Until their last breath: Persistence of a key ecological function of tropical sea cucumbers under ocean warming

Deposit feeding holothurians play a key role in many marine ecosystems by enhancing nitrogen remineralization, which supports primary production. Although these ecological functions are well documented, the impact of ocean warming on the roles of sea cucumbers in the nitrogen cycle remain unclear. We assessed the metabolic rates and inorganic nitrogen remineralization (production of NH₄⁺, NO₂^- and NO₃^-) of a tropical sea cucumber, Holothuria leucospilota, under predicted future temperatures (current, +3 °C, +5 °C) in seasonal (winter and summer) mesocosm experiments. Warming scenarios were based on IPCC projections for a tropical system, with moderate (+3 °C) and severe (+5 °C) increases above present-day temperatures. In summer, metabolic rates increased under both moderate and severe warming, while winter rates remained largely unchanged from present-day levels, mortality occurred only under severe summer warming. NH₄⁺ excretion was found to be consistent across both season and warming scenarios but showed greater variation under severe summer warming. Under severe warming, NO_x production increased compared to present-day levels in winter but declined in summer, whereas moderate warming did not affect NO_x production in either season. TIN production remained largely unaffected by warming across both seasons. These findings indicate that moderate warming is unlikely to compromise the ecological roles of tropical sea cucumbers in nitrogen remineralization, as their contribution to nitrogen production remains largely similar to present-day levels. However, severe warming may reduce population-level contributions through mortality and compromise NO_x production, potentially reducing the ecological contributions of sea cucumbers in tropical habitats.