Tolerance mechanism of Gymnostomum subrigidulum (Pottiaceae) to heavy metals in manganese mine waste areas.

Bryophytes have a strong absorption capacity for heavy metals (HMs). We investigated the physiological responses of the dominant bryophyte Gymnostomum subrigidulum to different levels of HM pollution in a manganese mine waste area, including changes in chlorophyll, malondialdehyde, and metallothioneins (MTs) content, as well as antioxidant enzyme activities. The cellular structure and subcellular distribution of HMs in G. subrigidulum from severely polluted areas were studied using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and transmission electron microscopy-energy dispersive X-ray spectroscopy techniques. The chlorophyll content of G. subrigidulum decreased significantly under the HM stress. HM pollution led to a significant increase in the content of MTs. The activities of peroxidase and catalase also increased significantly. The results showed that bryophytes reduced the toxic effects of HMs through a stress response mechanism, that involves adsorbing HMs onto their body surfaces through extracellular adsorption. Chloroplasts, mitochondria, and vacuole were the main damaged organelles in bryophyte cells under HMs stress. In heavily contaminated areas, the cell wall was significantly thickened, the chloroplasts were swollen, and more osmiophilic particles of the chloroplasts appeared. Pb, Cu, and Mn were deposited in the cell wall, whereas Cd was primarily distributed in the vacuole. Cell wall deposition and vacuolar compartmentalization were important mechanisms for HM detoxification in bryophytes.