Inhibitory effect of municipal sewage on symbiosis between mangrove plants and arbuscular mycorrhizal fungi

The impact of municipal sewage discharge on the symbiosis between arbuscular mycorrhizal fungi (AMF) and mangrove plants in different sections along 2 constructed mangrove belts was evaluated. Each section was 33 m long and 3 m wide and planted with Kandelia obovata or Aegiceras corniculatum, the 2 most common mangrove plants in South China. A greenhouse experiment comparing the colonization intensity of AMF among different mangrove plant species under wastewater treatment was also conducted. Typical arbuscular mycorrhiza structures were observed in most of the root samples collected from the constructed belts. In both belts, the AMF colonization intensities were significantly lower in the roots of plants close to the inlet than in those further away from the influent, suggesting that the colonization intensity of AMF was inhibited by municipal sewage discharge, and that inhibition was least pronounced in the effluent where the concentration of nutrients was lowest. AMF colonization intensities in the roots of A. corniculatum were significantly higher than those in the roots of K. obovata, which could be attributed to the fact that A. corniculatum provided more oxygen to support the AMF than did K. obovata, indicating the strong effects of the host species on AMF colonization. In both constructed wetland belts and greenhouse experiments, the AMF vesicle and arbuscular structures (the main functional structure of arbuscular mycorrhiza) appeared to be more sensitive to wastewater discharge than the hyphal structure, implying that sewage discharge would reduce the potential beneficial effects of AMF in mangrove ecosystems. This study provides useful information on the responses of AMF to sewage; this knowledge is important for the conservation and restoration of mangrove forests that are located close to human activities.