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

海洋在全球碳循环中扮演着中心角色。海洋对二氧化碳的吸收减少了因燃烧化石燃料和砍伐森林而增加的大气二氧化碳。人们很早就知道，海洋生物群在很长时间尺度(&gt；1000年)封存二氧化碳方面发挥着重要作用。最近的证据还表明，海洋生物群在较短的时间尺度(10-100年)上也发挥着重要作用。浮游植物光合作用和群落呼吸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氮的数量呢？与大多数浮游植物不同，固氮蓝藻的氮素供应是无限的，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，因为它们是氮的主要来源，为海洋营养最贫乏地区高达50%的初级生产力提供氮。固氮是调节海洋在10至10000年的时间尺度上隔离二氧化碳的能力的关键过程。限制固氮会导致其他初级生产者的氮素利用率降低，从而降低了营养稀少的海洋固碳的潜力。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的限制。虽然N是海洋浮游植物光合作用的最接近的限制营养物质，但最终的限制营养物质是P(或Fe)，因为这种营养物质限制了固氮量。这项建议将研究光、二氧化碳、磷限制和铁限制对固氮蓝藻的光合作用和固氮的影响。研究将在两个物种的特定培养条件下进行。其中一个物种，毛霉，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非常重要的。因此，第二个物种是这些单细胞固氮菌中的一种，Crocosphaera。这项研究的结果将为研究磷和铁限制蓝藻光合作用和固氮的机制提供新的见解。它还将为研究与光和二氧化碳等环境因素的相互作用提供新的见解。这项研究最终将有助于关于营养循环的海洋学研究的几个方面，并对海洋作为C下沉的未来重要性进行建模。

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.