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。在一定程度上，这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作用微浮游生物。海洋蓝藻是原氯球藻属和聚球藻属的近缘属，是光合作用微微浮游生物的原核成分。这些蓝藻不断生长和分裂，但它们也可以被病毒感染和死亡。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，最近随着对噬菌体异常丰富的认识及其在许多生物过程中的核心作用的认识，病毒具有了更重要的意义。噬藻体是一种能够感染一种细菌(蓝藻)的病毒，这种细菌通过光合作用过程利用光作为其主要能源。蓝藻的光合作用机制捕获光能，并将其转化为化学能，随后用于生长和复制。海洋地区的磷酸盐、氮和铁等营养物质的供应差异很大，这些营养物质对蓝藻的生长至关重要，可能会限制这些生物对二氧化碳的固定。营养物质的可获得性也可能影响噬藻体的复制，因为在感染期间，噬藻体依赖宿主为其提供足够的能量和资源，使其能够有效地复制。然而，在存在和不存在噬菌体感染的情况下，营养物质的有效性对海洋蓝藻固定二氧化碳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显著调节这一贡献。一个例外是，海洋聚球藻的磷酸盐限制已被证明能导致10%的细胞裂解产生的氰噬菌体数量减少80%。噬藻体感染饥饿的P细胞，但仍留在宿主内而不杀死它们，这种状态被称为“假溶源”。由于海洋系统经常缺乏磷(以及氮(N)和铁(Fe)等营养物质)，这表明这种感染动态很可能在自然环境中广泛存在。因此，在本提案中，我们将确定在存在和不存在噬菌体感染的情况下，营养限制生长对海洋蓝藻CO2固定率的影响。我们还将评估特定的噬藻体基因在这一过程中所起的作用，并确定调控“假溶源”的分子基础。此外，我们还将提供一个描述病毒感染的可靠的(实验得出的)数学公式，该公式将被纳入生态系统模型[ERSEM]，从而大大改进对海洋初级生产力的模拟。因此，该提议将提供直接的估计和机制基础，以了解营养物质和噬藻体感染在控制海洋初级生产力中的作用。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.