Many organisms synchronize their activities with natural light cycles, but artificial light at night (ALAN) from streetlights and other sources is increasing. While its effects on terrestrial organisms are well studied, research on marine organisms remains limited. This study examines how ALAN influences macroalgal growth and defense, focusing on a kelp Saccharina japonica and another brown algae Fucus distichus. The herbivorous isopod Idotea ochotensis was used to assess inducible defenses. A three-week indoor experiment was conducted using yellow LED light at 30 lux. Three treatments were established: (1) continuous illumination from sunset to sunrise, (2) partial illumination two hours after sunset and before sunrise, and (3) a control group with no nighttime lighting. Each treatment included conditions with and without the herbivore, totaling six groups. Growth was measured weekly by weighing macroalgae, and photosynthetic capacity was assessed using Diving-PAM. At the end of the experiment, tissue toughness was measured with a penetrometer, and a feeding assay was conducted to evaluate changes in palatability.
Results showed that F. distichus was more affected by ALAN than S. japonica, highlighting the species-specific sensitivity. While ALAN itself did not significantly impact growth, it interacted with grazing. In S. japonica, growth rate was significantly lower when exposed to continuous light during night and herbivory. In S. japonica, grazed algae indicated higher tissue toughness and increased palatability. In contrast, grazed F. distichus had lower tissue toughness, and there was no significant difference in palatability between the grazed and ungrazed algae. No inducible defense response to herbivory was detected, and increased tissue toughness did not function as a mechanical defense against I. ochotensis. Additionally, ALAN had no effect on these results. This study indicates that ALAN affects macroalgae differently depending on species, and interacts with grazing pressure. These findings suggest that ALAN may alter macroalgae-herbivore relationships by influencing growth and defensive responses.