Effects of nitrogen and phosphorus on arsenite accumulation, oxidation, and toxicity in Chlamydomonas reinhardtii

We studied arsenite (iAs(III)) accumulation, oxidation, and toxicity in the freshwater green alga Chlamydomonas reinhardtii under nutrient-enriched (+NP), phosphorus-limited (-P), and nitrogen-limited (-N) conditions. The -P alga (55.1 mu M) had a Michaelis constant (K-d) for uptake approximately one tenth of the +NP (419 mu M) and -N (501 mu M) cells, indicating iAs(III) uptake inhibition by extracellular phosphate. This conclusion was supported by the hyperbolic reduction in iAs(III) uptake rate (V) from 9.2 to 0.8 mu mol/g-dw/h when the extracellular phosphate concentration went up from 0 to 250 mu M. The maximal iAs(III) uptake rate (V-max) of the -N alga (24.3 mu mol/g-dw/h) was twice as much as that of the +NP (12 mu mol/g-dw/h) and -P (8.1 mu mol/g-dw/h) cells. It implies that more arsenic transporters were synthesized under the -N condition. Once accumulated, iAs(III) was oxidized and a higher proportion of arsenate (iAs(V)) was observed at lower [As](dis) or under nutrient-limited conditions. Nevertheless, iAs(III) oxidation mainly occurred outside the cells with the extent of oxidation reciprocal to [As](dis). Based on the logistic modeling of the concentration-response curves in the +NP, -P, and -N toxicity tests, iAs(III) had an [As](dis)-based EC50 of 1763, 13.1, and 1208 mu M and an intracellular arsenic concentration based EC50 of 35.6, 28.8, and 195 mu mol/g-dw, respectively. Higher toxicity to the -P cells occured because of their increased iAs(III) accumulation, whereas the underlying mechanisms why the -N alga was more tolerant need to be further revealed. Overall, both N and P had remarkable effects on the behavior and effects of iAs(III), which cannot be disregarded in the biogeochemical cycling research of arsenic. (C) 2014 Elsevier B.V. All rights reserved.