Influence of artificial light at night (ALAN) on the growth of two marine macroalgae, Gracilaria coronopifolia and Caulerpa lentillifera, in a tropical system.

Artificial light at night (ALAN) has increased worldwide along urbanised coastlines, yet the effects on marine photoautotrophs remain poorly understood. Macroalgae are habitat-forming species that provide essential ecosystem services in coastal environments and any factor impairing their performance may have far-reaching ecological consequences. This study investigated the effects of ALAN on growth rate, oxygen evolution, and palatability in two tropical macroalgae, Gracilaria coronopifolia (Rhodophyta) and Caulerpa lentillifera (Chlorophyta), from Penang, Malaysia. Samples were maintained under laboratory conditions for 14 days using white LED light with two regimes: 12 h of 2000 lux (34.93 μmol m-2 s-1) during the day followed by either No ALAN (0 lux) or ALAN (30 lux; 0.54 μmol m-2 s-1) at night. C. lentillifera exhibited enhanced growth under ALAN, but oxygen evolution was reduced compared with individuals kept in nocturnal darkness, indicating impaired photosynthetic performance. In addition, palatability increased following ALAN exposure. These contrasting responses suggest complex physiological effects, as enhanced growth coincided with reduced photosynthetic efficiency. In contrast, G. coronopifolia showed no statistically significant response to ALAN in any measured parameter. Our findings document that artificial light at night can influence the performance of marine macroalgae, highlighting the need to consider light pollution as a potential driver of change in coastal ecosystems.