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Organisms across all environments are typically assigned to one of the two categories: plants or animals. This binary classification, however, misrepresents the nature of the base of marine food webs, formed by plankton. Plankton are responsible for ~50% of the primary production in the planet and play a core role within marine trophodynamics. Among the protist plankton, many species are in fact mixotrophs.
Mixotrophy is the combination of photo-autotrophy and phago-heterotrophy within a single cell. Certain mixotrophic protists have an innate capacity for photosynthesis (the constitutive mixotrophs, CMs), while others acquire phototrophic capability from their prey (the non-constitutive mixotrophs, NCMs). NCMs can be classified into different functional forms based on how they acquire phototrophic capability. For instance, the microscopic “body-snatchers” (e.g., ciliates) can steal plastids from their prey while the “planktonic-greenhouses” (e.g., Rhizaria) enslave entire populations of their prey as endosymbionts. For the first time, a team of researchers have investigated the biogeography of these different forms of NCMs.
The study revealed that NCMs are ubiquitous and abundant in the global oceans. In addition, it showed that nearly half of what marine scientists have been labelling as “microzooplankton” (i.e., strictly heterotrophic plankton) are in reality NCMs. The data from OBIS played a key role in these investigations. Interrogation of over 100,000 OBIS distribution records and published literature across the global oceans showed different NCM functional forms to have different spatio-temporal distributions. The “body-snatchers” dominate high-biomass areas such as coastal seas while the “planktonic-greenhouses” are particularly dominant in oligotrophic open seas. Seasonally, within temperate seas, strict heterotrophic forms dominate during winter, the “planktonic-greenhouses” over spring and autumn, while the “body-snatchers” rule these waters during summer.
The findings from this study significantly changes the understanding of the functioning of the marine food web and thence the trophodynamics and the biogeochemical cycles in the oceans. This study shows that it is time to go beyond the binary ‘plant-animal’ classification and integrate the different forms of mixotrophy within marine ecology research.
Reference: Leles, S.G., Mitra, A., Flynn, K.J., Stoecker, D.K., Hansen, P.J., Calbet, A., McManus, G.B., Sanders, R.W., Caron, D.A., Not, F., Hallegraeff, G.M., Pitta, P., Raven, J.A., Johnson, M.D., Glibert, P.M., and Våge, S. (2017). Oceanic protists with different forms of acquired phototrophy display contrasting biogeographies and abundance. Proceedings of the Royal Society B 284, 20170664. https://doi.org/10.1098/rspb.2017.0664