Role of variable picoplankton cellular phosphorus turnover and allocation in marine phosphorus cycling

磷是生命的关键元素，在广阔的海洋中心地区，磷的含量通常很低，有时会达到限制生物生产力的浓度。微生物对溶解磷的吸收是控制微生物生产和海洋磷的命运和循环的重要杠杆。该项目将研究这样一个假设：在少营养环境中，微生物细胞的磷周转比细胞生物量周转更快，导致磷大量返回溶解池。这种磷的快速返回可以补充生物可利用磷库，影响微生物动力学并促进表面海洋中磷的显著循环。特别是，这项研究将使用实地样本来确定居住在马尾藻海（靠近百慕大的北大西洋中部地区）贫磷地表水中的主要小藻类和细菌群体的细胞磷和生物量周转率。该项目将由两位早期职业女性科学家共同指导。他们的教育工作将包括培训一名本科生助理，以及向K-12学生提供公共教育服务。生物地球化学循环研究的一个主要目标是将化学通量与生物体的活动联系起来。通过将海洋表面磷通量与特定微生物群的活动联系起来，并为控制这些通量的因素提供机制框架，这项工作将对理解磷的生物地球化学循环做出根本性贡献。这项研究的一个重要的新成果将是确定开放海洋中单个浮游生物群体的细胞磷周转率与生物量周转率的关系。为了建立对控制这些速率的过程的机制理解，该项目还将确定微浮游生物向细胞内生物化学物质中磷分配的变化。为了进行更详细的生化分析，将对马尾藻海中具有代表性的细胞分类群体进行无菌培养实验，以扩大现场测量。研究小组将利用一套独特的工具从环境样品中进行新的测量：流式细胞术和荧光活化细胞分选将与放射性同位素标记和生化分析相结合，以量化细胞特异性磷通量并表征这些通量的化学形态。细胞磷周转率与生物量周转率的高比值具有双重意义：(1)微生物生物量中的磷量将低估支持微生物生长所需的总磷需求量；(2)通过微生物的吸收和快速返回溶解池，海洋表面可能会发生溶解磷的大量循环。通过测量野外样品的这些速率，研究小组希望回答有关不同微生物群对海洋表面磷通量的相对影响以及驱动这些通量的细胞动力学的关键问题。

Phosphorus is a key element for life, often present in low amounts in the vast central regions of the ocean and sometimes reaching limiting concentrations for biological productivity. Microbial uptake of dissolved phosphorus is an important lever in controlling both microbial production and the fate and cycling of marine phosphorus. This project will investigate the hypothesis that in oligotrophic environments, microbial cellular turnover of phosphorus occurs more rapidly than cellular biomass turnover, leading to a significant return of phosphorus to the dissolved pool. This rapid return of phosphorus could resupply the pool of bioavailable phosphorus, impacting microbial dynamics and fueling significant recycling of phosphorus in the surface ocean. In particular, this research will use field samples to determine the rates of cellular phosphorus and biomass turnover in the dominant groups of very small algae and bacteria inhabiting the phosphorus-depleted, surface waters of the Sargasso Sea (the region of the central North Atlantic Ocean near Bermuda). The project will be co-directed by two early-career women scientists. Their educational efforts will include training of an undergraduate student assistant as well as public educational outreach to K-12 students.A primary objective in the study of biogeochemical cycles is linking chemical fluxes to the activity of organisms. This work will make fundamental contributions to the understanding of the biogeochemical cycling of phosphorus by linking phosphorus fluxes in the surface ocean to the activity of specific groups of microbes, and providing a mechanistic framework for the factors that control these fluxes. A significant, novel product of this research will be the determination of cellular phosphorus turnover rates relative to biomass turnover rates for individual picoplankton groups in the open ocean. To build a mechanistic understanding of the processes controlling these rates, this project will also determine the variation in picoplankton allocation of phosphorus into intracellular biochemicals. Field measurements will be augmented with experiments on axenic cultures, representative of the cell-sorted groups from the Sargasso Sea, to conduct more detailed biochemical analyses. The research team will utilize a unique suite of tools to make novel measurements from environmental samples: flow cytometry and fluorescence activated cell sorting will be combined with radioisotope labeling and biochemical analyses to quantify cell-specific phosphorus fluxes and characterize the chemical speciation of these fluxes. The significance of a high ratio of cellular phosphorus turnover rate to biomass turnover rate would be two-fold: (1) the amount of phosphorus in microbial biomass would underestimate the total phosphorus demand necessary to support microbial growth, and (2) significant recycling of dissolved phosphorus may occur in the surface ocean through microbial uptake and rapid return to the dissolved pool. By measuring these rates in field samples, the team expects to answer crucial questions about the relative impact of different microbial groups on surface ocean phosphorus fluxes and the cellular dynamics that drive these fluxes.