• 13C
• ALAN
• Cystoseira
• habitat-forming macroalgae
• macrofauna
• microbiota
• rockpool

Artificial light at night (ALAN) is a growing pollutant in coastal ecosystems, potentially disrupting marine organisms. A field experiment in Calafuria (Livorno, Italy) assessed ALAN’s impact on Cystoseira macroalgae, their microbiome, and associated macrofauna. Eight rockpools were subjected to ALAN or left unlit as controls. Half of the algal thalli were treated with Betadine to attempt microbiome removal. While Betadine failed to significantly alter the microbiome, ALAN affected macrofaunal community structure, reducing Ophiuroidea and potentially increasing Sphaeromatidae and Isopoda abundance. No physiological changes were detected in the algae themselves. These findings suggest that short-term ALAN exposure may alter certain macroinvertebrate groups without directly affecting algal function. The study underscores the need for longer-term investigations to better understand the combined effects of ALAN and microbiome disruption on coastal marine ecosystems.