Physiological response of small eukaryotic phytoplankton to differing nitrogen regimes in the North Pacific Subtropical Gyre

In the North Pacific Subtropical Gyre, the physical supply of nutrients to the mixed layer (ML) is relatively small, however, small injects of NO₃^- into the euphotic zone have been shown to stimulate eukaryotic phytoplankton blooms and enhance carbon export. To investigate the response of eukaryotic phytoplankton to inputs of new (e.g., NO₃^-) and recycled (e.g., NH₄⁺) nitrogen (N) sources, we conducted mesocosm experiments using seawater collected from within the ML and at the deep chlorophyll maximum (DCM) during late-summer. Various N regimes were simulated using assorted N amendments (as either NH₄⁺, NO₃^- or deep seawater), and metatranscriptomic responses of important phytoplankton groups were assayed. The physiological response of the ML and DCM communities differed, with the ML community expressing genes related to N acquisition regardless of N availability while the DCM community upregulated specific genes in response to N inputs. In addition, smaller cells (<5 μm) dominated biomass, primary productivity and generally utilized recycled N (f-ratio of 0.2±0.1) in the ML while larger cells (>5 μm) dominated at the DCM and relied more heavily on new N (f-ratio of 0.4±0.3). The study provides evidence for niche partitioning between the two layers: low-N supply, small cell dominance and constitutive gene expression in the ML contrasts pulsed N supply, large cell dominance and responsive gene expression at the DCM. These results highlight the physiological differences observed between the ML and DCM communities, and how they may respond to changing N supply mechanisms in current and future oceans.

Presentation Preference: Oral

Primary Presenter: Raquel Flynn, University of North Carolina at Chapel Hill (raqflynn@unc.edu)

Authors:

Meredith Meyer, School of Environmental Sciences, University of East Anglia, United Kingdom  (m.meyer@uea.ac.uk)

Julie Granger, Department of Marine Sciences, University of Connecticut, USA  (julie.granger@uconn.edu)

Angelicque White, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, USA  (aewhite@hawaii.edu)

Catherine Crowley, Department of Marine Sciences, University of Connecticut, USA  (catherine.crowley@uconn.edu)

Brandon Brenes, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, USA  (brenesb@hawaii.edu)

Adrian Marchetti, Earth, Marine and Environmental Sciences Department, University of North Carolina at Chapel Hill, USA  (amarchet@email.unc.edu)