Understanding Algae That Are Both 'Plant' And 'Animal'

We know that in terrestrial ecosystems, plants are the only living beings capable of producing their own food. This is only possible thanks to the chlorophyll and other pigments which can capture the sunlight energy.

With this energy and nutrients (e. g. nitrogen and phosphorus) from land and carbon dioxide from the atmosphere they produce organic material. Plants are thus producers and belong to the base of the food chain. Until recently it was believed that the same system worked for lakes and the sea, where phytoplankton (microalgae) served as food for zooplankton (microscopic animals), which in turns were eaten by small fishes, which were eaten by bigger fishes and then by humans and other top predators. However, nature is full of surprises! There are algae species that can act both as “plants” and as “animals” at the same time.

As “plants” the algae produce their own food and as “animals” they can eat other plants or even their own grazers. These organisms are called mixotrophs and their nutritional strategy is thus known as mixotrophy, in other words: “mixed nutrition”. This dual nutritional behavior affects the notion of food chain mentioned above. In a comparison, imagine if instead of a cow eating the grass, the grass grabs and eats the cow.

The thesis of Wanderson Carvalho had as one of the objectives to quantify in two mixotrophic species how much nitrogen and phosphorus are needed when they act as “plants” and as “animals”, respectively. For example, under nutrient (nitrogen and phosphorus) deficient conditions, mixotrophs can outcompete other algae species by eating them or utilizing the little available nutrients dissolved in the water. Wanderson also found out that “feeding as animals” can also provide carbon and energy to the mixotrophs if light is low or absent.

In absence of food, mixotrophs can use their photosynthetic capabilities to survive until suitable prey is available again. Mixotrophs can decrease competition since they can feed on their competitors and predators alike. Mixotrophs can survive adverse periods and because of that many mixotrophs form blooms, becoming potentially harmful to the environment.

Wanderson Carvalho thesis is entitled "The Role of Mixotrophy in the Ecology of Marine 'Phytoplankton.'"