Microalgae are the invisible engines of life: They sequester a significant portion of the planet's carbon dioxide through photosynthesis and form the basis of aquatic food chains. But when it gets too hot or too cold, this engine begins to sputter. These effects have been extensively studied in the green alga Chlamydomonas reinhardtii. This microalga serves as a model organism: because it is particularly easy to culture and genetically accessible, researchers use it to investigate more general biological principles of microalgae. The green alga's responses to particularly high or low temperatures have therefore already been studied on multiple occasions. Until now, however, there has been a lack of focus on smaller temperature fluctuations in water and soil — the habitat of C. reinhardtii — outside the range of extreme temperatures. In the context of climate change, however, temperature shifts within the relevant range of 18 to 33 degrees Celsius are increasing. A study by researchers from the Cluster of Excellence "Balance of the Microverse" now addresses this gap for the first time.
Even moderate changes in temperature can have a major impact
Even moderate temperature shifts lead to far-reaching changes in the gene activity and behavior of C. reinhardtii. Four research groups within the cluster, coordinated by Prof. Dr. Maria Mittag, have shown that about one-third of all protein-coding genes in the alga respond to temperature changes. These include genes from nearly all functional areas of the cell, ranging from photosynthesis and metabolism to locomotion and interaction with bacteria.
The consequences: If the temperature rises from 23 to 28 degrees Celsius, the algal population reaches a cell density that is twenty percent higher. At the same time, the cilia—the thread-like locomotive organs of the microalgae—shrink.
"Microalgae may be largely invisible to the naked eye, but they are closely attuned to their environment. We were surprised to find that the algae's swimming behavior adapts as early as 15 minutes after a temperature change," explains Dr. Prateek Shetty, the study's first author. "Even before their cell structure is remodeled, they reduce their speed and change direction more frequently."
But the microalgae's metabolism also changes: As temperatures rise, C. reinhardtii initially draws on organic carbon sources, thereby delaying the onset of photosynthesis by several days. Reproduction and interaction with other microorganisms are also affected by temperature changes.
Interdisciplinary Collaboration as the Key
A multi-level strategy formed the methodological core of the study. Four research groups within the Microverse Cluster contributed findings based on their respective areas of expertise. Through the collaboration of the research groups led by Prof. Dr. Maria Mittag, Dr. Markus Lakemeyer, Prof. Dr. Rosalind Allen, and Prof. Dr. Miriam Rosenbaum, a coherent picture emerged of the algae's gene activity, protein composition, motility, and photosynthetic performance.
"It was previously unclear that even gradual temperature shifts profoundly alter the behavior of algal cells. We owe this insight to the close and productive collaboration within the cluster: Only the interaction of several groups with different methodological strengths made this picture visible," emphasizes Prof. Dr. Maria Mittag, research group leader in the Cluster of Excellence and coordinator of the study.
Small Fluctuations, Big Waves
Microalgae such as Chlamydomonas reinhardtii are at the base of many aquatic food chains and contribute significantly to carbon dioxide sequestration and oxygen production. The algae's altered responses even to slight temperature shifts therefore suggest far-reaching implications in their associated ecosystems. The observed delay in photosynthesis could be particularly relevant. If this effect is confirmed under natural conditions, it could influence CO₂ sequestration and oxygen production in warming soils and water bodies.
"To fundamentally understand the effects of global phenomena such as climate change, we must start with the smallest actors in our complex ecosystems. This study excellently demonstrates how the combined research perspectives of our Cluster of Excellence make this invisible interplay visible," emphasizes Prof. Dr. Kirsten Küsel, spokesperson for the Cluster of Excellence "Balance of the Microverse."
