Community structures of cold and low-nutrient adapted heterotrophic sediment bacteria from the deep eastern tropical Atlantic

Colony counts on high and low-n'utrient agar media incubated at 2 and 20 "C, Acridine Orange Direct Counts and blomasses 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 O/o SWB, containing 125 mg 1-' peptone and 25 mg I-' 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 Alterornonas 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 BaciUus 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. ' Contribution No. 535 of the Alfred-Wegener-Institut fiirPolar-und Meeresforschung.Bremerhaven