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Differences in the uptake and translocation of differentially charged microplastics by the taproot and lateral root of mangroves
Summary
Researchers investigated how mangrove roots take up and transport microplastic particles of different sizes and surface charges. They found that taproots primarily absorbed negatively charged particles while lateral roots only took in positively charged ones, and that particle size limited how far the plastics could travel through the plant. Notably, the study observed microplastics being released from leaf surfaces through salt glands and stomata, revealing a previously unknown pathway by which mangroves may return intercepted microplastics to the environment.
The interception of microplastics (MPs) by mangrove roots plays an indispensable role in reducing the environmental risks of MPs. However, there remains limited research on the fate of the intercepted MPs. Hereby, the uptake and subsequent translocation of 0.2 μm and 2 μm PS MPs with different coating charge by the typical salt-secreting mangrove plants (Aegiceras corniculatum) were investigated. Compared to amino-functionalized PS with positive charge (PS-NH), the visualized results indicated that the efficient uptake of carboxy-functionalized PS with negative charge (PS-COOH) was more dependent on taproots. But for the lateral roots, it only allowed the entry of PS-NH instead of PS-COOH. The specific uptake pathways of PS-NH on the lateral roots could attribute to the release of H and organic acids by root hairs, as well as the relative higher Zeta potential. After entering the Aegiceras corniculatum roots, the translocation of PS MPs was restricted by their particle sizes. Furthermore, the release of PS MPs from Aegiceras corniculatum leaf surfaces through the salt glands and stomata was observed. And the decline in the photochemical efficiency of leaves under PS MPs exposure also indirectly proved the foliar emission of PS MPs. Our study improved the understanding of the environmental behaviors and risks of the retained MPs in mangroves.