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Abstract

Anoxygenic phototrophic sulfide oxidation by green and purple sulfur
bacteria (PSB) plays a key role in sulfide removal from anoxic shallow
sediments and stratified waters. Although some PSB can also oxidize
sulfide with nitrate and oxygen, little is known about the prevalence of
this chemolithotrophic lifestyle in the environment. In this study, we
investigated the role of these phototrophs in light-independent sulfide
removal in the chemocline of Lake Cadagno. Our temporally resolved,
high-resolution chemical profiles indicated that dark sulfide oxidation
was coupled to high oxygen consumption rates of similar to 9 mu M O-2
center dot h(-1). Single-cell analyses of lake water incubated with
(CO2)-C-13 in the dark revealed that Chromatium okenii was to a large
extent responsible for aerobic sulfide oxidation and it accounted for up
to 40% of total dark carbon fixation. The genome of Chr. okenii
reconstructed from the Lake Cadagno metagenome confirms its capacity for
microaerophilic growth and provides further insights into its metabolic
capabilities. Moreover, our genomic and single-cell data indicated that
other PSB grow microaerobically in these apparently anoxic waters.
Altogether, our observations suggest that aerobic respiration may not
only play an underappreciated role in anoxic environments but also that
organisms typically considered strict anaerobes may be involved.