Colors as Warnings: Red Algae May Use Structural Color for Inter-Species Communication

markrenngont

Certain red algae display striking structural colors, with their growth tips appearing blue and other parts, including reproductive structures, showing a white hue. A recent study from Kobe University suggests this coloration may serve a communicative purpose between species, marking the first such indication in red algae.

Though red algae typically appear red due to their light-harvesting pigments, marine phycologist Kawai Hiroshi of Kobe University noticed variations during his dives. "I had observed that some red algae appeared much whiter than expected underwater, and more recently, I noticed that the tips of young shoots in one species had a bluish tint. This made me curious about the mechanism behind these colors," said Kawai.

Studying the phenomenon posed several challenges. The algae in question live below the tidal zone, requiring scuba diving for collection. Many deep-sea species are also delicate and difficult to maintain for laboratory observation. Additionally, examining their internal structure required specialized techniques using electron microscopy. Despite these obstacles, the research team succeeded by building on earlier work with brown algae.

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The results, published in the European Journal of Phycology, revealed that gland cells within the red alga Asparagopsis taxiformis contain tightly packed microspheres made of light-reflecting materials. In the growth tips, the microspheres are uniform in size, reflecting blue light. As the cells mature, the microspheres become more irregular, causing them to scatter light of all wavelengths and appear white.

Structural coloration in shallow-water algae has been linked to protecting photosynthetic pigments or enhancing photosynthesis, but little was known about the function of such colors in deep-water species. The Kobe researchers now propose that these colors may serve as warnings or camouflage.

"The same gland cells that produce these colors also contain substances likely used to repel herbivorous fish," said Kawai. "The blue coloration at the tips may serve as a warning, signaling the presence of a chemical defense. In contrast, the white regions near the fruiting structures could help mask the algae’s red pigmentation, making it less visible to predators that rely on sight."

These adaptations are especially relevant in tropical waters, where most algae-grazing fish reside due to the high temperatures required for digestion. The visibility in these regions may also increase the need for such visual defenses. With climate change pushing tropical fish into higher latitudes, red algae in cooler waters—unprotected by these coloration traits—could face new threats.

Kawai concluded, "As ocean temperatures rise and warm-water fish expand their range, algae that have not evolved such defenses may become increasingly vulnerable."

The study was conducted in collaboration with a researcher from Hokkaido University.