Promotion of DNA Absorption onto Microplastics by Transition Metal Ions

Microplastics can absorb and spread a variety of pollutants in the ecosystem posing a threat to human health. One of the common pollution sources of environmental waters is metal ions, which not only absorb on microplastics but can also promote the absorption of other invasive species such as environmental DNA. Recently, we showed that environmentally abundant metal ions (Na+, Mg2+ and Ca2+) can promote the absorption of single-stranded DNA (ssDNA) onto micro-plastics. Herein, we investigated the effect of transition metal ions including Zn2+ and Mn2+ and compared them with Mg2+ for promoting DNA adsorption. To better mimic environmental DNA, we also used a salmon sperm double-stranded DNA (dsDNA) (~2000 bp). For both ssDNA and dsDNA, the transition metals induced a higher absorption capacity compared to Mg2+, and that correlated with the higher binding affinity of transition metals to DNA. Although metal-mediated interactions were vital for ssDNA adsorption, the dsDNA absorbed on the micro-plastics even in the absence of metal ions, likely due to the abundance of binding sites of the 100-times longer dsDNA. Finally, desorption studies revealed that hydrophobic interactions were responsible for dsDNA adsorption in the absence of metal ions.