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。具体过程包括海气二氧化碳交换、表面混合、深水排气、固碳、呼吸、碳酸钙的形成和沉积。最近的研究表明，光引发（光化学）过程也强烈影响海洋表面的碳循环，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光化学氧化的重要性的证据包括：1)经过无菌过滤的海水辐照导致COz、CO和生物可利用底物的高产率；2)低生产力的中央环流似乎是净异养的（即细菌C的利用率往往超过初级生产力）；3)据估计，细菌C的利用率与开阔海洋表层水中DOM光降解产生的生物不稳定底物的产量大致相同。4)表层水体中被开阔海洋细菌吸收的碳中有很大一部分是等渗的。尽管有这些强有力的证据，光化学在海洋碳循环中的定量重要性仍然未知。研究将集中在：(1)海洋DOM光化学生成CO2和CO； (2) DOC光化学损失；(3)光氧化DOM的微生物吸收和再矿化。结果将评估光化学对水柱中碳通量和有机碳质量平衡的影响，包括初级和细菌生产力、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、老化DOM的利用可能在很大程度上解释了在低生产力水域经常观察到的高细菌C利用率和低初级生产力之间的明显差异。本研究的另一个重要目标是开发基于作用谱、DOM吸光度和表面辐照度的算法，以预测地表水的光化学速率。这些算法与遥感DOM荧光相结合，用于预测海洋大面积区域的CO2和CO光化学产生和DOC光降解率。以前在各种海洋环境中研究光化学过程的成功为评估这些过程在海洋碳循环中的重要性提供了坚实的基础。

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 appare nt 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.