Mixotrophs are species of algae that act as "plants" when they produce their own food and as “animals” when they eat other plants.

Wanderson Carvalho from the University of Kalmar is studying these algae to understand their potential impact on the environment, the economy and public health issues.

In terrestrial ecosystems, plants are the only living beings capable of producing their own food, thanks to the chlorophyll and other pigments which can capture 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.

Maybe it doesn't always work that way

Mixotrophs instead literally have 'mixed nutrition.' The thesis of Wanderson Carvalho, entitled The Role of Mixotrophy in the Ecology of Marine “Phytoplankton”, had as one of the objectives to quantify in two mixotrophic species how much nitrogen and phosphorous 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.

Source: Kalmar University