Date of Award

Spring 2024

Project Type

Thesis

Program or Major

Oceanography

Degree Name

Master of Science

First Advisor

Robert T Letscher

Second Advisor

Elizabeth L Harvey

Third Advisor

Angela N Knapp

Abstract

Phytoplankton form the basis of the marine ecosystem and require phosphorus (P) for growth. Typically, phytoplankton utilize inorganic phosphate (PO43-) as it is the more biologically available form of P. Many regions across the global ocean have been found to exhibit vanishingly low concentrations of PO43- including the subtropical North Atlantic and North Pacific. Dissolved organic phosphorus (DOP) has been shown to serve as an alternative P source when PO43- concentrations are low. However, the concentration of PO43- which leads to significant phytoplankton DOP uptake has not been widely established. In this work, both experimental and field DOP data were analyzed to evaluate the role of DOP uptake by phytoplankton. GO-SHIP A20 and A22 transects were analyzed to assess the distribution of DOP concentrations in the Western North Atlantic Subtropical Gyre. For both transects, surface ocean DOP concentrations were generally elevated at the gyre edges with the lowest DOP concentrations located towards the center of the gyre, where PO43- concentrations were also lowest. This pattern largely fits within existing paradigms describing the lateral supply of DOP as an organic nutrient helping sustain subtropical gyre primary productivity. In addition to the DOP field data, phytoplankton batch cultures were performed to experimentally study the importance of DOP for sustaining autotrophic P demand under low PO43- conditions. Emiliania huxleyi (E. huxleyi) and Chaetoceros tenuissimus (C. tenuissimus) cultures were grown under five PO43- concentrations of increasing PO43- scarcity with an addition of DOP in each treatment. Measurements of alkaline phosphatase activity (APA), which is the enzyme used to access the P from DOP, and the consumption of PO43- and DOP were monitored over a 2-week period. Elevated APA rates were observed in the lowest PO43- treatments as expected. Growth rates for E. huxleyi were similar regardless of P source but C. tenuissimus unexpectedly grew faster when utilizing DOP compared to PO43-. C. tenuissimus also utilized DOP to satisfy a higher percentage of their P demand compared to E. huxleyi. Our experimental data suggest minimum DOP utilization rates for these two phytoplankton were 1.5 to 2.5 times greater than previous estimates, expanding the spatial domain over which autotrophic DOP utilization may occur in the ocean.

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