The research focuses on the fate of the carbon associated with a major group of phytoplankton, the diatoms. The major objective is to understand how diatom community composition and the prevailing nutrient conditions create taxonomic differences in metabolic state that combine to direct diatom taxa to different carbon export pathways. The focus is on diatoms, given their large contribution to global marine primary productivity and carbon export which translates into a significant contribution to the biogeochemical cycling of carbon (C), nitrogen (N), phosphorus (P), iron (Fe) and silicon (Si).
It is hypothesized that the type and degree of diatom physiological stress are vital aspects of ecosystem state that drive export. To test this hypothesis, combined investigator expertise in phytoplankton physiology, genomics and trace element chemistry will be used to assess the rates of nutrient use and the genetic composition and response of diatom communities, with measurements of silicon and iron stress to evaluate stress as a predictor of the path of diatom carbon export. The research leverages the NASA Export Processes in the Ocean from RemoTe Sensing (EXPORTS) field program that is using a variety of methods to quantify the export and fate of primary production in the upper ocean and will allow us to follow diatom fate through multiple export pathways including sinking of single cells, sinking aggregates and grazing. The research is focused in the subarctic North Pacific, the location of the first EXPORTS field campaign, with adaptive plans for sampling the location of the second field campaign either in the subarctic N. Pacific or the N. Atlantic.
The subarctic N. Pacific ecosystem is characterized as high nutrient low chlorophyll (HNLC) due to low iron (Fe) levels that are primary controllers constraining phytoplankton utilization of other nutrients. It has been a paradigm in low Fe, HNLC systems that diatoms grow at elevated Si:C and Si:N ratios and should be efficiently exported as particles significantly enriched in Si relative to C. However, Fe limitation also alters diatoms species composition and the high Si demand imposed by low Fe can drive HNLC regions to Si limitation or Si/Fe co-limitation. Thus, the degree of Si and/or Fe stress in HNLC waters can all alter diatom taxonomic composition, the elemental composition of diatom cells, and the path cells follow through the food web ultimately altering diatom carbon export.
Within each ecosystem state examined in the EXPORTS program, nutrient biogeochemistry, diatom and phytoplankton community structure, and global diatom gene expression patterns (metatranscriptomics) will be characterized in the lit ocean. Nutrient amendment experiments with tracer addition (14C,15N, 32Si) will be used to quantify the level of Si, N and Fe stress being experienced by the phytoplankton and to contextualize taxa-specific metatranscriptome responses for resolving gene expression profiles in the in situ communities.
The research will broadly impact our understanding of the biology of the biological pump by forming a mechanistic basis for predicting the export of diatom carbon. The proposed research will support the education of three graduate students, several undergraduates and a postdoctoral scholar, all who will be broadly cross-trained in participating in a large field program and in methods to combine nutrient biogeochemistry, physiology and genetic data. The proposed research will serve as the basis for week long camp activities aimed at junior high school girls in partnership with the Girl Scouts of Rhode Island, and the Women in Marine Science chapter at the University of Rhode Island, educational units for K-12 students through the " Ocean to Classrooms? and ?Floating Lab? programs at the University of California, Santa Barbara, and activities for the Oceanography Camp for Girls at the University of South Florida.