Collaborative Research: Activity and abundance of photoheterotrophs fueled by photochemically-produced substrates

智力优势：利用溶解有机物（DOM）和光的细菌，即光异养生物，将从根本上改变海洋中能量和物质如何处理的观点。然而，目前尚不清楚这些微生物在海洋中是否具有独特的作用，因为标准实验一直未能成功地证明光对假定的光合异养生物的生长和呼吸的积极影响。已知这些微生物是丰富的，其中一种类型（含有变形视紫红质的微生物）占海洋中所有微生物的50%或更多。但是，为什么这些微生物如此丰富是未知的，因为光合异养的生态优势仍然模糊。PI将使用一种新的方法和新的实验来研究光如何影响光合异养生物，并探索这些微生物对DOM通量的贡献。他们的工作是测试以下假设：光合异养生物的生态地球化学作用是使用低能量产生的DOM组分，如光化学反应的产物。这些反应涉及发色DOM（CDOM），它是碳循环的一个大的和动态的部分，特别是在沿海海洋。他们假设，光异养生物获得的光能将使这些微生物能够从使用光化学产生的化合物中受益，这些化合物本身不会产生太多能量。这一假设得到了实验室实验的支持，实验表明，只有当呼吸被抑制并且细胞受到能量限制时，蛋白紫质产生的能量才变得重要。其他实验室实验表明，含PR的细菌对CO2的回补固定受到光的刺激。这种固定是C1-C4化合物生长所必需的，包括许多由光化学反应产生的化合物。PI正在特拉华州河口进行实验，以验证这一假设，那里的CDOM在空间和季节上变化很大。他们正在研究光（PAR）对光化学产生的低分子量（LMW）有机化合物的吸收和呼吸以及光异养生物的基因表达（mRNA）的影响。重点是一氧化碳，丙酮酸，乙醛和乙二醛，这些化合物一起构成了海水中的光化学副产物的很大一部分。乙醇酸也在研究中，因为它在浮游植物排泄中的重要性，而且它与光化学反应产生的有机酸相似。这些化合物的摄取估计与14 C-示踪剂和HPLC测量浓度。然后将速率与通过QPCR测定法测量的在好氧无氧光合细菌中发现的蛋白紫质和pufM的丰度和mRNA水平进行比较。PI还研究了光和光化学产生的LMW有机化合物如何影响细菌呼吸和生长效率。他们正在研究回补CO2固定、光化学副产物的吸收以及特拉华州河口和昼夜研究中的光异养生物丰度和活动之间的关系。（Kirchman和Cottrell）和一名海洋生物地球化学家（Kieber），他们分别在光异养生物和光化学反应方面具有专长。这个跨学科项目支持研究生，也涉及暑期研究项目的本科生。研究结果将被纳入网站，并用于Kirchman和Kieber教授的课程。Kirchman实验室是向公众开放的实验室图尔斯之旅和海岸日的特色，海岸日是一个每年吸引大约10,000名游客的开放日。Kieber指导本科生，并为经济困难的高中生协调了一个项目。

Intellectual Merit: Bacteria that use both dissolved organic material (DOM) and light, i.e. photoheterotrophs, would fundamentally change views of how energy and material are processed in the oceans. However, it is still not clear if these microbes have unique roles in the oceans because standard experiments have not been successful in consistently demonstrating positive effects of light on growth and respiration of presumed photoheterotrophs. It is known that these microbes are abundant, with one type (those containing proteorhodopsin) alone constituting 50% or more of all microbes in the oceans. But why these microbes are so abundant is unknown as the ecological advantages of photoheterotrophy remain obscure.The PIs will use a new approach and novel experiments to examine how light affects photoheterotrophs and to explore the contribution of these microbes to DOM fluxes. Their work is testing the following hypothesis: The biogeochemical role of photoheterotrophs is to use low energy-yielding DOM components such as products of photochemical reactions. The reactions involve chromophoric DOM (CDOM) which is a large and dynamic part of the carbon cycle especially in coastal oceans. They have hypothesized that the light energy gained by photoheterotrophs would enable these microbes to benefit from using photochemically-produced compounds which alone do not yield much energy. This hypothesis is supported by lab experiments showing that proteorhodopsin-generated energy becomes important only when respiration is inhibited and cells are limited by energy. Other lab experiments demonstrated that anaplerotic fixation of CO2 by PR-containing bacteria is stimulated by light. This fixation is needed for growth on C1-C4 compounds, including many produced by photochemical reactions.The PIs are testing this hypothesis with experiments in the Delaware estuary where CDOM varies greatly spatially and seasonally. They are examining the effect of light (PAR) on the uptake and respiration of photochemically-produced low molecular weight (LMW) organic compounds and on gene expression (mRNA) of photoheterotrophs. The focus is on CO, pyruvate, acetaldehyde, and glyoxal; together these compounds constitute a large fraction of the photochemical-byproducts in seawater. Glycolate is also being examined because of its importance in phytoplankton excretion and because of its similarity to organic acids produced by photochemical reactions. Uptake of these compounds is estimated with 14C- tracers and HPLC measurements of concentrations. Rates are then compared with the abundance and mRNA levels of proteorhodopsin and pufM found in aerobic anoxygenic phototrophic bacteria as measured by QPCR assays. The PIs are also examining how light and the photochemically-produced LMW organic compounds affect bacterial respiration and growth efficiency. They are examining the relationships among anaplerotic CO2 fixation, uptake of photochemical byproducts, and photoheterotroph abundance and activity along transects of the Delaware estuary and during diel studies.The proposed work is being conducted by a team consisting of microbial oceanographers (Kirchman and Cottrell) and a marine biogeochemist (Kieber) with expertise in photoheterotrophs and photochemical reactions, respectively.Broader Impacts: This interdisciplinary project is supporting graduate students and also involves undergraduates in summer research projects. Results will be incorporated into web sites and used in courses taught by Kirchman and Kieber. The Kirchman lab is featured in lab tours open to the public and in Coast Day, an annual open house that attracts about 10,000 visitors. Kieber mentors undergraduates and coordinated a program for economically disadvantaged high school students.