Coral reefs harbor a huge marine biodiversity and have therefore been called the rainforests of the sea. However, despite their importance within marine ecosystems and the goods and services they provide to coastal populations, they are threatened by several anthropogenic stress factors such as chemical contamination, habitat destruction, overfishing and plastic pollution. These factors, in addition to Global Change effects (i.e., increasing water temperatures and ocean acidification), have recently led to a decline in coral health, observable in massive bleaching events and high mortality rates.
Plastic pollution in the ocean, for example, was first reported in the early 1970s. However, only in recent years it became of ecological interest. Plastic particles of less than 5 mm in diameter are known as microplastic (MP). Due to their small size and low density, they are easily dispersed in aquatic ecosystems and can be ingested by marine organisms at different trophic levels. Nonetheless, the effects of MP on animals and especially on corals are still poorly understood. MP ingestion by corals results in various responses such as mucus production or tissue overgrowth, and some coral species even show signs of bleaching or tissue necrosis. As all submerged surfaces, MP particles are subjected to bacterial colonization and recent studies showed that singular bacterial assemblages are formed on MP surfaces (the plastisphere). Therefore, the question arises whether MP itself and/or MP-colonizing bacterial assemblages are responsible for the impairment of coral health.
This microplastic project therefore focusses on the bacterial colonization of sterile MP and its potential pathogenic effect on corals. Scanning electron microscopy and molecular techniques showed that very specific and complex bacterial assemblages might be formed on MP particles. These assemblages were distinct from those on sterile sandy sediments particles, in detritus aggregates and in the ambient water. Various pathogenic bacteria such as Vibrio spp. were detected in the plastisphere. A broad-range cultivation approach enabled the isolation of several bacterial representatives of MP colonizers.
Likewise, bacteria next related to Vibrio coralliilyticus, a species associated with multiple coral diseases whose virulence factors are temperature-regulated, were isolated from MP, along with bacterial isolates next related to V. fortis, a pathogen of sea horses. Detailed genome-based studies will now enable a more detailed assessment of the pathogenicity of those MP colonizers. This study has helped to unravel the negative impact of bacterial communities associated to MP on reef building corals. It also supports the hypothesis that MP act as Trojan horse, spreading potential bacterial pathogens across marine ecosystems.