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

海洋在全球碳循环中发挥着核心作用。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。人们早就知道，海洋生物区系在很长一段时间（>1000年）的二氧化碳封存中发挥着重要作用。最近的证据还表明，海洋生物群在较短的时间尺度（10-100年）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的N含量呢？与大多数氮有限的浮游植物不同，固氮蓝藻具有无限的氮供应。这是溶解在海水中的N2气体。固氮蓝藻在海洋营养和生态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，在海洋营养最贫乏的地区提供高达50%的初级生产力。固氮作用是一个关键过程，在10至10 000年的时间尺度上调节海洋封存二氧化碳的能力。固氮作用的限制导致其他初级生产者的氮可用性降低，减少了贫营养海洋固碳的潜力。这就把我们带到了“是什么限制了海洋中固氮的数量？”“在可能限制固氮的环境因素中，有温度、光照、二氧化碳浓度和磷或铁限制。有人认为，而N是海洋中浮游植物光合作用的近限制营养素，最终限制营养素是P（或Fe），因为这种营养素限制了固氮量。本研究将探讨光、二氧化碳、磷限制和铁限制对固氮蓝藻光合特性和固氮作用的影响。研究将在规定的培养条件下在两个种属中进行。其中一个物种Trichodesmium被证明具有全球意义。此外，单细胞蓝藻固氮最近被认为是显着的。因此，第二个物种是这些单细胞固氮菌之一，Crocosphaera。这项研究的结果将提供新的见解的机制，磷和铁限制光合作用和固氮蓝藻。它还将为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营养物循环和模拟海洋未来的重要性，作为C汇的海洋学研究的几个方面。

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.