harmful algal blooms and scientific management of As in As-polluted water under organic phosphorus conditions.

Key words:organic phosphorus;N/P ratio;Microcystis aeruginosa;arsenic species;microcystins