Abstract
Root exudates play a pivotal role in the behaviors of polycyclic aromatic hydrocarbons (PAHs) in mangrove sediments, but the knowledge of how mangrove root exudates response to PAHs pollutants is limited. This study examined the root exudates of Bruguiera gymnorrhiza (L.) (B. gymnorrhiza) under exposure in phenanthrene, pyrene, and benzo[a]pyrene solution through a 45 days hydroponic cultivation. The results showed that the root exudates of B. gymnorrhiza were mainly hydrocarbon compounds. Tartaric acid was the dominant low molecular weight organic acids (LMWOAs) in root exudates. Under PAHs stress, the proportion of hydrocarbon compounds in root exudates decreased, while the proportion of amide compounds increased. At the first 15 days exposure, the amounts of dissolved organic carbon, soluble total sugars, total organic acids and LWMOAs all increased and reached the maximum values, subsequently, the amounts of root exudates had dropped. The degradation rates of PAHs followed the sequence of phenanthrene > pyrene > benzo [a] pyrene, and the presence of root exudates can significantly enhance the degradation of PAHs. The results illustrated that PAHs stress can significantly change the concentrations and species of root exudates. This study provides the scientific reference for understanding the ability of B. gymnorrhiza response to PAHs stress.
Highlights
Mangroves are important inter-tidal estuarine wetlands located along coastlines of tropical and subtropical regions
Comparing the change of the detected organic compounds in root exudates under polycyclic aromatic hydrocarbons (PAHs) stress, it can be seen that after 15 days of PAH stress (Table 1), the proportion of hydrocarbons significantly decreased (p < 0.05), the proportion of phenols and esters slightly decreased, while the proportion of amides significantly increased (p < 0.05), and which were the main components of organic compounds in root exudates
For each PAH, the most species of compounds produced under the medium concentration (1 mg/L), and comparing the stress under different PAH with the same concentration, phenanthrene stress showed the most species of compounds
Summary
Mangroves are important inter-tidal estuarine wetlands located along coastlines of tropical and subtropical regions. They serve important ecological functions because of high productivity, high return rate, high decomposition rate and high resistance to extreme weather events, and anthropogenic activities (Wang and Gu, 2021). They have important environmental protection functions because they can accumulate varieties of pollutants mainly derived from rivers or tidal waters (Zhang et al, 2014). Phytoremediation and biodegradation by indigenous degrading microbes are important processes of PAHs in contaminated mangrove sediments, and rhizosphere has been proved to effectively promote the depletion of PAHs in contaminated sediments in both cases (Tam and Wong, 2008; Lu et al, 2011)
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