Salinity tolerance of diatoms from thalassic hypersaline environments

Thirty-four benthic diatom strains were isolated from thalassic hypersaline marine environments and their salinity tolerance characterized in growth e xperiments conducted at salinities ranging from 0.5% to 17.5% (weight of to tal salts per volume, g.100 mL(-1)). The results were compared with the pat terns of diatom species distribution and abundance in hypersaline evaporati on ponds and tidal channels of Guerrero Negro, Baja California Sur, Mexico. The isolated strains were representative of the diatom assemblages present in the saltern ponds but were less so of natural assemblages in tidal chan nels, In general, we found a clear decreasing trend of diatom diversity in the field and in the isolated strains with increasing salinity. With some e xceptions, the upper limit of salinity tolerance in cultivated strains corr esponded to their distribution in field samples. However, the relative abun dance of species in the field was not correlated with growth rates achieved in culture for the same salinities, Most cultured strains exhibited extrem e euryhalinity growing well from brackish to hypersaline conditions, but th e particulars of salt tolerance were quite diverse among strains. The most halotolerant taxa, two Amphora species, Amphora cf, subacutiuscula Schoeman , Nitzschia fusiformis Grunow, and Entomoneis sp,, grew well ill salinities ranging from 0.5% to 15%, Three strains of Pleurosigma strigosum W, Smith that were unable to grow in salinities less than 5% total salts represent t he only true halophilic diatoms ever reported. The fact that many strains d isplayed a remarkable halotolerance, with optimal or near-optimal growth ra tes at salinities as high as three times that of seawater, implies that dia toms from hypersaline environments are evolutionarily highly adapted to suc h environments.