Protistan grazing and viral infection of marine picoplankton: a role for the host cell surface?

海洋在决定世界气候方面起着重要作用。在某种程度上，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的浮游生物。原绿球藻属和聚球藻属的海洋蓝藻是光合浮游生物的原核成分。这些蓝藻不断地生长和分裂，但它们也不断地被消耗。有两个主要的过程有助于消耗这些细胞。首先，它们可以被病毒感染和杀死，其次，它们可以被称为原生生物的小型单细胞食草动物用作食物。这是这两种死亡过程之间的相互作用，以及蓝藻已经形成的防御机制，这是本研究项目的重点。我们最近观察到，一种感染了病毒的海洋聚球菌菌株比未感染的培养物更容易放牧。这种捕食噬菌体感染细胞的独特偏好具有潜在的重要生态学意义，因为它通过在噬菌体破裂之前清除受感染的细胞来干扰噬菌体增殖，从而间接减少噬菌体的丰度。目前还不清楚是什么决定了猎物细胞的适口性。细胞表面特性可能起作用，我们有初步证据表明，细胞壁的脂多糖（LPS）成分在这方面至关重要。因此，本研究将比较原生生物对病毒感染、未感染和LPS修饰的聚球菌菌株的摄食，以明确LPS在猎物消化率中的作用。此外，我们将把我们的研究扩展到大西洋，以调查培养的原生生物对天然聚藻球菌群体的食草差异，并以此确定菌株选择的作用，或对噬藻感染细胞的偏好，在这个过程中。总的来说，该项目将为主要的生物损失过程提供新的机制信息，这些过程决定了关键的海洋光自养生物的生长速度和产量。这些信息对于定义和理解海洋光合作用的控制至关重要。

The oceans play a major role in determining the world's 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 are also continuously being consumed. There are two major processes that contribute to the consumption of these cells. Firstly, they can be infected and killed by viruses and secondly they can be used as food by small single celled grazing animals called protists. It is the interaction between these two processes of mortality, together with the defence mechanisms that the cyanobacteria have developed, which are the focus of this research project. We recently observed that a marine Synechococcus strain that was infected with a virus was more susceptible to grazing than the uninfected culture. This distinct preference for preying on phage-infected cells has potentially important ecological implications, as it interferes with phage proliferation by removing infected cells before they burst and could thus indirectly reduce phage abundance. It is not yet clear what determines the palatability of a prey cell. Cell surface properties are likely to play a role and we have preliminary evidence that lipopolysaccharide (LPS) components of the cell wall are critical in this respect. This proposal will thus compare protist feeding on virus-infected, uninfected and LPS-modified Synechococcus strains in order to specifically elucidate the role of LPS in prey digestibility. Moreover, we will extend our studies to the Atlantic Ocean to investigate whether cultured protists graze differentially on natural Synechococcus populations, and in so doing establish the role of strain selectivity, or preference for cyanophage-infected cells, in this processOverall, the project will provide fundamentally new mechanistic information on the major biological loss processes that dictate the growth rate and yield of a key marine photoautotroph, information which is critical for defining and understanding controls on marine photosynthesis.