Like an onion
To their surprise, they discovered that the concentration of fucoxanthin increases much more sharply near the phytoplankton cell than expected. This indicates that the phycosphere consists of multiple layers. These layers are wrapped like onion skin around the microalgae at the centre, creating progressively larger spherical shells. The chemical properties of the substances released determine how far each individual layer extends.
The researchers concluded that there were significant differences between the substances: only water-soluble substances such as sugars and amino acids behave as expected from simple diffusion. Their concentration decreases very gradually as the distance from the microalgae increases.
This differs from water-repellent substances such as fucoxanthin, a key photosynthetic pigment in many microalgae. This substance remains highly concentrated close to the microalgae, with its levels dropping very sharply within roughly ten micrometres of the cell surface. How is this possible? Zachary Landry and Riccardo Foffi, the two lead authors of the study, found that a thin layer of mucus surrounding the microalgae may be responsible.
The dual function of mucus
According to the researchers, a mucus-like coating envelops the microalgae and affects the spread of certain substances. “It helps water-repellent substances disperse in the water,” Foffi explains. At the same time, the mucus holds back these molecules, preventing them from drifting away from the cell and resulting in the significantly higher concentrations near the cell surface.
“Much stronger chemical signals are created as a result,” adds Landry. “This makes it easier for the bacteria to locate tiny microalgal cells in the ocean.” This marks a fundamental shift in our understanding of the phycosphere. The researchers highlight that their findings have significant implications for models of the interactions between phytoplankton and bacteria. They also emphasise that thorough experimental research remains crucial, even as the field becomes more reliant on computer-based models.
The study reveals that the area immediately surrounding microalgae is far more intricate than previously thought. These new insights could improve our understanding of the interactions between microalgae and bacteria – and thus of a crucial aspect of the oceans’ carbon cycle.