Mixotroph ecology: More than the sum of its parts

Abstract

Marine microbial ecosystems represent an important nexus in the Earth system, linking photosynthesis and biological productivity to global nutrient cycles and climate. Each year, marine biota export billions of tons of organic carbon into the deep ocean, maintaining an oceanic reserve that has a profound moderating effect on our climate (1). Our current understanding of these important microbial ecosystems has been shaped to a large extent by the terrestrial macroscopic world we see around us. In particular, the distinction between photosynthetic phytoplankton and heterotrophic zooplankton reflects a very familiar divide between plants and animals. While this distinction is intuitive, a great many species at the base of marine food webs defy such strict classifications (2). These flexible organisms, known as mixotrophs, not only use energy from the sun to take up nutrients and grow but they can also kill and eat other plankton. At present, we know that mixotrophy is the default lifestyle for many single-celled plankton, and we know that they often dominate marine communities (3). However, there remains considerable uncertainty as to how different environmental conditions select for mixotrophy across broad environmental gradients. While a number of hypotheses have been developed to explain the ecological niche of mixotrophs, we do not have a concrete understanding of how environmental factors shape the balance between different sources of nutrition in these communities. As such, it has been difficult to test the validity of hypotheses and to assess how the ecological role of mixotrophs might affect global ecosystem function, biogeochemical cycles, and climate. In PNAS, Edwards (4) presents a new synthesis of field observations and mathematical modeling that helps to address this knowledge gap.

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