Destination Unknown – my journey through the plankton world

Aditee Mitra writes about her research which started with protozooplankton and led her to the mixoplankton paradigm

January 2021

During my PhD years when I was modelling the, then rather neglected, protozooplankton community little did I know what was waiting behind the curtains for me. It was while trawling the literature for data for ciliates, that I first came across the works of Diane Stoecker and the enigmatic mixotrophic protists. A single celled organism which could hunt like animals and photosynthesize like plants in the one cell – to me this sounded like the perfect work-life balance in protist life-forms.

As I moved towards graduating with a PhD on Zooplankton Growth Dynamics from SAMSOU, I started planning and plotting. Aim – get funding to work with Diane, to learn to grow and experiment with mixotrophs and build a model to represent these perfect creatures of nature. Unfortunately, all my attempts to get funding for mixotroph work were met with a resounding “curious creatures but not really important” from the funders. As my microzooplankton funding came to an end, I had to be pragmatic (= £s for rent and food), I left science and went to work for the local government. However, over the next two years, I kept looking for opportunities to return to science.

In 2008 I discovered the Leverhulme International Network Grant – funding for folks to talk over ideas which no mainstream research funding body were willing to support. So we applied and got funded to talk about “Placing Marine Mixotrophs in Context”. We started with the Perfect Beast model based upon Diane’s conceptual models of mixotrophy.

After 4 meetings involving 30+ mixotroph protistologists we had gathered enough information to propose a revised functional classification of marine protists where mixotrophic protist plankton were just as important as the purists – phytoplankton and protozooplankton. We explored ecophysiology, we looked at the impact mixotrophic protists have on trophic dynamics, system stability and biological carbon pump.

Next on my wish list was to map the biogeography of the different mixoplankton functional groups on the Longhurst provinces – quite an undertaking. However, while I was back in research, my funding was still focused on zooplankton. Plus, as a new lecturer in a department dominated by “worshippers” of enigmatic megafauna, I had teaching modules to develop and, as the motto of my then employer was “everyone can teach everything”, my teaching modules were an eclectic mix of subjects from higher plant physiology to rudimentary taxonomy – so my hands were rather full.

At this time, I had a wonderful Diwali present – the festival of lights brought a bright star into my office all the way from Brazil – Suzana Leles came to Wales. Suzana was enthusiastic, ready to take on challenges. And, importantly she came with a background in biological oceanography, so the mapping plan seemed to be a good starting point.

Indeed, Suzana and the biogeography work were clearly meant for each other as she produced map after map of NCMs and CMs across Longhurst provinces.

Finally in 2017, after a decade since my first application for funding to work on mixotrophic protists, I got my first grant to work on these fascinating organisms as Project Coordinator! And, what a coup that was … €2.88 M funding to work with fellow mixotroph enthusiasts across Europe and employing 11 early stage researchers (ESRs) in the H2020-MSCA-MixITiN project.

At this time we also started discussing the meaning of the term mixotrophy in the context of plankton physiology and especially for protists which engage in phototrophy + phagotrophy. The problem was that all “phytoplankton” can be labelled as mixotrophs because they engage in phototrophy + heterotrophy through uptake of sugars and amino acids. But that was not what we meant by “mixotroph”! This led us to return to discussions as to whether such organisms should have their own identity like the virioplankton, bacterioplankton, phytoplankton, and, protozooplankton.

And, so the term “mixoplankton” – protist plankton capable of photoautotrophy and phagoheterotrophy in the one cell – came into being.

But all good things must come to an end. MixITiN is in its final year with MixITiN ESR contracts starting to end February 2021 onwards. Our original plan was to have a grand party celebrating all things mixoplanktonic. Unfortunately, the uninvited non-mixoplanktonic coronavirus took exception to this and like the bad fairy in Sleeping Beauty tried to put an end to our party plans. But we will not be stopped. If we cannot meet in the hallowed portals of Cardiff University, we will meet in silico on Zoom.

For two days this month, on 19 and 20 January, we will celebrate the ubiquitous enigmatic mixoplankton at the first Mixoplankton International Conference. And, what a party it will be. While we put together the book of abstracts, here are some teasers…

We have George McManus talking about green ciliates. MixITiN’s Maira Maselli will be introducing us to a new mixoplanktonic ciliate species from the Northern Hemisphere while Michaela Larsson and Martina Doblin will introduce us to a new mixoplankton-on-the-block from Australian waters. We also have Fernando Unrein who has explored 100s of published papers to bring together knowledge and understanding of mixoplanktonic phytoflagellates.

Dave Caron will be telling us all about my very favourite oceanic greenhouses – the fascinating and beautiful endosymbiotic mixoplanktonic rhizarians. These organisms have played and continue to play a vital role in planetary processes.

We have the redoubtable Beatriz Reguera talking about the importance of mixo-HABs bringing together field studies and policy.

And, of course we will have various modelling talks with Claudia introducing the first ever food-web with the Perfect Beast and an age-staged copepod model. Suzana will be discussing whether all the efforts of modelling Nature’s Perfect Beast is really worth the pain. Also, I shall be unveiling our Mixoplankton Magnum Opus which has been in the making for over nearly two years now!

And, while MixITiN comes to an end, I am embarking on a new journey with “MixoHUB“. But that is a story for another day!