Some red algae exhibit structural color that gives their growth tips a blue hue and the rest of their bodies including their fruiting structures a white hue. Moreover, since the color-producing structures are located together with anti-herbivory chemicals, the Kobe University discovery is the first to suggest that red algae use colors for inter-species communication.
Red algae are red due to the pigments they use to collect light. Kobe University phycologist KAWAI Hiroshi says: "Being a diver, I have long been aware that some red algae have a much whiter appearance than their usual red when observed in water. Also, in a recent diving survey I noticed that the tips of young shoots in one of the species seemed to have a blueish hue. These observations made me curious, and I wanted to clarify the mechanism of these colors."
Kawai describes the difficulties he faced while trying to investigate this phenomenon: "These species grow deep underwater below the tidal zone, which requires scuba diving for their observation and collection. In addition, many deep-water species are very fragile, and it is difficult to conduct detailed observations and experiments in the laboratory while they are still alive. Also, it requires special techniques to enable looking at the detailed structures with an electron microscope." However, building on their previous experiences with similar studies in brown algae, the Kobe University researcher managed to retrieve enough samples.
In the European Journal of Phycology, the team now publishes its findings. They discovered that a certain kind of cells of the red algae, called the "gland cells," contain bodies of tightly packed microspheres of a light-reflecting material. In the growth tips of the red alga Asparagopsis taxiformis, these microspheres are all of the same size, which allows light of a single, blue color to be reflected. As the gland cells mature, the microspheres lose their uniform size, which causes light of all colors to be reflected, resulting in a whitish hue. "The structural color of red and brown algae living in shallow water has been explained to protect the photosynthetic pigments or to optimize photosynthesis. However, little is known about the structural color seen in deep water species," explains Kawai.
What functions these colors serve may be related to where they are produced. Kawai says: "The inside of the structure that causes the coloration contains a substance that is highly reactive and may act as a feeding repellent to algae-eating fish. Such a repellant is more effective when combined with a warning color such as the blue hue at the algae's growth tips. And where these structures are white, around where the organism's fruiting structures are located, they may act as a camouflage of their original red color against grazers using sight to find their food."
The vast majority of algae-grazing fish occur in tropical waters, because of the higher temperatures needed to digest the food. This, together with the higher visibility there, may explain why structural coloration in red algae in deep water is more common to these regions. But this also has an implication for global warming. Kawai explains, "As warm-water fish migrate further north, they might pose a significant threat to the algae there that are not protected by such coloration."
This research was conducted in collaboration with a researcher from Hokkaido University.
Kobe University is a national university with roots dating back to the Kobe Higher Commercial School founded in 1902. It is now one of Japan's leading comprehensive research universities with nearly 16,000 students and nearly 1,700 faculty in 10 faculties and schools and 15 graduate schools. Combining the social and natural sciences to cultivate leaders with an interdisciplinary perspective, Kobe University creates knowledge and fosters innovation to address society's challenges.
