Award: OCE-1657757

This project investigated the factors that control the growth of nitrogen-fixing cyanobacteria in the ocean. These plankton help support the rest of the ocean food web by providing the essential nutrient nitrogen, since they have an unusual ability to obtain or 'fix' nitrogen from the air. However, they require resources to do this important function, and two of the resources in shortest supply in most oceanic areas are the nutrients iron and phosphorus. Because of this, many previous studies have tested how either iron or phosphorus can limit the amount of nitrogen fixation by marine cyanobacteria. This project focused on understanding what happens when both iron and phosphorus are limiting at once- a common situation in the ocean often called 'co-limitation'. This project investigated the consequences of iron and phosphorus co-limitation in key marine nitrogen fixing cyanobacteria using both laboratory and fieldwork approaches. For the culture component, the cyanobacteria were cultured under iron and/or phosphorus limitation, followed by measurements of gene and protein expression to identify molecular-level responses co- limitation. This allowed us to develop and apply molecular biomarkers to investigate co-limitation during a February 2018 research cruise transecting from relatively high-iron, low-phosphorus North Atlantic waters, to the relatively high-phosphorus, low-iron South Atlantic. This fieldwork component surveyed nitrogen-fixing cyanobacteria populations across this natural iron/phosphorus gradient, as well as testing their responses to iron and phosphorus additions in shipboard incubation experiments. Similar to the field work, the laboratory culture work used the unicellular nitrogen-fixing cyanobacterium Crocosphaera cultured under iron and/or phosphorous limitation to evaluate its gene and protein expression. Supported graduate student Nina Yang led this part of the project, and her work suggests that the relationship between gene expression and protein abundance play an important role in Crocosphaera's ability to adapt to low-nutrient environments in the ocean. The major contribution of this project to the field of marine science will be to start an open, inter-disciplinary conversation about the need to re-evaluate our ideas about nutrient limitation of nitrogen fixation in the open ocean, with potential implications for understanding the potential for ocean food webs to produce living resources like fish and take up fossil fuel carbon dioxide from the air. Last Modified: 04/22/2021 Submitted by: David A Hutchins