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

智力优势：同时利用溶解有机物质（DOM）和光的细菌，即光异养生物，将从根本上改变人们对海洋中能量和物质处理方式的看法。然而，尚不清楚这些微生物在海洋中是否具有独特的作用，因为标准实验尚未成功地始终证明光对假定的光异养生物的生长和呼吸的积极作用。众所周知，这些微生物非常丰富，仅一种类型（含有变形紫红质的微生物）就占海洋中所有微生物的50%或更多。但由于光异养的生态优势尚不清楚，这些微生物为何如此丰富尚不清楚。pi将使用一种新的方法和新的实验来研究光如何影响光异养生物，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：光异养生物的生物地球化学作用是利用光化学反应的产物等低产能的DOM成分。这些反应涉及到显色DOM (CDOM)，它是碳循环中一个很大的动态部分，特别是在沿海海洋中。他们假设，光异养生物获得的光能将使这些微生物受益于光化学产生的化合物，而光化学产生的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，变形紫红质产生的能量只有在呼吸受到抑制、细胞受到能量限制时才变得重要。其他实验室实验表明，含有pr的细菌对二氧化碳的异源固定受到光的刺激。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。ppi正在特拉华河口进行实验来验证这一假设，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对光异养生物基因表达（mRNA）的影响。重点是CO、丙酮酸、乙醛和乙二醛；这些化合物共同构成了海水中光化学副产物的很大一部分。由于乙醇酸在浮游植物排泄中的重要性以及它与光化学反应产生的有机酸的相似性，也正在对其进行研究。这些化合物的吸收是用14C示踪剂和HPLC测量浓度来估计的。然后将速率与通过QPCR测定的有氧缺氧光养细菌中发现的变形紫质和pufM的丰度和mRNA水平进行比较。pi还在研究光和光化学产生的低分子量有机化合物如何影响细菌呼吸和生长效率。他们正在沿着特拉华河口的横断面和在死亡研究期间，研究倒水二氧化碳固定、光化学副产物的吸收和光异养菌的丰度和活性之间的关系。拟议的工作由微生物海洋学家（Kirchman和Cottrell）和海洋生物地球化学家（Kieber）组成的小组进行，他们分别具有光异养和光化学反应方面的专业知识。更广泛的影响：这个跨学科的项目支持研究生，也包括本科生的暑期研究项目。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之旅和海岸日（Coast Day）的特色，海岸日每年都会吸引大约1万名游客。基伯指导本科生，并协调了一个针对经济困难高中生的项目。

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.