Abundance and community structures of heterotrophic and oligotrophic bacteria from the Sierra Leone Abbyssal Plain

Colony counts on high and low-nutrient agar media incubated at 2 and 20 °C, Acridine Orange Direct Counts and biomasses are reported for sediments of the Sierra Leone Abyssal Plain. All isolates from low-nutrient agars also grew in nutrient-rich seawater broth (100 % SWB). However, a greater proportion of the 2 °C than of the 20 °C isolates grew in 2.5% SWB, containing 125 mg/l peptone and 25 mg/l yeast extract. Only 14 strains or 12.7% of the 2 °C isolates, but none of the 20 °C isolates, grew in 0.25 % SWB. Psychrophilic bacteria with maximum growth temperatures below 12 °C, isolated at 2 °C, were predominant among the cultivable bacteria from the surface layer. They required seawater for growth and belonged mainly to the Gram-negative genera Alteromonas and Vibrio. In contrast to the earlier view that psychrophily is connected with the Gram-negative cell type, it was found that cold-adapted bacteria of the Gram-positive genus Bacillus predominated in the 4 to 6 cm layer. The 20 °C isolates, however, were mostly Gram-positive, mesophilic, not dependent on seawater for growth, not able to utilize organic substrates at 4 °C, and belonged mainly to the genus Bacillus and to the Gram-positive cocci. The majority of the mesophilic bacilli most likely evolved from dormant spores, but not from actively metabolizing cells. It can be concluded that only the strains isolated at 2 °C can be regarded as indigenous to the deep-sea.

Abundance of bacteria and endospores in surface sediments of Josephine Seamount area (Table 1)

During the 19th cruise of the research vessel "Meteor" between Madeira and Lisbon 260 strains of aerobic heterotrophic bacteria have been isolated from sediment samples collected from different depths. These strains have been identified mainly as members of the genera Marinovibrio, Pseudomonas, and Bacillus. The majority of bacteria isolated from shallow areas (Josephine Seamount) were sea water media requiring Marinovibrio and Pseudornonas spp. but in sediment samples taken from depths exceeding 1000 m the probably terrestrial sporeforming Bacillus spp. predominated. Further investigations in the same region during the 23rd cruise of the "Meteor" demonstrated that about 30 to 50% of the sporeforming bacteria found in the sediment samples could be isolated from dormant spores in situ. The remaining more than 50 % of sporeformers in the deep sea region examined are believed to be metabolic active cells.