NSF-BSF: The role of rhodopsin phototrophy in ocean's solar energy capture and its regulation in contrasting nutrient regimes

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This is a project jointly funded by the National Science Foundation's Directorate of Geosciences (NSF-GEO) and the Israel Binational Science Foundation (BSF) in accord with the language in the Memorandum of Understanding between the NSF and the BSF. This Agreement allows a single collaborative proposal, involving US and Israeli investigators, to be submitted and peer-reviewed by NSF. Upon successful results of the NSF merit review and recommendation by the cognizant NSF Program of an award, each Agency funds the proportion of the budget and the investigators associated with its own country. Rhodopsin phototrophy is the simplest light-dependent metabolism in nature. Since its discovery in marine systems over two decades ago, rhodopsin genes have been found in most microorganisms of the surface ocean. This implies that some of the solar energy that reaches the ocean could be channeled by metabolisms other than conventional chlorophyll-based photosynthesis. Despite the potential importance of rhodopsin phototrophy in marine energy fluxes and the global carbon cycle, its ecological role remains elusive. To study its relevance, this team has developed an analytical method to determine the concentrations of the light-sensitive pigment in rhodopsins, the chromophore retinal. First estimates suggest that rhodopsins are able to capture more sunlight energy than chlorophylls in marine regions where inorganic nutrients are very low, such as the Mediterranean Sea. Yet, these patterns seem to be opposite in the coastal north Pacific Ocean where high rhodopsin synthesis also occurs during the high-nutrient upwelling season. Therefore, it is still unclear which environmental variables are driving rhodopsin synthesis in the ocean. The overall goal of this project is to understand the effects of nutrient availability on marine rhodopsin production and light utilization in systems representative of vast areas of the world ocean. These data are key to further extrapolate the role of rhodopsins at a global scale and to predict their impact on ocean dynamics under different climate change scenarios. Broader impacts of this project include supporting the dissertation work of two graduate students and the active participation of undergraduate and underrepresented minority high school students in fieldwork and laboratory experiments.This project studies the effects of nutrient availability on microbial rhodopsin phototrophy in two marine regions which are representative of large areas of the world ocean: the oligotrophic P-limited Eastern Mediterranean Sea and the upwelling N-limited North Pacific. Monthly In situ rhodopsin quantifications coupled with metagenomics and metatranscriptomics are being used to identify the seasonal patterns of rhodopsin- based phototrophy in different microbial taxonomic groups and environments. These field data are complemented with seawater nutrient enrichment incubation experiments, which links nutrient availability to rhodopsin-mediated light capture in a controlled setting. Finally, studying the particular physiology of rhodopsin-containing bacteria isolated from the North Pacific coast and the Eastern Mediterranean allows the synthesis regulation strategies used by bacteria from different marine regions and lifestyles to be determined. Overall, this combination of observational and manipulative approaches will provide a more complete and comprehensive understanding of the effect of nutrient supply on rhodopsin distribution, synthesis regulation and energetic gain. The scientific and societal impacts of this project include understanding the consequences of a potential expansion of rhodopsin phototrophy in response to climate change forcing.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.