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Why Mixotrophy Matters: A Competitive Edge

This is Jim Fiorendino, your host for On the Ocean.

Runners often have a big meal of pasta, or another carbohydrate-rich food before a marathon. Carbohydrates provide lots of energy when broken down, which runners need to compete. A runner who has eaten lots of carbohydrates may have a competitive advantage over a runner who has not. In the same way, mixotrophic marine phytoplankton may have a competitive advantage over strictly autotrophic phytoplankton.

Phytoplankton in the ocean were once thought to be strictly autotrophic, which means they make their own food and do not need to eat. In contrast, heterotrophs like zooplankton obtain their food by consuming other organisms. Recently, the importance of mixotrophy, or the ability to derive nutrients and food from both autotrophic and heterotrophic pathways, has become apparent within the marine microbial world.

Phytoplankton are a numerous and diverse group present in all the world’s oceans. Despite their diversity, phytoplankton compete for the same resources, specifically light, carbon, and other nutrients such as nitrogen, phosphorous, and iron. There are so many different species of phytoplankton that it seemed impossible so many could exist while competing for the same resources; surely some species would outcompete the others and drive the losers to extinction. This problem has been the subject of extensive research in the past, and has been described as the Paradox of the Plankton.

Scientists now know that, though the marine environment may seem uniform, it is quite variable. Many phytoplankton are specialized, exploiting subtle differences in their environment that other species cannot, and ensuring their survival. If a species is to succeed over some other species, it must outcompete other organisms and maintain a steady growth rate. Mixotrophy may have offered a competitive advantage to certain species of phytoplankton by supplying an additional source of food that other species did not have. This may have allowed certain species to continue to be successful when conditions were unfavorable for strict autotrophs relying solely on photosynthesis.

This has been On the Ocean, a program made possible by the Department of Oceanography and a production of KAMU-FM on the campus of Texas A&M University in College Station. For more information and links, please go to and click On the Ocean.


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