By Kevin J Flynn
My research career on plankton ecophysiology has involved both empirical and modelling activities, on the basis that if we cannot model it adequately then we do not understand the subject adequately either. And of course, simulation models provide us with the only route to ‘see into the future’.
My involvement with iron (Fe) and plankton ecology stems from an interest in ammonium-nitrate-photosynthesis interactions in microalgal growth. We (Flynn & Hipkin 1999) built the first cybernetic description of these interactions, a model that was subsequently used to demonstrate that areas such as the mid N.Atlantic were close to being Fe-limited (Fasham et al. 2006). Around the same time, modelling work by others indicated that artificial Fe-fertilization of High Nitrate Low Chlorophyll (HNLC) areas would be unlikely to serve a useful purpose. Since then, however, significant developments in plankton research support an argument for a revisitation of the whole subject.
Plankton are not what we thought: why mixoplankton matter
Our understanding of the plankton food webs that supports all else in ocean ecology, traditionally cast as ‘phytoplankton-zooplankton’, has been upended through the mixoplankton paradigm that recognises that many ‘phytoplankton’ (including exemplar species – Mitra et al. 2023) perform both photosynthesis and phagotrophy. The consumption of bacteria (which are supremely adept at recovering nutrients from extremely low concentrations) provides mixoplanktonic microalgae with a mechanism to acquire micronutrients such as Fe and P (Mitra et al. 2014). This has the potential to retain Fe for longer in the photic zone, depending on the composition of the microalgal plankton communities. And we know that under climate change those communities are changing spatially and seasonally. The ramifications of these changes will be widespread through biogeochemistry and food webs. However, we understand little about them because we are not conducting the necessary experiments to determine rate processes and other data types required to build and test the required models.
Climate change has outpaced our ability to model plankton ecosystems
This, then, brings us to another development; the realisation that our plankton models are really not at all appropriate for the tasks we expect of them (Flynn et al. 2025). We need models that properly describe ecophysiology and food web dynamics; we need those for exploring many facets of marine research, and we certainly need them for the topic of Fe limitation and Fe-fertilization. Indeed, a combination of all the above raises profound questions over the conduct of earlier empirical and modelling studies on Fe and plankton, especially given that the world has literally changed over the 20-30 years since the last comprehensive studies were made of the topic.
From scepticism to support: reframing the problem
So, why do I support the work of Oceanry? I admit that I re-entered this arena as a strong sceptic. However, in addition to the foregoing topics, a major, arguably the major, new viewpoint concerns the term ‘Fe-fertilization’. Human activity, even setting aside that of anthropogenic influences on climate change, has long altered the chemistry of the ocean. This is most apparent in near-shore waters through eutrophication, but it is also apparent through changing patterns of weather altering inflows of nutrients from the land, including dust containing Fe, and even the removal of Fe by fishing.
‘Fe-replenishment’, the revitalization of the Fe-status of the ocean (or more specifically, those areas that are Fe-limited) is surely a worthy target of integrated research and, as/if appropriate, active amelioration.
About the author
Kevin J Flynn is a plankton ecophysiologist and ecologist, developing and exploiting cybernetic descriptions of these globally important systems to enhance research understanding and education. Flynn is one of Oceanry’s Advisors.
References
Fasham, M. J. R., Flynn, K. J., Pondaven, P., Anderson, T. R., & Boyd, P. W. (2006). Development of a robust marine ecosystem model to predict the role of iron in biogeochemical cycles: A comparison of results for iron-replete and iron-limited areas, and the SOIREE iron-enrichment experiment. Deep Sea Research Part I: Oceanographic Research Papers, 53(2), 333-366.
Flynn, K. J., & Hipkin, C. R. (1999). Interactions between iron, light, ammonium, and nitrate: insights from the construction of a dynamic model of algal physiology. Journal of Phycology, 35(6), 1171-1190.
Flynn, K. J., Atkinson, A., Beardall, J., Berges, J. A., Boersma, M., Brunet, C., … & Våge, S. (2025). More realistic plankton simulation models will improve projections of ocean ecosystem responses to global change. Nature Ecology & Evolution, 1-9.
Mitra, Aditee, Kevin J. Flynn, Joann M. Burkholder, Terrje Berge, Albert Calbet, John A. Raven, Edna Granéli et al. “The role of mixotrophic protists in the biological carbon pump.” Biogeosciences 11, no. 4 (2014): 995-1005.
Mitra, A., Caron, D. A., Faure, E., Flynn, K. J., Leles, S. G., Hansen, P. J., … & Tillmann, U. (2023). The Mixoplankton Database (MDB): Diversity of photo‐phago‐trophic plankton in form, function, and distribution across the global ocean. Journal of Eukaryotic Microbiology, 70(4), e12972.
