Impact of Photochemistry on Carbon Cycling in the Sea

物理和生物过程历来被认为是影响海洋碳循环的最重要因素。具体过程包括大气-海洋二氧化碳交换、表面混合、深水排气、固碳、呼吸作用、碳酸钙形成和沉积。最近的研究表明，光引发的(光化学)过程也强烈影响海面的碳循环，特别是在溶解有机碳(DOC)的光降解和细菌底物的产生方面。老化的、生物难降解的DOC的光化学氧化的重要性的证据包括：1)无菌过滤海水的照射导致高COZ、CO和生物可利用底物的产生，2)低生产力中心旋涡似乎是净异养的(即细菌C的利用率往往超过初级生产力)，3)细菌C的利用率估计与开放海洋表面水域DOM光降解产生的生物不稳定底物的产生速率大致相同，以及4)在表层水域被开放的海洋细菌同化的很大一部分碳是等渗老的。尽管有这些强有力的证据，但光化学在海洋碳循环中的数量重要性仍不得而知。研究将集中在：(1)海洋DOM光化学产生CO2和CO，(2)DOC的光化学损失，(3)微生物对光氧化DOM的吸收和再矿化。结果将评估光化学对水柱中的碳通量和有机碳质量平衡的影响，包括初级生产力和细菌生产力、微生物呼吸以及在BATS站的颗粒碳通量。我们预计，细菌对光氧化陈化DOM的利用可能在很大程度上解释了高细菌C利用率和低初级生产力之间的明显差异，这在低生产率的水中经常被观察到。这项研究的另一个重要目标是开发基于作用光谱、DOM吸光度和表面辐照度的算法来预测地表水中的光化学速率。这些算法与遥感DOM荧光相结合，可以预测海洋大片区域的二氧化碳和一氧化碳的光化学产量和DOC的光降解率。以往在研究各种海洋环境中的光化学过程方面取得的成功为评估这些过程在海洋碳循环中的重要性奠定了坚实的基础。

Physical and biological processes are historically considered to be the most important factors affecting the carbon cycle in the ocean. Specific processes include air-sea CO2 exchange, surface mixing, venting of deep waters, carbon fixation, respiration, calcium carbonate formation and sedimentation. Recent studies now suggest that light-initiated (photochemical) processes also strongly impact carbon cycling at the sea surface, particularly with respect to the photodegradation of dissolved organic carbon (DOC) and the production of bacterial substrates. The evidence for the importance of photochemical oxidation of aged, biologically refractory DOC includes: 1) irradiation of sterile-filtered seawater results in high production rates of COz, CO and biologically utilizable substrates, 2) low productivity central gyres appear to be net heterotrophic (i.e., bacterial C utilization rates often exceed primary productivity rates), 3) bacterial C utilization rates are estimated to be about the same as the production rate of biologically labile substrates from DOM photodegradation in open oceanic surface waters, and 4) a significant fraction of the carbon assimilated by open ocean bacteria in surfaces waters is isotonically old. Despite this strong evidence the quantitative importance of photochemistry in the oceanic carbon cycle is still unknown. Studies will focus on: (1) the photochemical production of CO2 and CO from marine DOM, (2) the photochemical loss of DOC, and (3) the microbial uptake and remineralization of photooxidized DOM. Results will assess the impact of photochemistry on carbon fluxes and organic carbon mass balances in the water column, including primary and bacterial productivities, microbial respiration and, at the BATS station, particulate carbon fluxes. We expect Bacterial utilization of photooxidized, aged DOM may largely explain the apparent discrepancy between high bacterial C utilization rates and low primary productivity rates that have been frequently observed for low productivity waters. Another important goal of this study is to develop algorithms, based on action spectra, DOM absorbance and surface irradiance, to predict photochemical rates in surface waters. These algorithms in combination with remotely sensed DOM fluorescence to Predict CO2 and CO photochemical production and DOC photodegradation rates over large areas of the ocean. Previous successes in studying photochemical processes in a variety of marine environments gives a strong foundation for assessing the importance of these processes in the oceanic carbon cycle.