High incidence of halotolerant bacteria in Pacific hydrothermal-vent and pelagic environments

The abundance of halotolerant microorganisms in hydrothermal-vent and pelag ic waters in the North and South Pacific was estimated by the most probable number (MPN) technique using a heterotrophic 16% NaCl medium incubated at 20-24 degrees C. Based on these MPNs and direct counts with epifluorescence microscopy to enumerate the total microbial population, salt-tolerant micr obes comprised from < 0.01 to > 28% of the total microbial community. Fourt een isolates from these MPN enrichments were identified by sequencing a por tion of the 16S rRNA gene, and all were found to belong to the genera Halom onas and Marinobacter. The response to salt of mesophilic hydrothermal-vent microbial isolates obtained without selecting for salt tolerance was also examined. Forty-one of 65 strains cultured from hydrothermal plume waters, low-temperature hydrothermal fluids, sulfide rock and an animal specimen at similar to 2000-2200 m depth from the Endeavour Segment of the Juan de Fuc a Ridge were subjected to increasing concentrations of NaCl, and over half grew at a NaCl concentration that is lethal to many commonly isolated marin e bacteria. At least 36 of the 65 isolates (greater than or equal to 55%) g rew in the enrichment medium supplemented with 10% NaCl; at least 30 of 65 (greater than or equal to 46%) grew with 16% NaCl; at least 20 of 65 (great er than or equal to 31%) tolerated 22% NaCl. Based on phylogenetic analysis of the 16S rRNA gene in nine of these 65 isolates, four belonged to the ge nus Halomonas. These Halomonas strains tolerated 22-27% NaCl. It is possibl e that a majority of the other 16 isolates which grew with 22% NaCl are als o Halomonas based on their degree of halotolerance, morphology, and apparen t abundance as revealed by MPN enrichments. The four Halomonas strains obta ined without selecting for halotolerance were further characterized physiol ogically and metabolically. Overall, they grew between -1 degrees C and 40 degrees C, were facultative aerobes, oxidized between 49 and 70 organic com pounds according to Biolog plate substrate utilization matrices, grew with oligotrophic quantities of carbon (0.002% yeast extract) in liquid media, r educed nitrate to nitrite, and tolerated up to 0.05-3 mM Cd2+. Halomonas is one of the most abundant culturable organisms in the ocean, and its succes s may be attributed to its metabolic and physiological versatility. (C) 200 0 Federation of European Microbiological Societies. Published by Elsevier S cience B.V. All rights reserved.