How Microalgae Reveal Ecosystem Health from France to the Sub-Antarctic
In the frozen, windswept landscapes of the sub-Antarctic islands, and in the freshwater streams of rural France, silent architects have been building glass palaces for millions of years. These architects are diatoms—microscopic, single-celled algae encased in intricate silica walls, whose beauty is only matched by their ecological significance. Though barely visible to the naked eye, these organisms produce 20-30% of the oxygen we breathe and form the base of aquatic food webs worldwide 1 .
Diatoms are responsible for approximately 20-30% of all oxygen production on Earth, making them as important as tropical rainforests for our planet's atmosphere.
The recent publication of "Diatom taxonomy and ecology. From France to the sub-Antarctic islands" celebrates the work and life of Prof. Dr. René Le Cohu on the occasion of his 80th birthday. Edited by Bart Van de Vijver, Loïc Tudesque, and Luc Ector, this volume honors a remarkable scientist whose work transcended the boundaries of taxonomy and ecology, connecting freshwater systems from his native Brittany to the remote islands of the Southern Ocean 2 .
This special volume, published as a tribute to Prof. Le Cohu, represents what scholars call a Festschrift—a collection of scientific papers honoring a respected academic. The book emerged from the deep appreciation of Le Cohu's colleagues and students for his decades of contributions to diatom research and education 2 .
Born in Groix, Brittany, in 1936, Le Cohu began his academic career at the University of Rennes in 1959, later moving to Paul Sabatier University in Toulouse, where he continued working as a Professor Emeritus for 20 years after his formal retirement 2 .
The volume reflects Le Cohu's interdisciplinary approach to phycology (the study of algae), bringing together research on diatom taxonomy, ecology, and biogeography. True to Le Cohu's own scientific journey, the contributions span from local discoveries in European water bodies to global patterns of diatom distribution in some of the planet's most remote ecosystems 2 .
What makes this volume particularly significant is its celebration of a scientist who balanced taxonomic precision with ecological application 2 .
Scientific Articles
Taxa Described
Species Named in His Honor
Diatoms belong to a large group of organisms called heterokonts, which include both autotrophs (like golden algae and kelp) and heterotrophs (like water moulds) 3 . These microscopic algae are unique for their silica-based cell walls, called frustules, which persist in sediments long after the organisms die. These intricate frustules often resemble beautifully crafted glass boxes with detailed patterns unique to each species.
Diatoms are classified based on their symmetry into two main groups: centric diatoms (radially symmetrical) and pennate diatoms (bilaterally symmetrical) 3 . Among the pennate diatoms, some possess a specialized structure called a raphe—a slit-like opening through which the diatom can move, while others lack this feature 3 .
| Classification Level | Major Groups | Key Characteristics |
|---|---|---|
| Overall Division | Bacillariophyta | All diatoms; silica cell walls |
| Main Classes | Coscinodiscophyceae | Centric diatoms; radial symmetry |
| Fragilariophyceae | Pennate diatoms without raphe | |
| Bacillariophyceae | Pennate diatoms with raphe | |
| Habitat Types | Planktonic | Free-floating in water column |
| Benthic | Living on bottom surfaces | |
| Epiphytic | Growing on other plants/algae |
The sub-Antarctic islands, including the Kerguelen Islands, Crozet Archipelago, and Prince Edward Islands, represent some of the most remote and pristine ecosystems on Earth. These islands function as natural laboratories where scientists can study ecological patterns and processes with minimal human disturbance. Prof. Le Cohu recognized this potential early in his career, conducting extensive research on the freshwater diatoms of the Kerguelen Islands 2 5 .
Research from these regions reveals fascinating patterns. For instance, a study on Ile de la Possession (Crozet Archipelago) identified 210 diatom taxa belonging to 32 genera, with the most abundant genera being Fragilaria, Achnanthes, and Navicula 5 . Similarly, the Prince Edward Islands host at least 214 diatom taxa from 60 genera .
In these remote ecosystems, diatom communities are primarily shaped by pH levels and chloride concentration. Statistical analyses of samples from the Crozet Archipelago revealed that these two factors divided the sampling sites into three categories: "rivers, more acid lakes, and lakes with higher chloride content" 5 .
The diatom flora of these sub-Antarctic islands shows remarkable similarity across the southern Indian Ocean, forming what scientists recognize as a distinct biogeographical entity separate from other parts of the Antarctic region .
| Environmental Factor | Effect on Diatom Communities | Example Habitats |
|---|---|---|
| pH | Divides acid-tolerant from circumneutral species | Acid lakes vs. circumneutral lakes |
| Chloride Content | Separates freshwater from brackish communities | Coastal pools vs. inland waters |
| Habitat Type | Differentiates flowing from standing water | Rivers vs. lakes |
| Water Hardness | Influences mineral-dependent species | Mineral-rich vs. soft water systems |
| Moisture Availability | Distinguishes aquatic from terrestrial taxa | Moss habitats vs. open water |
While early diatom ecology relied heavily on observational studies, Prof. Le Cohu understood the importance of integrating multiple approaches. Modern diatom research combines field observations with controlled laboratory experiments to establish causal relationships between environmental factors and diatom growth.
One pioneering approach, outlined by Cox (1993), advocates for developing "an experimental approach as an aid to interpreting field data" 4 . This methodology allows researchers to investigate the effects of specific variables under controlled conditions, testing predictions generated from field observations and improving understanding of the constraints on growth of particular species.
In a compelling example of this experimental approach, researchers studied four benthic diatom species—Synedra ulna, Meridion circulare, Pinnularia viridis, and Eunotia exigua—under contrasting light, temperature, and pH regimes 4 . The results revealed ecological insights that field observations alone could not provide.
| Species | Light Response | Temperature Sensitivity | pH Tolerance |
|---|---|---|---|
| Synedra ulna | Positive response | Positive response; wide tolerance | Not specified |
| Meridion circulare | More sensitive to temperature at high light | High sensitivity at elevated temperatures | Not specified |
| Pinnularia viridis | Tolerant of varied conditions | Wide tolerance range | Wider than field data suggested |
| Eunotia exigua | Tolerant of varied conditions | Wide tolerance range | Wider than field data suggested |
| Tool/Method | Primary Function | Application in Diatom Research |
|---|---|---|
| Light Microscopy | Initial examination and counting | Routine identification and enumeration |
| Scanning Electron Microscopy (SEM) | Detailed ultrastructural analysis | Species description and taxonomy |
| Molecular Genetics | DNA sequencing for phylogenetic studies | Resolving evolutionary relationships |
| Culture Experiments | Isolating environmental variables | Testing physiological responses |
| Multivariate Statistics | Analyzing complex environmental data | Identifying species-environment relationships |
| Diatom Indices | Quantifying ecological status | Water quality assessment |
Prof. Le Cohu's impact on diatom research extends far beyond his publications. As one of the founding members of the Association of French-speaking Diatomists (ADLaF), he helped build a collaborative community of researchers dedicated to advancing the field 2 . He served in various leadership positions within this organization from 1980 to 2001, helping to shape the direction of diatom research in Francophone countries.
Perhaps one of the most touching measures of Le Cohu's impact is the number of diatom species that have been named in his honor. To date, 16 species of diatoms have been dedicated to him, including the genus Lecohuia described by his colleague and friend Prof. Dr. Horst Lange-Bertalot 2 .
Born in Groix, Brittany
Began academic career at University of Rennes
Leadership roles in Association of French-speaking Diatomists
Professor Emeritus at Paul Sabatier University, Toulouse
Continued research well into his 80s
Those who worked with Le Cohu remember not only his scientific acumen but also his personal qualities. He was described as having "a sharp intellect combined with a strong sense of humor and clever puns that were so uniquely his own" 2 .
Beyond the laboratory, he was an accomplished athlete in his youth, having been "champion of the 400-meter hurdles at the International Junior and was a football player at the highest amateur level for 3 years" 2 .
The tribute volume honoring Prof. Dr. René Le Cohu represents more than just a collection of scientific papers—it symbolizes the continuing vitality of diatom research and its critical importance in understanding and protecting our planet's aquatic ecosystems. From the remote sub-Antarctic islands to local European waterways, diatoms continue to serve as sensitive indicators of environmental change, their silent glass houses recording ecological history in exquisite detail.
As we face increasing environmental challenges, from climate change to biodiversity loss, the integrated approach championed by Le Cohu—combining meticulous taxonomy with ecological experimentation and observation—becomes increasingly valuable. These microscopic organisms, which he studied with such passion throughout his long career, have stories to tell us about the health of our planet, if we only learn how to listen.
The legacy of researchers like Prof. Le Cohu ensures that we continue to develop the tools, knowledge, and scientific community needed to understand these stories, protecting the invisible diversity that sustains visible life on Earth.