Environmental regulation of ciliates in corals: A secondary pathogen in coral disease

Ciliate invasion in corals is an emerging concern, yet the mechanisms behind their role in disease progression remain unclear. In this study, ciliates were isolated from pigmented lesions of Porites lutea and Acropora muricata and examined for their possible mode of interactions with coral tissues and bacteria. Histopathology revealed active bacterial feeding by ciliates along lesion margins, highlighting their potential role in worsening tissue damage. Microcosm experiments, designed to mimic seasonal conditions at the sampling site, showed that ciliate growth was most favorable during pre-monsoon conditions (32 °C, 36 ‰ salinity), while nutrient-rich post-monsoon environments (25 °C, 32 ‰) also supported proliferation. Importantly, maximum bacterial feeding occurred under pre-monsoon conditions, suggesting that elevated temperatures and mucus release from thermally stressed corals provide ideal resources for ciliates. This feeding may further extend to host-damaged tissue containing Symbiodiniaceae, disrupting coral recovery processes and increasing risks to reef biodiversity. These findings emphasize the importance of environmental factors and microbial interactions in shaping ciliate behaviour and highlight their role as secondary pathogens in coral disease dynamics.