Funded Research

Quantifying the carbon export potential of the marine microbial community: coupling of biogenic rates and fluxes with genomics at the ocean surface

Marchetti, Adrian: University of North Carolina at Chapel Hill (Project Lead)

NRA: 2016 NASA: Ocean Biology and Biogeochemistry   

We seek to quantify and relate primary productivity, remineralization and net community production (NCP) in the mixed layer across different ecosystem/carbon cycling states (ECCs). More specifically, our measurements will contribute towards the determination of upper ocean ecosystem characteristics that are important in controlling the vertical transfer of organic matter to ocean depths. Through a combination of biogenic O2 gas and inorganic carbon and nitrogen inventory measurements, we will quantify the biological rate processes and fluxes in both autotrophic and heterotrophic members of the marine microbial community. Estimates of Gross Primary Productivity (GPP) and respiration rates will be used in conjunction with mixed-layer integrated rates of NCP to quantify the overall carbon export from the mixed layer. In addition to the composition of the microbial community present (i.e., phytoplankton, bacteria and archaea) there is increasing evidence that their physiological status is also important in predicting carbon export. Thus, in tandem with our rate measurements, we will determine the marine microbial community composition through targeted DNA sequencing as well as sequencing of environmental RNA to determine gene expression that can be used to infer the physiological status of the microbial plankton community. Together, the integration of these measurements will allow for an unprecedented ability to examine how the form and function of upper ocean microbial ecosystems shape the carbon export potential across different ECCs. Using our combined measurements, we will test the following hypotheses: i) Specific phytoplankton and bacteria contribute disproportionately to NCP, ii) NCP is highest when there is low coupling between members of a given marine microbial community, iii) GPP and respiration rates in the mixed layer co-vary in tightly coupled microbial communities, and iv) the physiological status of phytoplankton will have a large influence on the NCP and is an important component to predicting NCP. Our group brings a unique set of skills to address the first scientific question (SQ1) of NASA’s EXPORTS program, more specifically SQ1a, “how does plankton community structure regulate the export of organic matter from the surface ocean”. PI Marchetti’s research expertise is in phytoplankton ecophysiology and genomics. He will be responsible for the phytoplankton community characterization and primary productivity measurements. These include measurements of phytoplankton biomass and photophysiology (using FIRe fluorometry), size-fractionated new versus regenerated production (using 15N stable isotope techniques) and primary productivity (using 13C stable isotope techniques), and eukaryotic plankton differential gene expression analysis via metatranscriptomics. Co-PI Gifford’s research expertise is in marine bacterial metabolism and genomics. He will be responsible for quantifying respiration rates and bacterioplankton composition and activities. Respiration rates will be measured via O2 drawdown incubations using oxygen optodes. Bacterial taxonomic composition and functional potential will be determined by metagenomics, while expressed functional activities will be identified via metatranscriptomics. Co-PI Cassar’s research expertise is in marine biogeochemistry and ecophysiology. He will be responsible for eukaryotic and prokaryotic plankton taxonomic composition (using 18S and 16S ribosomal DNA sequencing) and the high-frequency measurements of biological O2 (O2/Ar) fluxes. The O2/Ar observations will be performed using a new generation of equilibrator inlet mass spectrometers.