Filterable marine bacteria found in the deep sea: Distribution, taxonomy, and response to starvation

Abstract

A significant number of viable colony-forming bacteria were recovered from deep-ocean bottom water samples passed through a 0.45μm filter. However, these bacteria small enough to pass through a 0.45μm membrane filter and termed “filterable bacteria” were less abundant in open-ocean surface water and coastal water samples. The reduced size of bacterial cells present in deep-ocean bottom water samples was documented by scanning electron microscopy. The concentration of ATP in the water samples was found to be correlated with results of direct counts of bacteria. Numerical taxonomy of bacterial strains isolated from water samples collected at two stations in the deep sea yielded taxonomic clusters grouped according to sample and size fraction. The generic composition of bacterial populations of bottom water filtrates was compared with that of bacteria retained by 0.45μ m filters. Strains ofAlcaligenes, Flavobacterium, Pseudomonas, andVibrio spp. were identified among those retained by, as well as passing through, 0.45μm filters. Two marine isolates obtained from the filtrate of a deep-ocean water sample were incubated for 9 weeks in nutrient-free artificial seawater, during which the cells became rounded and reduced in size. After the 9-week incubation period, more than 10% of the viable cells of both cultures were able to pass through a 0.4μm filter. The viable count at 9 weeks wasca. 10% of that of the initial population, although from direct counts the total population number remained relatively constant throughout the incubation period. From the observed reduction in cell size and increased starvation resistance of cells held under low nutrient conditions, it is concluded that a significant relationship exists between decreased cell size and increased survival of marine bacteria in the deep sea.