Aerobic anoxygenic photoheterotrophs (AAPs), and their characteristic pigment, bacteriochloropyll a (BChla), were recently discovered throughout the surface waters of the open ocean (Kolber, 2000; Kolber, 2001). AAPs comprise upwards of about 11 % of the total microbial community, they are globally distributed in the euphotic zone, and display high levels of photosynthetic activity. More than twenty isolates have been obtained from different ocean basins. These organisms are facultative photoheterotrophs, and represent a potentially significant, but hitherto unrecognized component of the marine microbial community. Their potential role in the ecology of the upper ocean is determined by their contribution to the energy and carbon fluxes, and by their interactions with other components of marine ecosystems.
To elucidate this role, this project is investigating mechanisms of photosynthesis, carbon fixation, and respiration in AAPs, to identify factors controlling expression of phototrophy vs. heterotrophy, and to estimate the extent of AAP-specific energy, carbon, and redox under varying environmental conditions. AAPs are readily cultivated on organic-poor media. The investigators are establishing a library of the open ocean isolates, and characterizing the phylogenetic relationship between AAPs from different geographical regions. AAPs use photosynthesis to produce ATP and to fix CO2. The investigators are characterizing the photosynthetic properties of AAPs, evaluating the effect of light on their carbon metabolism, and investigating the effect of photosynthesis on the incorporation of dissolved organic material (DOM). Where DOM is abundant, AAPs perform mostly heterotrophic metabolism, respiring the available organic carbon. When DOM is limited, AAPs synthesize photosynthetic pigments and reaction centers, shifting their metabolism toward phototrophy. The investigators are assessing the extent and the factors controlling these transitions. Unable to oxidize water, AAPs rely on other substrates to serve as electron donor(s). Although most of the photosynthetic electron transport in AAPs is cyclic, external electron donors are necessary to replenish the electrons lost to photosynthetic carbon fixation, or to external electron acceptors. The researchers are investigating which of the organic/inorganic reductants present in the upper ocean serve as potential electron donors to AAPs, and explore the AAP-mediated redox reactions, with particular emphasis on oxidation of the reduced sulfur species, and reduction of iron and nitrogen species.
Phototrophy alleviates AAPs' dependence on the organic carbon, reducing the rates of carbon remineralization in the upper ocean, thus affecting the extent of new production. If effective in reducing iron and nitrogen species, AAPs will also affect the physiology of oxygenic phototrophs. By addressing these questions, this project is providing the most basic knowledge regarding the potential role of AAPs in the upper ocean ecology.
