Anoxygenic Photosynthesis in Cyanobacteria

Understanding the evolution of photosynthesis, the process of using light energy to convert carbon dioxide (CO2) into biomass, remains an outstanding question. A number of factors confound our ability to trace the evolution of photosynthesis: associated observed chemical limitations are not clearly defined in genetic information and horizontal gene transfer (the transfer of genetic information between microorganisms) hinders phylogenetic approaches. In the absence of characterized deeply-branching microorganisms, a robust phylogenomic framework, and undisputed biosignatures in the rock record, facultative anoxygenic photosynthesis in extant cyanobacteria represents an opportunity to better understand and constrain the evolution of photosynthesis. In this project, the investigator will characterize photosynthesis in an emerging model cyanobacterium, Leptolyngbya sp. strain hensonii, isolated from an anoxic, sulfide-rich sinkhole. This research will evaluate the genetic response of Leptolyngbya sp. strain hensonii to the presence of sulfide (which has been linked to inhibition of oxygenic photosynthesis) and the potential for a cyanobacterium to express a single photosystem based on environmental conditions. Collectively, the data will provide insight into the physiology and potential success of cyanobacteria prior to the evolution of oxygenic photosynthesis. The study will examine the role of life in the transformation and evolution of Earth's geochemical cycles and the evolution of photosynthesis, which is an outstanding question in geobiology. The project will train a graduate student and an undergraduate student. In addition, the research team will develop a demonstration for Market Science to broadly disseminate findings at Farmers' Markets in and around the Twin Cities area of Minnesota.

The ability to harvest light and fuel cellular processes through phototrophy is arguably one the most important biological innovations in Earth history. Yet, understanding the evolution of photosynthesis remains an outstanding question in geobiology. Oxygenic photosynthesis is often cited as the most important microbial innovation having tipped the scale from a reducing early Earth to an oxygenated world that eventually lead to complex life. However, oxygenic phototrophs use two reaction centers: Photosystem II and Photosystem I for light-driven oxidation of water to fuel primary productivity. In extant sunlit environments where low oxygen concentrations and sulfide persist, some cyanobacteria can use sulfide as the electron donor to photosystem I, performing anoxygenic photosynthesis. In the absence of characterized deeply-branching isolates, a robust phylogenomic framework, and irrefutable biosignatures in the rock record, facultative anoxygenic photosynthesis in extant cyanobacteria represents a tractable system for examining the evolution of photosynthesis including the potential for an early evolving one-photosystem cyanobacterium. The proposed research plan will integrate a set of physiological studies coupled with systems biology approaches—transcriptomics and proteomics—to characterize an emerging model cyanobacterium isolated from a sulfidic, anoxic environment. The following objectives will guide this work: 1) define the molecular machinery necessary for anoxygenic photosynthesis; 2) characterize the effects of sulfide on photosystem II during anoxygenic photosynthesis; 3) determine oxidation kinetics of sulfide during anoxygenic photosynthesis; 4) examine the enhancement of carbon fixation in the presence of sulfide. The proposed research will examine the role of life in the transformation and evolution of Earth's geochemical cycles and the evolution of photosynthesis.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.