Revealing a mechanistic understanding of the role of viruses and host nutrient status in modulating CO2 fixation in key marine phototrophs

海洋在决定世界气候方面发挥着重要作用。这部分是由于非常小的单细胞生物产生氧气和消耗二氧化碳，这些生物被称为光合微型浮游生物。海洋蓝细菌的密切相关的属原绿球藻和聚球藻是光合微型浮游生物的原核组成部分。这些蓝藻不断生长和分裂，但它们也可以被病毒感染和杀死。感染细菌的病毒（噬菌体）为我们目前对分子生物学和遗传学的理解提供了基础，并且最近随着噬菌体的异常丰富及其在许多生物过程中的核心作用的认识而具有更大的意义。噬藻体是一种病毒，能够感染一种细菌（蓝细菌），这种细菌通过光合作用将光作为其主要能源。蓝细菌的光合机制捕获光能并将其转化为化学能，随后用于生长和复制。海洋区域在其营养物供应方面差异很大，例如磷酸盐，氮和铁，这些营养物对蓝细菌的生长至关重要，可能限制这些生物体对CO2的固定。营养物质的可用性也可能影响噬藻体的复制，因为在感染期间，噬藻体依赖于其宿主为它们提供足够的能量和资源，以使它们能够有效地复制。然而，在存在和不存在噬菌体感染的情况下，营养物质的可用性对海洋蓝藻CO2固定的影响在很大程度上是未知的。这一点很重要，因为海洋蓝藻是全球二氧化碳固定的关键贡献者，这些生物的病毒感染可能会显着调节这种贡献。一个例外是海洋聚球藻的磷酸盐限制已显示导致产生的噬藻体数量减少80%，而细胞裂解<10%。噬藻体感染P饥饿的细胞，但留在宿主体内而不杀死它们，这种状态称为“假溶血”。鉴于海洋系统往往是耗尽营养物质，如P（以及氮（N）和铁（Fe））表明，这种感染动力学可能是广泛流行的自然environment.Hence，在这个建议中，我们将确定的作用，营养有限的生长海洋蓝藻CO2固定率在噬菌体感染的存在和不存在。我们还将评估特定的噬藻体基因在这一过程中的作用，并确定调节“假溶血性”的分子基础。此外，我们亦会提供一个可靠的（实验所得的）数学公式，用以描述病毒感染，并将其纳入一个生态系统模式[ERSEM]中，从而大大改善对海洋初级生产力的模拟。总的来说，这项建议将为了解营养物质和噬藻体感染在控制海洋初级生产力方面的作用提供直接的估计和机理基础。数据和概念随后将用于ERSEM，以完善海洋光合作用和随后的C循环的控制点。

The oceans play a major role in determining world climate. In part, this is due to the production of oxygen and the consumption of carbon dioxide by very small, single celled organisms, which are referred to as the photosynthetic picoplankton. Marine cyanobacteria of the closely-related genera Prochlorococcus and Synechococcus are the prokaryotic components of the photosynthetic picoplankton. These cyanobacteria are continually growing and dividing, but they can also be infected and killed by viruses. Viruses that infect bacteria (bacteriophage) have provided the basis of our current understanding of molecular biology and genetics and have recently assumed a much greater significance with the recognition of the extraordinary abundance of bacteriophages and their central role in many biological processes. Cyanophages are viruses that are specifically capable of infecting a type of bacteria (cyanobacteria) that utilises light as its primary energy source through the process of photosynthesis. The cyanobacterial photosynthetic machinery captures light energy and transfers it to chemical energy which is subsequently used for growth and replication. Oceanic regions vary considerably in their supply of nutrients e.g. phosphate, nitrogen and iron, that are critical for the growth of cyanobacteria, potentially limiting CO2 fixation by these organisms. The availability of nutrients may also affect cyanophage replication, since during infection cyanophage rely on their hosts to provide them with enough energy and resources to allow them to replicate efficiently. However, the effect of nutrient availability on marine cyanobacterial CO2 fixation in the presence and absence of phage infection is largely unknown. This is important because marine cyanobacteria are critical contributors to global CO2 fixation and virus infection of these organisms may significantly modulate this contribution. One exception is that phosphate limitation of marine Synechococcus has been shown to cause an 80% reduction in the number of cyanophage produced with <10% of cells lysing. Cyanophage infect P starved cells but remain inside their hosts without killing them, in a state known as 'pseudolysogeny'. Given that oceanic systems are often depleted in nutrients such as P (as well as nitrogen (N) and iron (Fe)) suggests such infection dynamics are likely widely prevalent in the natural environment.Hence, in this proposal we will determine the role that nutrient limited growth plays on marine cyanobacteria CO2 fixation rates in the presence and absence of phage infection. We will also assess the role that specific cyanophage genes contribute to the process, and determine the molecular basis regulating 'pseudolysogeny'. Moreover, we will also provide a reliable (experimentally-derived) mathematical formulation describing viral infection which will be incorporated into an Ecosystem Model [ERSEM] providing a substantially improved simulation of oceanic primary production.Overall, the proposal will therefore provide direct estimates, and a mechanistic basis, for understanding the role of nutrients and cyanophage infection in controlling marine primary production. Data and concepts will subsequently be used in ERSEM to refine control points for marine photosynthesis and subsequent C cycling.