The functional roles of marine organisms are often assessed in relation to abiotic stressors such as warming, acidification and pollution. However, biotic contexts, including host-associated pathogens and food-resource context, may also shape sub-lethal functional responses but remain less well understood.
Here, we tested empirically how exposure to the Brown Ring Disease-associated bacterium Vibrio tapetis and two contrasting microalgal diets, Phaeodactylum tricornutum and Nannochloropsis gaditana, affect the physiology, behaviour-mediated ecosystem processes and nutrient-cycling in the Manila clam Ruditapes philippinarum. Our results show that Vibrio tapetis exposure did not impact survival, condition index or total haemocyte abundance, but caused a transient increase in oxygen uptake within the first few days after exposure, before returning to normal levels after 11 days. Under V. tapetis exposure, oxygen uptake was higher in clams fed P. tricornutum than in those fed N. gaditana. Mean sediment reworking depth and bioirrigation was not affected by either V. tapetis exposure or diet. However, among-individual variability in median particle-mixing depth increased when the P. tricornutum diet was combined with V. tapetis exposure, while individual variability in bioirrigation increased under V. tapetis exposure. Water-column [NH₄-N] did not differ among treatments. In contrast, [NO_x-N] and [PO₄-P] concentrations were higher in clams fed P. tricornutum than in those fed N. gaditana. However, this dietary contrast was weaker under V. tapetis exposure: relative to unexposed clams, exposed clams tended to have lower nutrient concentrations under P. tricornutum but slightly higher concentrations under N. gaditana.
Overall, these findings show that sub-lethal bacterial exposure and microalgal diet influenced different parts of the pathway linking individual responses to ecosystem functioning. Pathogen exposure altered metabolic demand and the reliability of behaviour-mediated processes before any detectable decline in survival or body condition, whereas microalgal diet mainly shaped nutrient-cycling outcomes. These findings highlight the need to consider interacting biotic contexts and to look beyond visible health metrics when assessing how environmental change affects ecosystem functioning.

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