Bacillus arsenicoselenatis, sp nov, and Bacillus selenitireducens, sp nov:two haloalkaliphiles from Mono Lake, California that respire oxyanions of selenium and arsenic

Two gram-positive anaerobic bacteria (strains E1H and MLS10) were isolated from the anoxic muds of Mono Lake, California, an alkaline, hypersaline, ar senic-rich water body. Both grew by dissimilatory reduction of As(V) to As( III) with the concomitant oxidation of lactate to acetate plus CO2. Bacillu s arsenicoselenatis (strain E1H) is a spore-forming rod that also grew by d issimilatory reduction of Se(VI) to Se(IV). Bacillus selenitire-ducens (str ain MLS10) is a short, non-spore-forming rod that grew by dissimilatory red uction of Se(IV) to Se(0). When the two isolates were cocultured, a complet e reduction of Se(VI) to Se(0) was achieved. Both isolates are alkaliphiles and had optimal specific growth rates in the pH range of 8.5-10. Strain E1 H had a salinity optimum at 60 g l(-1) NaCl, while strain MLS10 had optimal growth at lower salinities (24-60 g l(-1) NaCl). Both strains have limited abilities to grow with electron donors and accepters other than those give n above. Strain MLS IO demonstrated weak growth as a microaerophile and was also capable of fermentative growth on glucose, while strain E1H is a stri ct anaerobe. Comparative 16S rRNA gene sequence analysis placed the two iso lates with other Bacillus spp. in the low G+C gram-positive group of bacter ia.