Arsenite toxicity and tolerance in three microalgae, synechococcus leopoliensis, oscillatoria limnetica and Chlorella pyrenoidosa

Abstract

The cyanobacteria Oscillatoria limnetica and Synechococcus leopoliensis manifested concentration-dependent growth lags in presence of arsenite above 200pM. There were no detectable growth lags in the Chlorophyte Chlorella pyrenoidosa in presence of arsenite concentrations above ImM and arsenite affected the log rate of growth. C. pyrenoidosa was less sensitive to arsenite than the two cyanobacteria. Arsenite toxicity in ~ leopoliensis was affected by pH of the medium. It decreased with an increase in pH. ImM potassium acetate also alleviated toxicity' in cells grown in presence of 200}lM arsenite. Acetate did not alleviate toxicity of ~ pyrenoidosa grown in presence of 3mM arsenite. Cells of the two cyanobacteria grown in presence of 10pM arsenite for up to 12 hours became tolerant to 200pM arsenite with growth being similar to that of normal cells. 1.OpM,--arseni te was also adequate to induce arsenite tolerance but not 0.lpM arsenite. ~l.'re-exposureof C. pyrenoidosa to SOOpM arsenite for 12 hours indpced some "tolerance" to arsenite concentration of up to 4mM. The amounts of arsenic in wnole air-dried tolerant and sensitive cells of s. leopoliensis,were similar after exposure to 200pM arsenite for 8 or 9 hours. The cyanobacteria accumulated arsenic to about 18-40 times the medium concentration. The relative amounts of arsenite ~~and arsenate in ice-cold water extract from tolerant cells were the same as in sensitive cells. However in recovered cells (after a 30-hour growth lag) almost all arsenic in the ice-cold water extract was in"form of arsenate. The me cha ni sm of tolerance in this culture was apparently due to conversion of arsenite to arsenate. C. pyrenoidosa grown in non-toxic concentration of arsenic (SOOpM) accumulated a"rsenic to about 80 times the medium concentration after 11 hours. Possible mechanisms of arsenite tolerance in S. leopoliensis were investigated. The data did not support a tolerance mechanism based upon exclusion of arsenite, decreased uptake of arsenite, external or internal conversion of arsenite to the less toxic arsenate form, (except in recovered cells), or increase in any identified sulfhydryl compounds. An alternative pathway for synthesis of acetyl-CoA in which pyruvate dehydrogenase complex is not involved should be considered.