NSF Award
1024913
Current estimates indicate that approximately half of the annual photosynthetic production of organic matter on Earth takes place in the euphotic zone of the water column in marine environments, making the ocean a major component of the global carbon cycle. Nearly all of the bloom-forming phytoplankton in the modern ocean, including the diatoms, are members of the chlorophyll c (Chl c) lineage. It is now well accepted that chlorophyll c-containing algae share a common ancestry where a eukaryote enveloped and domesticated a red algae, incorporating biochemical entities from both organisms. The subsequent diversification of this lineage has yielded an astounding diversity of ecologically dominant phytoplankton. Ultimately carbon export in marine ecosystems is governed and balanced by assimilation of nitrogen by phytoplankton. Among marine phytoplankton, diatoms are often the most responsive to mixing events where nutrient laden water is injected into the sunlit euphotic zone, yet the cellular basis for this competitive advantage is poorly understood. <br/><br/>The global state-of-knowledge concerning nitrogen metabolism in photosynthetic eukaryotes is largely based on biochemistry and genome sequence data from a few well-studied model green algae and vascular plants. Genome sequences, physiological and biochemical studies, and gene and protein expression data indicate that diatoms and other Chl c algae utilize a variety of biochemical components previously only observed in metazoans or bacteria. A complete metazoan-like urea cycle, for example, appears to function in concert with genes of bacterial and red or green algal origin in a functionally expanded and heretofore unobserved fashion. In metazoans the urea cycle has a function related to cellular detoxification and export of fixed nitrogen. The presence of the urea degrading enzyme urease strongly suggests an alternative function in diatoms.<br/><br/>This research is addressing the hypothesis that in marine diatoms, which are frequently subjected to nitrogen limitation, the urea cycle and associated mitochondrial enzymes function as an inorganic carbon and nitrogen recycling and repackaging hub; ultimately serving to redistribute catabolically derived NH4+ to arginine and other non-amino-acid nitrogen compounds. Based on recent advancements in diatom reverse genetics and other recently developed resources for genome enabled research, basic hypothesis concerning the role of mitochondria in nitrogen assimilation and metabolism in marine diatoms is being evaluated.<br/><br/>Broader Impacts<br/>Anthropogenic coastal loading of organic and inorganic nitrogen is expected to increase and climate change-induced shifts in oceanic mixed layer depth and mixing frequency are predicted to alter the delivery of nutrients into the euphotic zone. As a result, a key research focus is to facilitate prediction of how diatoms and other Chl c algae will respond to changing frequency and intensity of nutrient delivery events. This research will contribute to a more accurate depiction of nitrogen metabolism in marine algae, which is critically important for predictive ecosystem modeling.<br/><br/>As part of the research project, a high school biology teacher from the Escondido Union High School District in San Diego will be recruited to work on a specific topic related to the proposed research. Upon completion of his/her paid internship, in collaboration with the PIs, the teacher will design a classroom activity and curriculum installment related to the rapidly emerging field of marine genomics, also including associated topics in marine biogeochemistry, for use the following school year.
