Rhodopsins are the simplest energy-harvesting photoproteins and community metagenomics have revealed that their synthesis genes are ubiquitous throughout the world oceans. These include microbial rhodopsin (proteorhodopsin (PR)), which occur in an estimated 75% of marine bacteria and archaea in oceanic surface waters. The discovery of this abundant and widespread photoprotein in the surface ocean has challenged the notion that solar energy can only be converted into chemical energy for growth in marine ecosystems through chlorophyll-based photosynthesis. Although the potential of light-driven energy flux in ocean ecosystems through PR could be significant, the physiological and ecological functions of this type of rhodopsin remains undetermined, mainly due to the lack of a technique for a direct measurement of this photoprotein. To evaluate the ecological relevance of PR in the marine environment, The investigators have developed a new analytical technique to measure the concentrations of the light-sensitive pigment in the PR, the chromophore retinal. Because rhodopsins have a single retinal chromophore associated with the polypeptide opsin, the total number of retinal molecules is equivalent to the total number of PR. Intellectual Merit: This project will employ the PI's newly developed protocol to examine the effects of light, organic carbon and trace metals availability on PR and bacteriochlorophyll synthesis using field and laboratory manipulations. Such experiments will establish the impact of abiotic factors on the two known bacterial photoheterotrophic metabolisms. The laboratory studies will be complemented with the analyses of those pigments in field samples collected along spatial and temporal gradients in light intensity, organic carbon and trace metals in different oceanographic regimes. Gene expression patterns will be determined in concert with changes in retinal and bacteriochlorophyll concentrations and microbial growth responses in the field and in the laboratory. Therefore, the combination of observational and manipulative approaches, will address fundamental questions in regard to the impact of retinal-based photochemical energy transformation in the ocean, a process that still is not well understood. Broader Impacts: Undergraduate and graduate education at USC will be furthered through active participation in the joint laboratory experiments and field work. The PIs research program includes a commitment to undergraduate student training by requesting support to target qualified underrepresented students from other L.A.-area schools, as well as active mentoring of undergraduate senior research theses at USC. In support of this effort Sanudo-Wilhelmy is starting a new initiative for minority recruitment at USC, using the Society for Advancement of Chicanos and Native Americans in Science meetings and publications as a platform. This project will also completely support the dissertation work of one USC graduate student and provide the funding for publication of his results, as well as to allow him to present his work at national meetings. The scientific and societal impacts of this project include elucidating the ecological importance of energy-transforming PR, without which we will never have a complete understanding of all of the essential mechanisms sustaining major biological processes in the world ocean.
