Two anaerobic, psychrotolerant, syntrophic strains were enriched from permanently cold, shallow anoxic marine sediments in Skan Bay, Alaska. One strain, AK-BT, oxidized butyrate syntrophically and was isolated in defined coculture with a H2-using methanogen or in a dixenic coculture that also contained an acetate-scavenging methanogen. The other enrichment culture syntrophically oxidized propionate. The growth of these syntrophic cultures was very slow: approximately 1 year for cocultures of strain AK-BT to form colonies and >1 year for the propionate-oxidizing enrichment to form colonies. Neither culture grew axenically when supplied with the catabolic substrates crotonate, pyruvate, malate, or sulfate plus butyrate or propionate. Strain AK-BT catabolized iso-butyrate in syntrophic coculture but did not catabolize valerate or caproate. Phylogenetic analyses of the 16S rRNA gene sequence suggested that strain AK-BT was only distantly related to cultivated sulfate-reducing bacteria, and that this strain represented a new genus. We propose Algorimarina butyrica, with strain AK-BT (=OCM 842T), as the type strain. This report is the first description of psychrotolerant as well as marine butyrate- and propionate-oxidizing syntrophic organisms. [ABSTRACT FROM AUTHOR]