Effect of light intensity on photophysiology and growth dynamics of crustose coralline algae (CCA): implications for the loss of canopy-forming algae

The disappearance of canopy-forming algae in coastal habitats exposes crustose coralline algae (CCA) to varying light conditions during the transition from foliose macroalgal habitats to barren ground by physical, chemical, or biological interaction. We hypothesized that the sudden increase in light intensity, resulting from the absence of canopy-forming algae, leads to photodamage in CCA related to inhibition of growth. To test this, a 4-week indoor incubation experiment was conducted under different light intensities (20-, 60-, 120-, and 250-μmol photons m⁻² s⁻¹). Results revealed that CCA can acclimate to limited light environments, with the efficiency of PSII significantly reduced at higher light levels. Photoprotective mechanisms were activated under persistent stress, leading to a reduction in encrusting area and CCA bleaching. This suggests that CCA coexist with canopy-forming algae in environments with sufficient light exposure, benefiting from the shade provided. However, if directly exposed to light due to the disappearance of canopy-forming algae, CCA faces extreme light stress and potential disappearance.