Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Related Publications
• Related Datasets
• Parameters
• Deployments
• Project Information
• Funding

Shipboard incubation experiments were carried out at 15 stations in the tropical Atlantic with trace metal clean-collected bulk seawater to examine iron (Fe) and phosphorus (P) co-limitation of dinitrogen (N2) fixation (see TriCoLim Station coordinates metadata). Treatments were: Co-limited control (no additions, “Colimit”), + 2nM Fe (“Fe”), + 50 nM P (“P”) and Replete (+Fe+P, “Rep”). All incubations were carried out at ambient (“Lo”) temperature and ambient temperature +3C (“Hi”) for 2-4 days on deck before measuring 15N2 and 13CO2 fixation rates and particulate organic carbon (POC), nitrogen (PON) and phosphorus (POP). See related datasets.

POP measurements were conducted as follows: ~500 ml of bulk water were filtered onto pre-combusted GFF filters and rinsed two times with 2 ml each of 0.17M sodium sulfate (Na2SO4). Filters were placed in pre-combusted borosilicate vials capped a combusted aluminum foil cap and soaked in 2 ml of 0.017M magnesium sulfate (MgSO4) and left to dry in an oven at 60 deg C for 2-3 days.

Samples were processed colorimetrically following previously established methods (Fu et al., 2005).

BCO-DMO Processing Notes:

• names

NSF abstract:

Marine cyanobacteria are able to use or "fix" atmospheric nitrogen gas, and so supply much of the essential nutrient nitrogen that supports open ocean food chains. Oceanographers have usually thought that the growth of these nitrogen-fixing cyanobacteria is limited at any particular time and place by the supply of either iron, or of phosphorus. Preliminary experiments have shown, though, that these nitrogen fixers instead grow best when both iron and phosphorus are scarce at the same time. In this project, the researchers will use cellular indicators that are specific for iron and phosphorus limitation to determine how important this type of "balanced limitation" of nitrogen-fixing cyanobacteria is in controlling the productivity of ocean food chains in the tropical Atlantic Ocean. Two graduate students will be trained at the University of Southern California (USC) and Woods Hole Oceanographic Institution, as well as a postdoctoral researcher at USC. Educational outreach efforts will take place at a Los Angeles inner city high school with a student body that is over 98% Hispanic and African-American, and with underrepresented undergraduates in the USC Global Environmental Microbiology course. In addition, two Research Experiences for Undergraduates students will be supervised for summer research projects to help them learn about science career options.

The researchers will investigate the biological and biogeochemical consequences of this unique balanced iron/phosphorus-limited phenotype, using both laboratory and fieldwork approaches. During the first year of this project, the nitrogen-fixing cyanobacteria will be cultured under iron and/or phosphorus limitation, followed by application of proteomics and transcriptomics to identify genes that are potential diagnostic biomarkers for iron/phosphorus balanced limitation. Preliminary work has already identified one promising candidate biomarker in one cyanobacterium, an EzrA protein domain that appears to be associated with the cell size decreases seen specifically under balanced limitation, and the researchers have identified numerous other potential candidates for similar biomarkers. During the second year, these new co-limitation biomarkers and others previously validated for iron limitation (IsiB) and phosphorus limitation (SphX) will be used to investigate balanced limitation during a research cruise transecting from relatively high-iron, low-phosphorus North Atlantic waters, to the relatively high-phosphorus, low-iron South Atlantic. This fieldwork component will survey nitrogen fixing cyanobacteria populations across this natural iron/phosphorus gradient for genetic, proteomic, and physiological indicators of balanced limitation, as well as testing their responses to iron and phosphorus manipulations in shipboard incubation experiments. The third year will be devoted to sample analysis, and publications exploring the responses of oceanic nitrogen fixers to simultaneous limitation by both iron and phosphorus.