Precious and fragile microalgae are under close observation
Like a doctor tending to a patient, Sarah Janin is by the side of diatoms, those precious and fragile microalgae that are constantly exposed to various environmental stresses. Since January 2025, she has been conducting research funded by the Ocean and Climate Priority Research Programme, with the aim of identifying the nature and intensity of these stresses and their impact on diatom communities.
by Carole Saout-Grit (for Connected Oceans) – 19/10/2026
Her love of the marine environment and her commitment to protecting living organisms undoubtedly stem from her origins. Born in New Caledonia, Sarah Janin quickly shifted her focus from human biology to marine biology. It was her initial interest in corals and how climate change impacts their health that led her to discover microalgae, which are the beginning of the food chain and are the subject of her close manufacturing.
diatom populations under severe pressure
Microalgae are found in all aquatic environments but are often invisible to the naked eye, except when they proliferate so extensively that they turn the sea water green or red. Marine biologists refer to these algal blooms as ‘blooms’. These micro-organisms play a major role in life on land and at sea: through photosynthesis, they use sunlight to convert water and carbon dioxide (CO₂) into organic matter and oxygen. Microalgae thus form the basis of the marine food chain and contribute to biogeochemical cycles on a global scale. The health of ecosystems – and our own – depends on their well-being.
Among microalgae, diatoms stand out for their unusual shape: their cells are surrounded by a silica wall called a frustule, which resembles glass. This group comprises a vast diversity of species, which colonise a wide range of habitats: oceans, lakes, rivers, sediments and even sea ice. It is estimated that diatoms alone account for around 20 per cent of global primary production (the organic matter produced through photosynthesis).
However, in the ocean, diatoms must constantly cope with fluctuations in numerous factors. Some of these are abiotic, such as light, water temperature or the availability of nutrients. Others are biotic and stem ly from interactions with other organisms, such as viral infections or allelopathy – that is, the release of chemical compounds by other species. Taken together, these factors constitute what scientists call the exposome, and variations in this are exacerbated by climate change and certain human activities. Understanding how diatom communities respond to these stresses, which fluctuate over time and space, remains a major challenge today. It is indeed difficult to assess, in a non-invasive manner, both the nature and intensity of the stresses, and the responses that the cells mount to cope with them.
photosynthesis and innovative techniques for measuring the impact of stress
Just as a doctor would take a patient’s pulse, Sarah uses photosynthesis as her main diagnostic tool for assessing the physiological state of cells, in order to characterise the response of diatoms to environmental changes.
As the primary source of energy for microalgae, photosynthesis is rapidly affected by abiotic and biotic stresses. It is therefore fairly easy to monitor its changes in vivo without altering it, particularly thanks to the fluorescence of chlorophyll-a, a pigment involved in the capture of solar energy by microalgal cells.
During photosynthesis, a chlorophyll molecule that has absorbed a photon enters an excited state. This energy can then be dissipated via three competing pathways: photochemistry (where the energy is used for photosynthesis and fuels the cell’s metabolism), thermal dissipation (excess energy is released as heat, which acts as a photoprotective mechanism against damage caused by excessively intense light) and fluorescence (a small fraction of the energy is re-emitted as light).
Measuring fluorescence therefore provides indirect information on the other two pathways: photosynthetic activity and photoprotection, which are closely linked to the cell’s physiological state. In addition to being non-invasive, fluorescence- e measurements are sensitive enough to be carried out on low-concentration field samples, as well as at the level of a single cell within a population.
Photophysiological measurements on diatoms in the laboratory and in mesocosms during a field mission.
multidisciplinary research for marine bioodiversity
To identify, through photosynthesis, the nature and intensity of the various stresses experienced by diatoms across multiple scales, Sarah conducts research in the laboratory, in the field and at the single-cell level. The idea is first to test different stress factors under controlled conditions in the laboratory, then to apply them under natural conditions in the field, and finally to explore the response at the single-cell level by studying the heterogeneity of responses within a population. With this in mind, Sarah is primarily interested in a biotic phenomenon known as allelopathy: certain dinoflagellates (and other single-celled microalgae) release chemical molecules that inhibit photosynthesis in diatoms.
In the laboratory, Sarah therefore begins by establishing ‘photosynthetic signatures’ by subjecting populations of diatoms to individual stressors (light, temperature, nutrient deficiency, etc.) in order to analyse how each stressor, taken separately, alters the mechanism of photosynthesis. Then, in the field, she tests these signatures under natural conditions on a diatom bloom that has been collected and placed in large, isolated and controlled bags of natural seawater. Finally, at the single-cell level, she uses these signatures to study cells trapped individually in microfluidic chips, which allows her to measure the fluorescence of each cell within the population. Ultimately, all these experiments should make it possible to better identify diatom populations that are potentially more resistant to stress, and then to pinpoint the mechanisms and molecular players that enable them to respond effectively to such stress.
Sarah is undertaking her PhD as part of the ‘Complexity of Life’ Doctoral School at Sorbonne University. In practice, she divides her time between three laboratories:
The Photobiology and Physiology of Plastids and Microalgae (P3M) laboratory at the Institute of Physico-Chemical Biology (IBPC Paris), which specialises in photosynthesis, where she cultivates her favourite diatoms and develops the methodology for fluorescence measurements; ;
The Interdisciplinary Laboratory for the Energies of Tomorrow (LIED), where microfluidic chips are designed to isolate microalgae and measure fluorescence at the single-cell level;
The Adaptation and Diversity in the Marine Environment Laboratory at the Roscoff Biological Station, which houses a large collection of microalgae (Roscoff Culture Collection, RCC) and contributes its expertise in marine ecology, based on long-term monitoring of local planktonic communities.
This research is co-supervised by Benjamin Bailleul, a CNRS research fellow at P3M; Catherine Villard, a CNRS research director at LIED; and Stephan Eberhard, a researcher at P3M and a senior lecturer at Sorbonne University. Combining microalgal biology, biophysics and hands-on work in the fabrication of microfluidic chips, Sarah is making a significant contribution to advancing scientific understanding of the impact of environmental stresses on the health of diatoms. After completing her PhD, she hopes to return to New Caledonia with these new skills to contribute to the development of her island and the preservation of its unique biodiversity.
three questions for sarah janin
Why did you decide to do a PhD in marine sciences?
I have always been concerned about environmental conservation and, having been born in New Caledonia, I have a particular attachment to the marine environment. During my degree in life sciences engineering – which was more focused on human biology – I realised that I could apply my scientific skills to the subject I was most passionate about: the sea. So I changed direction, starting with an internship on corals and the impact of climate change. That’s when I discovered the world of microalgae and their vital ecological role, and I’ve been hooked ever since!
What made you want to apply for this PhD topic? What were your motivations?
One thing that really appealed to me was the multidisciplinary nature of the topic: it combines microalgal biology, biophysics and hands-on work involving the fabrication of microfluidic chips. I enjoy applying different disciplines to the study of living organisms. The opportunities for mobility and fieldwork also really appealed to me: I wanted to give meaning to my work by linking it to the real-world environment in which these microalgae live. In fact, I initially worked on this project as an engineer. When my supervisors suggested I continue it as a PhD, I didn’t hesitate for very long: it was an opportunity to explore the subject in greater depth whilst continuing with the same team and the same laboratories – a truly ideal environment.
How do you envisage your future after completing this PhD?
I think I’ve found fulfilment in scientific research and, in the longer term, I’d like to return to New Caledonia to put my skills and energy towards the island’s development, and above all towards preserving its unique biodiversity. I’d also like to get more involved in raising public awareness of these issues, and of science in general. I’m convinced that we protect better what we understand…
Reference : Sarah Janin, « The response of phytoplankton in French oceans and coastal waters to their exposome: using photosynthesis as an indicator of stress », phD 2025-2028
Contact : janin@ibpc.fr
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