Collaborative Research: Rhizosolenia Mats as a Source of Nitrogen Flux into the Surface Waters of the Pacific Ocean: Fe stress, N excretion and basin-scale distribution patterns

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Collaborative research: Rhizosolenia mats as a source of nitrogen flux into the surface waters of the Pacific Ocean: Fe stress, N excretion and basin scale distribution patternsThis project will quantify nitrogen cycling dynamics in the euphotic zone by vertically migrating Rhizosolenia mats, and will provide the first physiological data on mats below diver accessible depths. These macroscopic diatom assemblages sink below the euphotic zone to acquire nitrate, store it in their vacuole, and then return to the surface for photosynthesis. This new production is based entirely on a biologically, rather than physically mediated transport of N. These fragile associations require specialized collection techniques such as SCUBA and remotely operated vehicles (ROVs). As a result, their biology and biogeochemical importance has been largely overlooked. Other taxa also vertically migrate, and this project will conduct the first enumeration of this entire community in order to understand the broader role of vertical migration in oceanic nitrogen cycling.In-situ video imaging techniques will be used to quantify mats in the eastern central N. Pacific gyre. Depth specific (MOCNESS) sampling will be used to quantify the remaining taxa. A ROV will be used to collect mats below diver accessible depths in order to compare deep mats with surface mats. An existing computer model will be used to calculate transport/flux rates with these revised estimates. In addition to transporting N, Rhizosolenia mats under Fe and/or light stress may excrete both inorganic and organic nitrogen. This will be evaluated using both laboratory and field Rhizosolenia. Based on previous results, oceanic Rhizosolenia mats in nature appear to be chronically Fe-stressed. However, ferredoxin, a common and convenient in situ marker of cellular Fe status, may not be produced in these oceanic taxa and thus may not be a valid means of characterizing Fe stress in open ocean Rhizosolenia. As part of our research, we will evaluate this indicator for Fe stress in Rhizosolenia isolated from the Pacific Ocean. Our goals are to assess the Fe quotas of rhizosolenid diatoms, document the validity of the ferredoxin index as a measure of Fe stress in large oceanic diatoms, and determine N excretion rates by these taxa. A migration model will be rigorously tested by examining deep mats. These data will provide additional insight into the synergistic relationship between trace nutrient limitation and macronutrient acquisition and assimilation, an important developing theme of modern biological oceanography. The rates of biological N import and release for the eastern half of the central N. Pacific gyre by the entire migrating community will be assessed. This is a fundamental to understanding oceanic N cycles, and has direct relevance for both carbon and nitrogen cycling in the upper ocean.