Microbial responses to crude oil and cadmium pollution in a tropical coastal environment, Gulf of Guinea

Abstract

Crude oil and cadmium (Cd) are common pollutants in Ghana's coastal ecosystems, where the cyanobacterial phytoplankton Synechococcus sp. serves as the primary producer and forms the base of the marine food web alongside small grazers. We hypothesized that cadmium and crude oil would disrupt microbial community structure and function, with the strongest effects under combined exposure. This study investigates the toxic effects of Oil (2 mL L⁻¹), Cd (4.4 μg L⁻¹), and their combined impact (Cd + Oil) on functional groups within the coastal microbial community, including Synechococcus sp., heterotrophic bacteria, nanoflagellates, eukaryotic phytoplankton, ciliates, and dinoflagellates, as well as on copepod nauplii and copepodite development during six-day incubations. We observed acute toxic effects on heterotrophic ciliates and dinoflagellates, with >50 % reductions in abundance within 6 h and a marked decrease in diversity. Phytoplankton showed growth within the first 24 h due to nutrient replenishment from the protist decay, however, their growth continued to decline after 24 h, with Synechococcus being particularly sensitive to Cd and less affected by Oil. In contrast, heterotrophic bacteria increased in abundance across all treatments, likely benefiting from organic matter released during phytoplankton decay and their high adaptability. Notably, the bacterial genera Marivivens and Rhodovulum became dominant mainly in the Oil-amended treatments. Overall, the microbial groups exhibited diverse responses to the pollutants, with the combined Cd + Oil treatment exerting the strongest negative effects, while crude oil alone had the least impact. These findings highlight the vulnerability of tropical microbial food webs, typically dominated by Synechococcus and microbial grazers, to combined pollutant stress, with potential cascading effects on higher trophic levels and coastal ecosystem productivity. This highlights the need for comprehensive monitoring and conservation efforts in these globally significant, yet understudied, regions.