Heterotrophic bacterial flora of the Antarctic Ocean

Abstract

Vertical distribution of heterotrophic bacteria was surveyed at six sampling stations in the Antarctic and the Indian Oceans. Seawater samples were collected at various water depths, ranging from 0 to 2000m. Total bacterial counts were determined by the direct microscopic method. Plate counts were carried out by the surface spreading method and the filter method using ORI agar medium. At a sampling station in the Antarctic Ocean, the enumeration of oligotrophic bacteria was also made using an MPN method. The total bacterial counts per ml of seawater in the Antarctic region ranged from (10)^4 to (10)^5,whereas the plate counts by the filter method were (10)^0 to (10)^1 at the upper 500m layers and (10)^ to (10)^0 at the deeper water lyers. The plate counts at 2℃ incubation temperature were higher than those at 20℃ by 1-2 orders of magnitude for the samples at stations in the Antarctic region. In contrast, the 20℃ counts for seawater from the Indian Ocean were slightly higher than 2℃ counts. The surface spreading method gave bacterial counts that were one order of magnitude higher than the filter method, and the counts with the MPN method using diluted medium were greater still, reaching about 10% of total bacterial counts. The results suggested that a fairly large proportion of the bacterial population in Antarctic seawater is in the actively growing state. More than 87% of the bacterial strains isolated from surface water of the Antarctic Ocean were orange- and yellow-pigmented bacteria. Among the pigmented bacteria, Gram-negative, non-motile, orange-pigmented rods that appeared to constitute a single species belonging to Flavobacterium-Cytophaga predominated