Coral reefs are among the most biodiverse and economically valuable ecosystems on Earth, providing essential services such as coastal protection, fisheries, and tourism. However, these ecosystems are increasingly threatened by global stressors such as ocean warming and acidification, as well as local anthropogenic pressures including chemical pollution. Among these, herbicides like Diuron, widely used in agriculture and as an antifouling agent, represent a major concern due to their ability to inhibit photosynthesis in the symbiotic algae living within coral tissues.

This study investigates the toxicological effects of environmental (0.5 µg/L) and sub-lethal (5 µg/L and 50 µg/L) concentrations of Diuron on the health and physiology of the soft coral Coelogorgia palmosa over an 18-day exposure period. C. palmosa represents a particularly interesting model species as it is a taxonomically isolated, monotypic species that remains understudied compared to reef-building hard corals. A multidisciplinary approach was applied to evaluate coral health through different biomarkers. Morphological observations were conducted to monitor visible signs of stress including color changes (bleaching), tissue shrinkage, tissue attenuation, and mucus production.

Physiological performance was assessed using oxygen respirometry to quantify net photosynthesis and respiration rates. At the cellular level, the density of algal symbionts was measured, and the activity of the antioxidant enzyme Superoxide Dismutase (SOD) was evaluated to quantify oxidative stress responses. In addition, the coral volatilome was investigated through Volatile Organic Compound (VOC) analysis using Headspace Solid-Phase Microextraction (HS-SPME) coupled with Gas Chromatography–Mass Spectrometry (GC–MS) to identify metabolic shifts induced by chemical stress.

The results revealed a clear dose-dependent impact of Diuron on C. palmosa. Morphological data showed significant tissue shrinkage and attenuation in all treated groups starting from day 6 of exposure. Respirometry measurements indicated a critical shift from a net autotrophic to a net heterotrophic state in all Diuron-exposed corals, as evidenced by negative net respiration values compared with the positive values observed in the control group. Symbiont density showed a marked reduction only at the highest concentration (50 µg/L), whereas the intermediate dose (5 µg/L) triggered a significant increase in SOD activity, suggesting an active but energetically costly antioxidant response. Conversely, at 50 µg/L SOD activity decreased, indicating a potential breakdown of compensatory mechanisms under severe stress. VOC analysis identified 1137 molecules and showed that high Diuron concentrations lead to a simplified volatilome with reduced chemical diversity, potentially reflecting constrained metabolic capacity.

These findings highlight that morphological changes and VOC profiles can represent early indicators of chemical stress, detectable within a short timeframe and preceding more severe physiological impairments. However, these indicators should be integrated with additional physiological biomarkers to obtain a comprehensive assessment of coral health.

Overall, this study demonstrates that even environmentally relevant concentrations of Diuron can impair the physiological and metabolic integrity of C. palmosa. The combined use of morphological observations, VOC profiling, and enzymatic biomarkers provides a robust and integrative approach that may be applied not only to other coral species but also to the assessment of different chemical contaminants in marine ecosystems.