Arsenic efflux from Microcystis aeruginosa under different phosphate regimes

Phytoplankton plays an important role in arsenic speciation, distribution, and cycling in
freshwater environments. Little information, however, is available on arsenic efflux from
the cyanobacteria Microcystis aeruginosa under different phosphate regimes. This
study investigated M. aeruginosa arsenic efflux and speciation by pre-exposing it to 10
μM arsenate or arsenite for 24 h during limited (12 h) and extended (13 d) depuration
periods under phosphate enriched (+P) and phosphate depleted (−P) treatments.
Arsenate was the predominant species detected in algal cells throughout the
depuration period while arsenite only accounted for no greater than 45% of intracellular
arsenic. During the limited depuration period, arsenic efflux occurred rapidly and only
arsenate was detected in solutions. During the extended depuration period, however,
arsenate and dimethylarsinic acid (DMA) were found to be the two predominant
arsenic species detected in solutions under −P treatments, but arsenate was the only
species detected under +P treatments. Experimental results also suggest that
phosphorus has a significant effect in accelerating arsenic efflux and promoting
arsenite bio-oxidation in M. aeruginosa. Furthermore, phosphorus depletion can
reduce arsenic efflux from algal cells as well as accelerate arsenic reduction and
methylation. These findings can contribute to our understanding of arsenic
biogeochemistry in aquatic environments and its potential environmental risks under
different phosphorus levels.