Ecological Paradox in the bloom formation of mixotrophic algae

有些浮游藻类，如冰藻和隐藻，被称为混合营养藻类，因为它们可以吸收细菌。混合营养藻类具有异养和自养两种不同的营养模式，是一种多面手。在湖泊和沿海水域，这些混合营养藻类有时在浮游生物群落中占主导地位，形成密集的水华。琵琶湖就是这样一个例子，那里的美洲尾藻在春天形成密集的开花。有人认为，混合营养藻类的水华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（自养和异养）共享时，通才（混合营养体）不一定是优越的竞争对手。本研究的目的是阐明为什么美洲杉可以在琵琶湖形成密集的水华。野外生长和摄食试验研究表明，琵琶湖美国藻等混合营养藻类通过摄食细菌获得营养不足，在营养缺乏条件下具有较强的竞争优势。然而，这些混合营养藻类在细菌资源竞争方面不如异养鞭毛虫（HNF）。因此，当混合营养体与藻类和HNF共存时，它们不是优势竞争对手。捕食实验表明，至少美国的浮游动物不像HNF那样容易被捕食，因为它们的种群规模很大。这些结果表明，琵琶湖美洲花的形成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.