Ecological Paradox in the bloom formation of mixotrophic algae

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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.