MixITiN: Bringing the paradigm for marine pelagic production into the 21st century; incorporating mixotrophy into mainstream marine research

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The conceptual basis for marine science, upon which management tools for our oceans, seas and coasts have operated, are out-of-date. Current management tools and policies operate within a paradigm that builds on a simple division – between “plant-like” phytoplankton and their main consumers, the “animal-like” microzooplankton. Thus, the microscopic eukaryotes (protists) that comprise the phytoplankton and microzooplankton are divided neatly (Fig.1).

Fig.1 Traditional division of protist plankton groups between “plant-like” phytoplankton and their main consumers, the “animal-like” microzooplankton. Adapted from Flynn et al. 2013.
Fig.2 The revised nutritional description of protist plankton groups traditionally divided between phytoplankton or microzooplankton. Compare with Fig.1. Adapted from Flynn et al. 2013.

This conceptual model forms the base of the aquatic food chain leading to fisheries. We have recently shown that this “plant-animal” paradigm represents at the least a gross simplification, if not a falsehood. The revised paradigm recognises that most protist phytoplankton and half the microzooplankton actually combine plant-like photosynthesis with animal-like consumer activity within the one cell. This form of nutrition is termed, “mixotrophy”.  The pattern shown in Fig.1 is thus far more complex in reality (Fig.2, Fig.3).

Fig.3 The form of mixotroph protist can be determined by following this key. Modified from Mitra et al. 2016.
Fig.4 Differences between the traditional paradigm and the mixoplankton paradigm. In the former, organic molecules leaking from protist phytoplankton support the growth of bacteria which then compete with the phytoplankton for common nutrients. In the mixoplankton paradigm the bacteria are farmed (eaten) by the protist so giving the protist access to nutrients that bacteria alone are capable of taking up from solution. Adapted from Mitra et al. 2014.

Mixotrophy supports the growth of organisms important for marine food chains and also for the biogeochemical cycles removing atmospheric CO2. These organisms are also causative agents of harmful algal blooms. The way that they function can radically alter the way that we view flows of energy and nutrients between organisms. Thus, in Fig.4, we see that in the traditional paradigm bacteria compete with protist phytoplankton for common nutrients, while in the new paradigm, mixotrophic protists “farm” bacteria for the nutrients that they contain. In consequence of this revised paradigm, laboratory and field research approaches, management policies and allied computer modelling tools, are arguably no longer fit for purpose.  Certainly their veracity needs to be confirmed.

The MSCA funded ITN-ETN MixITiN is training an innovative team of Early Stage Researchers (ESRs) to develop new methodologies for researching, monitoring and managing our marine environment according to the recently revised paradigm for marine pelagic production.

MixITiN brings together world-class European research and training centres from 9 different countries, with skillsets from molecular biology, physiology and computer modelling, to marine and coastal zone management, public and media engagement. MixITiN ESRs are thus being trained at multinational and multidisciplinary centres of excellence, with intersectoral engagement to best equip them for employment in various sectors such as consultancy, government, academic, or allied industries. The synergy of training approaches being provided to the ESRs raises their capabilities and also progresses science beyond the current state-of-the-art to enable improved management of marine resources.