Coastal habitats have been suggested to serve as a sink for unaccounted plastic debris, i.e., "missing plastic" in the sea, and hence, a hotspot of plastic pollution in the marine and coastal environments. Although the accumulation of plastic debris may pose significant threats to coastal ecosystems, we know little about the fate of these plastic debris and their ecological impacts due to the lack of studies on plastic-microbe interactions in coastal habitats, especially for the tropical marine and coastal environments. In this study, we collected plastic debris from 14 sites consisting of various coastal ecosystems (seagrass meadows, mangrove forests, and beaches), and marine ecosystem (coral reef) around Singapore and characterized the prokaryotic and eukaryotic microbial communities colonized on them. Our results showed that the composition of plastisphere communities in these intertidal ecosystems was predominantly influenced by the sediment than by the plastic materials. Compared with surrounding sediment and seawater, the plastic debris enriched potential plastic degraders, such as Muricauda, Halomonas, and Brevundimonas. The plastic debris was also found to host taxa that play significant roles in biogeochemical cycles (e.g., cyanobacteria, Erythrobacter), hygienically relevant bacteria (e.g., Chryseobacterium, Brevundimonas), and potential pathogens that may negatively impact the health of coastal ecosystems (e.g., Thraustochytriaceae, Labyrinthulaceae, Flavobacterium). Taken together, our study provides valuable insights into the plastic-microbe interactions in tropical coastal and marine ecosystems, highlighting the urgent need for plastisphere studies to understand the fate and ecological impacts of plastic debris accumulated in coastal habitats.

Competitive interactions between corals and macroalgae play an important role in determining benthic community structure on coral reefs. While it is known that macroalgae may negatively affect corals, the relative influence of contact- versus water-mediated macroalgal interactions on corals – such as via an influence on coral-associated microbiomes – is less well understood. Further, the impacts of macroalgae on corals that have persisted in a heavily urbanized reef system have not been explored previously. We examined the effects of the macroalgae Lobophora sp. and Hypnea pannosa on the physiology and microbiome of three Indo-Pacific coral species (Merulina ampliata, Montipora stellata, and Pocillopora acuta) collected from two reefs in Singapore (Pulau Satumu and Kusu Island), and compared how these effects varied between direct contact and water-mediated interactions. Direct contact by Lobophora sp. caused visible tissue bleaching and reduced maximum quantum yield (F_v/F_m) in all three coral species, while direct contact by H. pannosa only led to slight, but significant, suppression of F_v/F_m. No detrimental effects on coral physiology were observed when corals were in close proximity to the macroalgae or when in direct contact with algal mimics. However, both direct contact and water-mediated interactions with Lobophora sp. and H. pannosa altered the prokaryotic community structures in M. stellata. For M. ampliata and P. acuta, the changes in their microbiomes in response to algal treatments were more strongly influenced by the source reefs from which the coral colonies were collected. In particular, coral colonies collected from Kusu Island had proportionately more initial abundances of potentially pathogenic bacteria in their microbiomes than those collected from Pulau Satumu; nevertheless, coral fragments from Kusu Island had the same physiological responses to macroalgal interactions as corals from Pulau Satumu. Overall, our results reveal that, for the species tested, the coral microbiomes were sensitive to both direct contact and water-mediated interactions with macroalgae, while coral physiology was only compromised when in direct contact. Further, the presence of high levels of potentially pathogenic bacteria in some of the coral samples did not lead to the corals being more susceptible to impacts from macroalgae.