Sulphide-dependent anoxygenic photosynthesis in the cyanobacterium Oscillatoria limnetica

Abstract

PROFOUND differences in structure and function separate the prokaryotic cyanobacteria (blue-green algae) from eukaryotic algae and plants, whereas oxygenic photosynthesis is considered to distinguish the cyanobacteria and eukaryotic plants from the other phototrophic bacteria. The prevailing concepts on the cyanobacteria emphasise their prokaryotic nature and oxygenic photosynthesis. This combination of properties, among others, led to the suggestion that the cyanobacteria represent a group of possible progenitors of chloroplasts¹. While cyanobacteria typically exhibit oxygenic photosynthesis with two photosystems using electrons from water, it has been demonstrated that photosystem I of certain cyanobacteria can function in vivo independently in supporting virus production², heterocyst functions³ and even photoheterotrophic growth⁴. We have demonstrated anoxygenic photosynthesis in a cyanobacterium Oscillatoria limnetica, also capable of oxygenic photosynthesis, which was isolated from the anaerobic H₂S rich hypolimnion layer of the monomictic, hypersaline Solar Lake located on the desert margin of the Gulf of Elat, Israel⁵. During winter stratification, this layer is located below two plates of phototrophic sulphur bacteria (Chromatium violescens and Prosthecochloris sp.), and exhibits a dense cyanobacterial bloom with very high rates of primary production. We report here evidence for the photosystem I-driven CO₂ photoassimilation in O. limnetica with H₂S serving as sole electron donor through oxidation to elemental sulphur.