Abstract
Marine microplastic pollution presents a significant threat to coastal systems, where anthropogenic activities have resulted in the accumulation and abundance of pollutants in this environment. Although a number of studies have reported the presence of microplastics on macroalgae, there is a lack of in-depth information regarding microplastic adhesion and retention to these important primary producers, and subsequent morphological or physiological effects. Although microplastics can interact with harmful chemical compounds such as polycyclic aromatic hydrocarbons, exacerbating or mitigating their toxicity, no research currently exists on the combined effects of these two pollutant groups in marine macroalgae. Consequently, this study aims to provide a comprehensive insight into microplastic adhesion, retention, and effects on marine macroalgae, and to examine the effects of microplastic exposure in combination with phenanthrene, a common polycyclic aromatic hydrocarbon. Disks were excised from the blades of red, green, and brown macroalgae to allow for comparisons between species. Macroalgae disks were exposed to different concentrations of 1 – 5 um fluorescent thermoset amino formaldehyde microplastics across several exposure durations, followed by wash cycles and imaging. Adhesion and retention trends were identified using fluorescence microscopy and the imaging processing software ImageJ. While a significant increase in adhesion to the green macroalgae U. pertusa was observed between the 1-hour and 24-hour exposure treatments, no significant increase occurred between the 24-hour exposure and longer exposure treatments, suggesting adhesion occurs within 24 hours of exposure. Furthermore, although wash cycles were effective at removing a significant percent of the adhered microplastics, a considerable proportion of microplastics were retained. Biochemical analysis results also showed a dose-dependent effect on microplastic toxicity, where concentrations of 50,000 mp/mL resulted in a significant increase in enzymatic antioxidant activities (APOX, CAT, GPOX, GR, and SOD) and markers of oxidative stress (Lprox and PC) in U. pertusa and the red macroalgae Gigartina sp. Microplastic adhesion trends and subsequent effects were also species-specific, with the brown algae U. pinnatifida exhibiting lower microplastic adhesion and retention compared to U. pertusa and Gigartina sp., and no change in enzymatic antioxidant activities and markers of oxidative stress. Exposure to phenanthrene at concentrations of 0.1 uM and higher resulted in an increase in U. pertusa enzymatic antioxidant activities and markers of oxidative stress, with higher doses presenting qualitative observations of cellular damage and a decline in macroalgal growth. However, when 0.1 uM phenanthrene was combined with 1–4 um HDPE microplastics at 25,000 mp/mL, a mitigating effect was observed. These findings suggest that microplastic sorption can reduce the available phenanthrene in the media and subsequent toxicity. In summary, this study comprehensively examines the adhesion and retention of microplastics to three different macroalgae species, and provides novel insights into the effects of microplastic and phenanthrene toxicity in a marine macroalgae model.