Ecological Paradox in the bloom formation of mixotrophic algae

一些浮游藻类，如隐藻纲和隐藻纲的物种，因为它们可以摄取细菌而被称为混合营养藻类。兼养藻类具有异养和自养两种不同的营养模式，因此可以被看作是多面手。在湖泊和沿海沃茨中，这些兼养藻类有时在浮游群落中占主导地位，并形成密集的水华。这样的例子可以在琵琶湖看到，那里的Uroglena Americana在春天形成密集的花朵。研究表明，兼养藻类水华的形成与其从细菌中获取必需物质和营养物质的能力有关。然而，最近的理论研究表明，当所有的资源都与不同的专家（自养生物和异养生物）共享时，多面手（混合营养生物）不一定是上级竞争者。本研究的目的是澄清为什么美国的。美洲花在琵琶湖能形成密集的水华。通过生长和摄食试验表明，包括U.在营养缺乏的条件下，琵琶湖的美洲藻通过内生细菌获得了缺乏的营养，因此竞争力优于其他藻类的上级。然而，这些兼养藻类在对细菌的资源竞争中不如异养鞭毛虫（HNF）。因此，当混合营养体与藻类和HNF共存时，它们不是上级竞争者。捕食实验表明，至少U。美洲是不太容易受到浮游动物放牧比HNF，因为他们的大殖民地规模。这些结果表明，水华形成的U。琵琶湖美洲虎不仅与资源竞争有关，而且与放牧压力有关。

Some planktonic algae, such as species of Crysophyceae and Cryptohyceae are called mixotrophic algae because they can ingest bacteria. The mixotrophic algae can be viewed as generalists because they have two different nutritional modes, heterotrophy and autotrophy. In lakes and coastal waters, these mixotrophic algae sometimes dominate in planktonic communities and form a dense bloom. Such an example can be seen in Lake Biwa where Uroglena Americana form dense bloom in spring. It has been suggested that the bloom formation of the mixotrophic algae is related with their ability to acquire essential substance and nutrients from bacteria. However, recent theoretical studies suggest that generalist (mixotrophs) is not necessarily superior competitor when all of the resources are shared with different specialists (autotrophs and heterotrophs). The object of the present study is to clarify why U. Americana can form dense bloom in Lake Biwa. Field research with growth and feeding experiments showed that mixotrophic algae including U. Americana in Lake Biwa acquired deficient nutrients by ingesting bacteria, and therefore competitively superior to other algae under nutrient deficient conditions. However, these mixotrophic algae were inferior to heterotrophic flagellates (HNF) in resource competition for bacteria. Thus, the mixotrophs are not superior competitor when they coexist with both algae and HNF. Predation experiments showed that at least U. Americana is less vulnerable to zooplankton grazing than HNF, because of their large colony size. These results suggest that the bloom formation of U. Americana in Lake Biwa is related with not only resource competition but also grazing pressure.