Nickel and cobalt disrupt the microbiome and metabolic function of the photosymbiotic sea slug Berghia stephanieae (Á. Valdés, 2005)

The rapid global expansion of electromobility has increased demand for nickel and cobalt, which are key components in lithium-ion batteries. As a result, mining activities are intensifying, yet their ecological impact on marine ecosystems remains poorly understood. In this study, we examine the effects of nickel and cobalt on the microbiome composition and bacterial metabolic pathways of the marine photosymbiotic gastropod Berghia stephanieae. Preliminary toxicity tests showed concentration-dependent metal uptake, with behavioural stress responses such as lethargy, cessation of feeding, and shedding of cerata occurring at high concentrations (around 18 mg/L for nickel and cobalt). Microbiome analysis across three concentrations (1, 3, and 17 mg/L for nickel and cobalt) revealed significant changes in community composition and function. At 1 mg/L, the observed effects were concentration-dependent rather than metal-specific, as indicated by a decline in alpha diversity and a shift in community composition, characterised by a reduction in Mycoplasma sp. and an increase in Pseudofulvibacter and Endozoicomonas sp. At 3 mg/L, Shewanella spp. abundance increased and metal-specific effects emerged, with a strong downregulation of the metabolic pathways denitrification and lignin degradation for cobalt and an upregulation of sulfur oxidation for nickel. At 17 mg/L, many functional pathways were downregulated, aligning with observed host stress and pointing to a possible loss of functional redundancy. Overall, nickel and cobalt exposure led to distinct microbiome alterations, with intermediate levels eliciting the strongest metabolic response.