Effect of light, CO2 and nutrient limitation on photosynthesis in marine diazotrophic cyanobacteria.

海洋在全球碳循环中起着核心作用。海洋对二氧化碳的吸收减少了大气中因燃烧化石燃料和砍伐森林而增加的二氧化碳。人们早就知道，海洋生物群在很长时间尺度（大约1000年）内对二氧化碳的隔离起着重要作用。最近的证据还表明，海洋生物群在较短的时间尺度（10-100年）内也发挥着重要作用。浮游植物光合作用和群落呼吸作用之间的平衡决定了海洋吸收二氧化碳的能力。氮通常被认为是限制浮游植物光合作用的营养物质。但是是什么限制了海洋中的氮含量呢？与大多数受氮限制的浮游植物不同，固氮蓝藻有无限的氮供应，这是溶解在海水中的氮气。固氮蓝藻在海洋营养和生物地球化学循环中发挥着重要作用，因为它们是N的主要来源，在海洋最营养贫乏的地区为高达50%的初级生产力提供N。固氮是调节海洋在10到1万年的时间尺度上隔离二氧化碳的能力的关键过程。固氮的限制导致其他初级生产者的氮素有效性降低，降低了低营养海洋固碳的潜力。这就引出了一个问题：“是什么限制了海洋中的固氮量？”可能限制固氮的环境因素包括温度、光照、二氧化碳浓度和磷或铁限制。有人认为，虽然N是海洋浮游植物光合作用的近似限制养分，但最终限制养分是P（或Fe），因为这种养分限制了固氮量。本研究将探讨光、二氧化碳、限磷和限铁对固氮蓝藻光合特性和固氮的影响。研究将在两个物种的规定培养条件下进行。其中一个物种，Trichodesmium，被记录为具有全球意义。此外，单细胞蓝藻的固氮作用最近被认为是重要的。因此，第二个物种是这些单细胞固氮生物之一，鳄鱼。本研究结果将为揭示磷和铁限制蓝藻光合作用和固氮的机制提供新的见解。它还将为与环境因素（如光和二氧化碳）的相互作用提供新的见解。这项研究最终将有助于营养循环的海洋学研究的几个方面，并模拟海洋作为碳汇的未来重要性。

The ocean plays a central role in the global carbon cycle. Uptake of carbon dioxide by the oceans has reduced the increase in atmospheric carbon dioxide that has arisen from fossil fuel burning and deforestation. It has long been know that the ocean biota play a major role in sequestering carbon dioxide on very long time scales (>1000 years). Recent evidence also suggests that the ocean biota play an important role on shorter time scales (10-100 years) as well. The balance between phytoplankton photosynthesis and community respiration determines the ability of the oceans to take up carbon dioxide. Nitrogen is generally considered to be the nutrient that limits phytoplankton photosynthesis. But what limits the amount of N in the ocean? Unlike most phytoplankton, which are N-limited, nitrogen fixing cyanobacteria have an unlimited supply of N. This is the N2 gas that is dissolved in seawater. Nitrogen-fixing cyanobacteria play a significant role in ocean nutrient and biogeochemical cycles as they are a major source of N, providing N for up to 50% of primary productivity in the most nutrient impoverished regions of the ocean. Nitrogen fixation is a key process that modulates the ability of the oceans to sequester carbon dioxide on time scales of 10 to 10,000 years. Limitation of nitrogen fixation results in lowered N availability for other primary producers reducing the potential of oligotrophic oceans to sequester carbon. This brings us to the issue of 'What limits the amount of nitrogen fixation in the ocean?' Amongst the environmental factors that may limit nitrogen fixation are temperature, light, carbon dioxide concentration and P- or Fe-limitation. It is argued that whereas N is the proximate limiting nutrient for phytoplankton photosynthesis in the sea, the ultimate limiting nutrient is either P (or Fe) because this nutrient limits the amount of nitrogen fixation. This proposal will examine the effects of light, carbon dioxide, P-limitation and Fe-limitation on photosynthetic properties and nitrogen fixation of nitrogen-fixing cyanobacteria. Research will be conducted under defined culture conditions in two species. One of these species, Trichodesmium, is documented to be of global significance. In addition, nitrogen fixation by unicellular cyanobacteria has recently been recognized to be significant. Therefore, the second species is one of these unicellular nitrogen fixers, Crocosphaera. The outcomes of this study will provide new insights into the mechanisms by which phosphorous and iron limit photosynthesis and nitrogen fixation in cyanobacteria. It will also provide new insights into the interaction with environmental factors such as light and carbon dioxide. This research will ultimately assist with several aspects of oceanographic studies on nutrient cycling and modeling the future importance of the oceans as C sinks.