MixITiN

The conceptual basis upon which management tools for our ocean, seas and coasts operate are out-of-date and do not adequately address current challenges. We spoke to Dr Aditee Mitra and Dr Xabier Irigoien about implications for ocean health, policies, aquaculture and fisheries under climate change in this UN Ocean Decade.

The way marine systems have historically been thought to function closely parallels that of land-based systems, where plants produce food and animals consume it. Thus, in marine systems, the traditional view is that phytoplankton (microalgae) produce food which zooplankton consume, which in turn, are consumed by larger animals through to fish. “Actually, the situation is quite different,” says Dr Mitra, project coordinator of MixITiN. “What have traditionally been labelled as primary producers are also often consumers, and what have been labelled as consumers are also primary producers – life is complicated!” These organisms that combine plant-like and animal-like characteristics in the one cell are termed mixoplankton. “In essence, over decades of research, mixoplankton have been mislabelled and misunderstood,” she explains. This forms the backdrop to the work of the MixITiN project.

The MixITiN project focused on the development and deployment of new research methods for application in ocean sciences to establish a better picture of how mixoplankton contribute to marine ecology. This interdisciplinary project encompassed diverse approaches, including laboratory and field work drawing on a range of disciplines, from molecular biology right through to coarse grain systems biology techniques. The project’s wider goal – an improved understanding of indicators impacting ocean health, has major implications for the design of ocean ocean management policies and planning for a sustainable future.

Mixoplankton are not new discoveries as such, rather until recently they were just not recognised as a major component of the plankton community. “Indeed, we found that most of what are traditionally thought to be phytoplankton, and around 50% of protozooplankton, are actually mixoplankton,” says Dr Mitra. Well-known phytoplankton that are now known to be mixoplankton include: the chalk-producing coccolithophore Emiliania huxleyi which, like the minute ocean-dwelling phytoflagellates, can eat bacteria to obtain vital nutrients; toxin-producing Alexandrium and Dinophysis, whose blooms (ironically termed Harmful Algal Blooms, HABs) result in shellfish contamination and aquaculture closures; Karlodinium and raphidophytes that cause mass mortalities of farmed and wild fish.

Mixoplankton are a diverse group which can be broadly divided into two types: constitutive mixoplankton (CM) – these have their own chloroplasts and can also eat (‘plants that eat’); non-constitutive mixoplankton (NCM) – these are like bodysnatchers, acquiring phototrophy through ‘stealing’ chloroplasts from their prey or capturing and maintaining their prey as symbionts (‘animals that photosynthesize’). Mixoplankton are an enigmatic group; for example, the CM group include the fish-killing Prymnesium on one hand and the fisheries supporting Tripos (Ceratium) on the other. Likewise, the NCM group include the fishsupporting Strombidium and also the bane of shellfisheries, the toxic Dinophysis.

The wider backdrop to this is concern that with climate change we will see changes in biodiversity as well as on food security and sustainability, and Dr Mitra says plankton are fundamental in this respect. “Microbial plankton drive life in the ocean, so it is essential that we get the basics right,” she stresses. This need has been the driver of the MixITiN project.

The UN has designated 2021-30 as the Decade of Ocean Science for Sustainable Development, an initiative designed to support ocean research and facilitate communication with stakeholders, including policy makers, ecosystem managers and the fisheries industry. The Ocean Decade has its roots in a wider recognition of the importance of the ocean systems and the ecosystem services they provide, along with a commitment to reversing the long-term decline in their overall health and supporting the UN’s ‘Sustainable Development Agenda’. “Issues like eutrophication, climate change and the occurrence of HABs all affect the health of the oceans. Critically, the mixoplankton paradigm strikes at the heart of this effort, by bringing into question core scientific assumptions,” explains Dr Irigoien, Scientific Director of AZTI.

Mixoplankton combine multiple trophic strategies for acquiring nutrition and energy. However, traditional climate change models are focussed on single strategies – photosynthesis versus predation in different organisms. This means that the conceptual basis of marine systems within climate change simulations are at best a gross simplification and at worst fundamentally flawed. Coarse grain systems biology modelling undertaken within MixITiN has shown that the exclusion of mixoplankton could potentially have serious consequences for future predictions.

Integration of facets of mixoplankton science in MixITiN. Working clockwise from 12 o’clock these include field studies and monitoring, determination of rate processes, taxonomic and genomic analyses, chemical analyses leading to stoichiometric ecology, trophic dynamics, ecosystem services and sustainability/profitability. The centre denotes the focussing of all these activities through modelling for hypothesis testing and predictive purposes. © Aditee Mitra

The shift in perception of the food web under the mixoplankton paradigm dramatically changes how marine systems should be viewed and studied. That we got our understanding so fundamentally wrong should make us reexamine the whole subject of plankton ecology.

The mixoplankton paradigm brings into question our basic understanding of the energy fluxes in the ocean, from carbon fixation to fish production.

This in turn affects our understanding of how energy moves in the marine trophic chain, from microbes to fish. These issues fall within the aegis of ocean management including policies such as the EU Marine Strategy Framework Directive, Dr Irigoien points out “mixotrophy does not change fisheries or water quality operational management, as we act on indicators. But it substantially changes our understanding about the carrying capacity of the ecosystem and its response to anthropogenic impacts. We need to revise those indicators, as the ecosystem tipping points might be different than what we thought.”

MixITiN has highlighted the importance of revising the indicators of the health of marine ecosystems in line with the mixoplankton paradigm. This is especially important for predictions of algal blooms and their impacts on aquaculture. “Most HAB species are mixoplanktonic, what controls their growth is not what we thought it was – it is not just light and plant-food. Competitors and even grazers are actually potential food,” explains Dr Mitra. And, new types of mixoplankton blooms are appearing, such as green Noctiluca which are growing across coastal oceans with climate change. In the Arabian Sea, these blooms are leading to the collapse of the traditional phytoplankton-zooplankton-fisheries link in the food web with severe food security and socio-economic hardships to a population of over 140 million people. Other mixoplankton affect recreational activities and the property market - discolouration of water caused by Karlodinium blooms have been known to result in a decrease in prices of highly soughtafter waterside properties.

“There is also a potential interaction with aquaculture, as fish farms release both the nutrients needed for photosynthesis, but also the organic matter used by mixoplankton. Algal blooms are a major issue for aquaculture all around the globe and including mixoplanktonic activity helps us to understand what controls such blooms. Indeed, the mixoplankton paradigm helps plug gaps in our understanding of what controls many algal blooms,” emphasizes Dr Irigoien.

Current monitoring methods need to reflect the complexity of the reality we now better understand. Dr Irigoien points out that “most routine field phytoplankton sampling techniques currently used in ocean monitoring, based on chlorophyll, are not well adapted to provide quantitative data on mixoplankton”. For example, the presence of chlorophyll - used as an indicator of phytoplankton biomass in surveys and ecosystem monitoring - is actually not just an indicator of the presence of phytoplankton. “It may also indicate the presence of mixoplankton, which are not just primary producers, but also consumers, and include harmful species,” explains Dr Mitra. It is important that plankton monitoring programmes take into account the mixoplankton communities; their proliferation is not driven solely by light and inorganic nutrients as is that of phytoplankton. Therefore, they have a much wider and diverse impact on marine trophic dynamics.

One of the wider aims of MixITiN has been to raise awareness, enhance ocean literacy, educate and train; this means not just the 11 researchers employed on the project, but also school pupils and the wider public. Mixoplankton are key drivers of life in the ocean, and the MixITiN researchers have been keen to raise the profile of these enigmatic organisms. Without mixoplankton, life on Earth would not function as it does.

The importance of mixoplankton is not currently recognised in educational books, environmental management frameworks nor in the simulation models used to guide climate change policies, issues that Dr Mitra and her colleagues in the project are keen to address. “With this in mind, we have produced various open access manuals and reports to aid understanding and development of new protocols,” says Dr Mitra. These include a fieldwork guide, a functional group classification guide to establish whether a HAB species is mixoplanktonic or not, a manual for isolation and establishment of cultures and a simple mixoplankton food web model for teaching. These materials and other information can be accessed via www.mixotroph.org.

Mixoplankton are important organisms helping to maintain planetary homeostasis and therefore could play a fundamental role under climate change. “We have looked at their biogeography, and we have seen that mixoplankton are absolutely everywhere, but the question of which type occurs where and how it relates to factors like seasonality, warming waters, and changing availability of nutrients, remains unclear,” says Dr Mitra. The project MixITiN has come to the end of its journey (Oct 2017-Sept 2021). However, there is still much to learn about mixoplankton.

Dr Mitra and colleagues are currently working to build a comprehensive mixoplankton database to establish a clearer picture of which organisms are mixoplanktonic, and which species eats what. Various planktonic species have not been documented as mixoplanktonic, because no one has been looking for this characteristic routinely. Now is the time to look specifically for them. “Once again on our marine planet, we need to re-start from the basic questions, how many, where, when,” says Dr Irigoien.

The overarching aim of MixITiN has been to explore the underlying principles of marine ecology under the new mixoplankton paradigm, a concept that reimages over a 100 years of science. The project has developed a suite of novel techniques for investigation and monitoring of our ocean under this paradigm.

The MixITiN project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 766327.

Dr Aditee Mitra School of Earth and Environmental Sciences Cardiff University E: MitraA2@cardiff.ac.uk W: www.mixotroph.org

Mitra & Flynn (2021) HABs and the Mixoplankton Paradigm. UNESCO Harmful Algae News no. 67 https:// zenodo.org/record/5109703 Mitra et al. (2021) Novel Approaches for investigating marine planktonic mixotrophy. MixITiN Report 3.8 http://doi.org/10.5281/Zenodo.5148500 Leles et al (2021) Differences in physiology explain succession of mixoplankton functional types and affect carbon fluxes in temperate seas. Prog Oceanogr 190:102481 https://doi.org/10.1016/j.pocean.2020.102481 Flynn et al. (2019) Mixotrophic protists and a new paradigm for marine ecology. J Plankton Res 41:375 https://doi.org/10.1093/plankt/fbz026

Dr Aditee Mitra

Dr Xabier Irigoien

Dr Aditee Mitra is a Research Fellow in Cardiff University, UK. Her research focuses on plankton life in the single largest ecosystem of Earth, the ocean. She has been a key driver of the mixoplankton paradigm in marine ecology. She is a keen advocate of ocean literacy. Dr Xabier Irigoien is Scientific Director at AZTI, a scientific and technological centre based in northern Spain. He is also an IKERBASQUE Research Professor. His main research interests lie in biological oceanography, plankton ecology and the physics of plankton-fish interactions.