Habitats

The discovery of obligately aerobic bacteria containing Bchl a was first reported by T. Shiba et al. (148), in whose work the isolation and enumeration of these microorganisms on seaweed and in seawater, sand, and bottom sediments of Tokyo Bay and adjacent areas were described. Sixteen strains of aerobic pink or orange bacteria that contained Bchl a were isolated from these aerobic marine environments with a rich medium (148) and were found to be abundant on thalli of Enteromorpha linza and Sargassum horneri and in beach sand. The proportions of these bacteria among the species that formed colonies on the medium employed ranged from 0.9 to 1.1% in the seaweed samples and from 1.2 to 6.3% in the beach sand samples (148).

Subsequent reports broadened the geographical area and ecological niches in which obligately aerobic Bchl a-containing bacteria are found. The presence of aerobic heterotrophic Bchl a-synthesizing strains in high proportions (10 to 30%) of the total heterotrophic bacterial strains cultivated was described for marine environments on the west and east coasts of Australia (146) and at the Pacific Ocean inlet English Bay, in Vancouver, Canada (209).

Investigation of samples taken from freshwater cyanobacterial mats in hot springs of the Bol’shoi River valley (Lake Baykal region) and from Neskuchninskii Spring situated on Southern Kurily in Russia led to the discovery of freshwater strains of obligately aerobic bacteria that synthesize Bchl a (212, 214-216). The isolation and growth of freshwater species were obtained in a rich organic medium (212). The mats from which these isolates came were largely composed of cyanobacteria (e.g., Oscillatoria subcapitata), diatoms, and the purple phototrophic bacteria Thiocapsa roseopersicina and Rhodopseudomonas palustris. The mats were located at the boundary of anaerobic and aerobic zones at alkaline pH values ranging from 8.0 to 9.4 and hydrogen sulfide concentrations from 0.6 to 7.4 mg/liter, depending on the spring (213, 217). The samples of these mats contained up to 106 cells of aerobic bacteria containing Bchl a per ml. Some strains were isolated from environments with considerably hot temperatures: strain KR-99 was isolated from an environment with a temperature of about 40°C and strains RB3 and RB7 came from a site with a temperature of 54°C (214, 215). However, in pure laboratory cultures all of these strains demonstrated typical mesophilic properties and grew optimally at 28 to 30°C (214, 215). Such thermotolerancy has been found for purple nonsulfur bacteria such as R. palustris, Rhodomicrobium vannielii, and Rhodopseudomonas viridis, which have also been detected in high-temperature environments and which have temperature optima of 30 to 35°C in pure culture (21, 57, 58). Why and how these latter species and obligately aerobic species survive in thermal environments are unclear.

Mildly thermophilic (growth temperature optima at 42 to 45°C) species of purple phototrophic bacteria, Rhodospirillum centenum, Rhodopseudomonas cryptolactis, and Rhodopseudomonas strain G1, have been isolated (21). Two moderately thermophilic or thermotolerant aerobic anoxygenic representatives, strain OT3 and strain JF-1, were discovered recently (63, 228). Strain OT3 was isolated from bacterial mats in the brackish Usami hot spring (Japan). The temperature at the sampling site was 42.7°C, the pH was 5.8, and the bacterial mat consisted mainly of a dark green layer of thermophilic filamentous cyanobacteria. The new isolate OT3 grew at temperatures up to 50°C, and optimal growth occurred at 40 to 48°C (63). Aerobic anoxygenic phototrophic strains containing Bchl a were discovered in hydrothermal black smoker plume waters of the Juan de Fuca Ridge in the Pacific Ocean (208). Water samples taken from about 2,000 m beneath the ocean surface were found to contain aerobic bacteria producing Bchl a in numbers of 20 to 40 cells/ml of the samples, about 30% of the pigmented strains that formed colonies on the rich medium used. The representative strain JF-1 revealed a broad tolerance for culture conditions such as salinity, temperature, and pH. Thus, growth was obtained in a freshwater medium and a medium supplemented with 10% NaCl, at temperatures ranging from 5 to 42°C and at pH values of 5.5 to 10.0. Therefore, JF-1 is a salt- and pH-tolerant and thermotolerant strain (222).

Several strains of pelagic bacteria were purified from the surface of a freshwater subtropical pond in Australia (48), and acidophilic heterotrophic bacteria that synthesize Bchl a were isolated from an acidic mine drainage system (190). Aerobic phototrophic bacteria were detected in high numbers relative to the numbers of other heterotrophic strains in the North Adriatic Sea, where they comprised 5 to 55% of the total cells cultivated (103).

The strains isolated from freshwater cyanobacterial mats in Russia are obligately freshwater species. For example, the growth of strain RB16-17 was strongly inhibited by salt concentrations higher than 1% (212). Salt-tolerant strains (T1 through T7) were isolated from a cyanobacterial mat located in the supralittoral zone on the West Frisian island of Texel in The Netherlands and comprised 2 to 23% of the aerobic pigmented strains that formed colonies on the medium used (231). This microbial mat was known to be flooded twice a month by the North Sea. Because of alternating heavy rainfall and evaporation, the salinity of this environment varies from 8 to 10‰ to more than 100‰. The organisms isolated from this mat are able to grow over a broad salinity range, from 5‰ (freshwater) to 96‰ (Table ​(Table1).1). This ability may reflect an adaptation to an environment with fluctuating salinity. Similarly salt-tolerant strains of obligately aerobic Bchl a-containing bacteria (strains 15s.b. and 23s.b.) were isolated from English Bay in Vancouver. The strains were found on the surfaces of seaweeds and sand alternately exposed to air or covered by water during low or high tides, respectively. During summer low tides, the bacterial environment is dried for several hours, consequently presenting an econiche with fluctuating salinity (207). Such salt-tolerant strains can be described as facultatively marine or freshwater organisms.

In summary, most strains of aerobic anoxygenic phototrophic bacteria isolated so far inhabit a wide variety of eutrophic aquatic environments and seem to comprise a significant part of the aerobic heterotrophic bacterial population. An exception is strain JF-1, isolated from apparently oligotrophic deep-sea hydrothermal vent plume waters. In spite of the broad geographical distribution of aerobic phototrophic bacteria in different ecological niches and their presence in high numbers, the ecological importance of this group of organisms (their role in microbial populations) has not been studied.