In our high school studies we learned that in Nature there are two fundamental ways of earning a living. One is to make their own food, and the other is to take advantage of the work of the former and steal it. No, I’m not talking about depraved human beings, there are, I’m talking about the most elementary ways of surviving that all living beings on this planet have. Plants, algae and, in general, all those organisms, large or small, that extract nutrients directly from mineral sources, using the energy provided by the Sun, belong to the former. These beings are known as “autotrophs”. On the opposite side are the “heterotrophs”, that is, those who extract the nutrients made by others, recycling their organic matter for our own benefit, among them are us, other animals, fungi and many other creatures.
This is how things were taught to us, but, as is usually the case with Science when it advances based on new discoveries, there are creatures that use both strategies, that is, they are capable of capturing the energy of the Sun to manufacture their own food from inorganic compounds and , when the occasion is propitious, they can feed on the already elaborated organic matter that surrounds them. These are the “mixotrophs”.
We might think that mixotrophs are “weird bugs”, a minority, but as we discovered thanks to the explanations of María del Carmen Muñoz Marín, our guest on Talking to Scientists, not only do they abound but we owe our lives to them.
Mari, who is known as such in the Department of Biochemistry and Molecular Biology at the University of Córdoba, says that these mixotrophic organisms are cyanobacteria, that is, microorganisms that abound in oceanic systems. These microscopic beings, despite being bacteria, have chlorophyll, like plants or algae, and convert carbon dioxide into organic compounds thanks to solar energy. The importance of cyanobacteria is enormous, so much so that we owe 50% of the oxygen we breathe to them.
For a long time it was thought that cyanobacteria fed only autotrophically, but in the last 15 years a series of studies have been published that show a much more complex reality. Certain genera, such as Prochlorococcus and Synechococcus are adapted to be both autotrophic and heterotrophic. And between both genera of cyanobacteria they contribute 25% of the primary production of the ocean, that is, they are responsible for a quarter of the organic matter that is generated in them and therefore serve as a base in the trophic chain.
María del Carmen Muñoz Marín had published in 2020 an article in the magazine NAME whose title translates as Mixotrophy in marine picocyanobacteria: use of organic compounds by Prochlorococcus and Synechococcus. In that work, Mari and her colleagues described the main findings on mixotrophy and its ecological relevance, an article that I invite those who want to deepen what they have learned by listening to the interview to read.
Now, Mari is back with a new article published in Nature Microbiology, entitled Mixotrophy in Depth. On this occasion, she comments on the importance of the research carried out in the waters of the Mediterranean Sea and the Pacific Ocean by Zhen Wu and his team from the WITH (Massachusetts Institute of Technology). These researchers have carried out expeditions to collect water samples at different depths, samples in which they have been able to quantify the Prochlorococcus populations and determine to what extent they feed through photosynthesis or heterotrophically through the absorption of organic carbon compounds.
The in situ discovery of two types of Prochlorococcus that develop at different depths, a shallower one where sunlight is abundant and photosynthesis dominates, and a deeper one with less light and where the cyanobacteria feed mainly on organic matter, has led to a step forward in understanding the importance and pervasiveness of mixotrophy in marine ecosystems.
I invite you to listen to María del Carmen Muñoz-Marín, researcher at the Department of Biochemistry and Molecular Biology at the Campus of International Agri-Food Excellence of the University of Córdoba,
References:
Muñoz-Marín, MC, Gómez-Baena, G., López-Lozano, A. et al. Mixotrophy in marine picocyanobacteria: use of organic compounds by Prochlorococcus and Synechococcus. NAME J 14 , 1065–1073 (2020).
Muñoz-Marín, MdC. Mixotrophy in depth. Nat Microbiol 7, 1949–1950 (2022).
Wu, Z., Aharonovich, D., Roth-Rosenberg, D. et al. Measurements and single cell modeling reveal substantial organic carbon acquisition by Prochlorococcus. Nat Microbiol 7 , 2068–2077 (2022). https://doi.org/10.1038/s41564-022-01250-5