Bacteriochlorophyll a in the ocean: Is anoxygenic bacterial photosynthesis important?

Abstract

Some groups of marine bacteria within the Α‐proteobacteria are capable of anoxygenic photosynthesis in oxic environments. Their primary photosynthetic pigment is bacteriochlorophyll a (BacChl a). The impact of these bacteria on flows of energy and carbon in the ocean has been difficult to ascertain in the past. Recently, however, Kolber et al. (2001) reported that such bacteria are abundant in the upper ocean and that these might contribute significantly to photosynthetically driven electron transport since measured and inferred ratios of BacChl a and chlorophyll a (Chl a) were about 0.8% in coastal environments and as high as 10% in the Eastern Tropical North Pacific. The authors suggested that the globally averaged BacChl a/Chl a ratio could be as high as 5 to 10%. To determine whether such high values are representative of other marine environments, concentrations of BacChl a were measured in samples collected in eutrophic nearshore and mesotrophic and oligotrophic offshore environments off Southern California. The average BacChl a/Chl a ratio was 1.1% in nearshore and 0.5% in mesotrophic shelf and oligotrophic offshore environments. Assuming that rates of photosynthesis scale with concentrations of photosynthetic pigments, these data suggest that the contribution of BacChl a driven anoxygenic bacterial photosynthesis to energy production in coastal eutrophic and offshore oligotrophic areas of the California current system is small, substantially smaller than the suggested global average of 5 to 10%.