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The availability of phosphate plays a critical role in limiting primary production in large regions of the oceans, including major ocean gyres. Phosphorus is a primary limiting nutrient in the surface ocean, where phosphate availability controls the extent of primary production and carbon export fluxes. In phosphate-limited regions of the ocean, organisms rely on efficient recycling of nutrients to meet metabolic demand when nutrient concentrations are low. However, the process of recycling and transformation of reduced organic matter to dissolved nutrient forms is poorly understood. This thesis investigates the mechanisms responsible for nutrient availability and nutrient cycling of phosphorus in the surface ocean at the microbial level. The microbial loop of phosphate is primarily comprised of uptake, release, and regeneration fluxes, although release and recycling of phosphate remains to be quantified. To resolve the release of phosphate and enzyme-mediated reactions involved in nutrient regeneration, detailed laboratory experiments with model bacteria were completed alongside field investigations with natural surface ocean microbial populations. In this study, the role of cell lysis as a mechanism for nutrient release is assessed and quantified along with nutrient regeneration, or remineralization, catalyzed by enzymes to better understand how organic matter is transformed to dissolved nutrient forms in the surface ocean. First, this work presents a refined procedure for tracking phosphorus cycling in the ocean and identifying the important modes of nutrient cycling. An improved technique for the isolation and subsequent measurement of the stable isotopic composition of phosphate provides insight towards the significant enzyme-mediated reactions carried out by microbes. Second, the importance of cell lysis as a mode of phosphate release and nutrient stress relief was assessed both in the laboratory and the surface ocean. A primary finding of this study is the role of cell lysis in relieving nutrient stress in nutrient starved regions of the ocean. Following lysis, cells release a pulse of phosphate in organic and inorganic dissolved forms. This flux is supplemented by a continued release of phosphate from organic and particulate reservoirs of phosphorus that are degraded in the pool of cellular debris. Continued enzymatic activity in the lysate is responsible for the breakdown of particulate and organic phosphate well after cell death. This mode of enzymatic regeneration is significant, resulting in the generation of more inorganic phosphate than the immediate release after lysis. Moreover, the flux of phosphate subsequent to lysis is on par with phosphate demand by microbes in oligotrophic regions of the ocean. The transformation of organic forms of phosphate after cell death has potential to produce a larger, additional flux of phosphate available for uptake. In addition, the continued enzymatic breakdown of phosphorus after cell death serves to explain the observed decoupling of carbon and phosphorus in the ocean. Characterization of release and transformation fluxes of phosphate is essential in determining the role of phosphate in controlling primary production, carbon export, and ocean biogeochemistry. This work highlights the importance of nutrient regeneration pathways on biogeochemical dynamics in marine ecosystems and ultimately the amount of carbon exported to the deep ocean.


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