Dissolved oxygen at four times normal saturation inhibited growth and metabolism of summer planktobacteria in surface waters of alpine oligotrophic Mountain Lake (Giles County, Virginia). Data included viable colony counts, D-[U-¹⁴C]glucose incorporation into extractable lipid of colonies, and respiration-assimilation of D-[U-¹⁴C]glucose by lake water samples. Significant (P<0.05) differences were not detected in either colony counts or ¹⁴C-lipid when superoxide dismutase (30 U ml⁻¹) or catalase (130 U ml⁻¹) were added to the medium. The upper waters of Lake Hoare, Antarctica, contain dissolved oxygen at ≥42 mg liter⁻¹ (=HDO). HDO did inhibit D-[U-¹⁴C]glucose assimilation-respiration compared with normal atmospheric dissolved oxygen (=ADO) in Lake Hoare water. D-[U-¹⁴C]glucose was assimilated and respired optimally at 12°C in Lake Hoare. Colony formation was inhibited in both lakes. Colonies represented <1% of the fluorochrome-stained direct counts in Lake Hoare. Lake Hoare planktobacteria were smaller than the planktobacteria in Mountain Lake. ATP size fractionation revealed that 39% of ATP biomass was <0.5 Hm in Lake Hoare. Five microbial isolates were selected from Lake Hoare by growth under very high oxygen (=VHO, 4.6x in situ HDO, 55 lb in⁻² of added oxygen). One isolate was selected under ADO from shallow benthic mat underlying HDO waters. Isolates were examined for physiological characteristics which might enhance their survival in the HDO environment. While HDO incubation produced <36% of ADO incubated CFU, VHO was more selective producing <1%. Bacterial isolates were motile Gram negative rods, catalase and oxidase positive, differing in their growth response to temperature and nutrient concentration. One VHO isolate was a yeast. HDO reduced the maximal cell density in three isolates tested at higher nutrient concentrations, however, all three exhibited less repression as nutrients were lowered from 1000-10 mg liter⁻¹ in comparison to ADO grown controls. One isolate actually produced a cell density 3x that of the ADO control. Four of five bacterial isolates demonstrated HDO inducible superoxide dismutase (SOD). The inducible and constitutive SOD were the manganese type and had the same electrophoretic mobilities in respective isolates. All VHO isolates contained carotenoids. Pigmentation of the bacterial isolates differed due to the types and relative proportions of the constituent carotenoids. A carotenoid-negative mutant of one isolate grown under HDO exhibited a lengthened lag phase, decreased growth rate, maximal cell density and thereafter increased lysis compared to the same ADO grown strain and the carotenoid containing parent strain. The mutant and parent strain produced catalase and indistinguishable specific activities of SOD. Microorganisms in the high oxygen Lake Hoare waters may be protected from oxygen toxicity by the lake’s oligotrophic nature as well as a combination of cellular defenses. / Ph. D.