Haoliang Lu

Kandelia obovata (S, L) Yong tolerance mechanisms to Cadmium: Subcellular distribution, chemical forms and thiol pools

In order to explore the detoxification mechanisms adopted by mangrove under cadmium (Cd) stress, we investigated the subcellular distribution and chemical forms of Cd, in addition to the change of the thiol pools in Kandelia obovata (S., L.) Yong, which were cultivated in sandy culture medium treated with sequential Cd solution. We found that Cd addition caused a proportional increase of Cd in the organs of K. obovata. The investigation of subcellular distribution verified that most of the Cd was localized in the cell wall, and the lowest was in the membrane. Results showed sodium chloride and acetic acid extractable Cd fractions were dominant. The contents of non-protein thiol compounds, Glutathione and phytochelatins in K. obovata were enhanced by the increasing strength of Cd treatment. Therefore, K. obovata can be defined as Cd tolerant plant, which base on cell wall compartmentalization, as well as protein and organic acids combination.

Silicon Alleviation of Cadmium Toxicity in Mangrove ( Avicennia marina ) in Relation to Cadmium Compartmentation

Mangrove plants seem to be highly tolerant of high levels of heavy-metal pollution. Recently, some researchers have focused on the mechanisms involved in their metal uptake and tolerance. However, the important mechanisms involved are still only partly understood. This investigation studied whether silicon (Si) affected cadmium (Cd) subcellular distribution in the leaves and root tips of Avicennia marina (Forsk.) Vierh seedlings, resulting in the amelioration of the toxicity of Cd. The results showed that Si partly overcame the reduction in growth due to Cd. This amelioration was correlated with a reduction in Cd uptake and alteration of Cd subcellular distribution. The mechanisms of Si amelioration of Cd stress were tissue dependent. In the leaves and root tips, Si reduced Cd concentration in subcellular fractions, Cd mobility, and the concentration of biologically active Cd in the cell wall active space. Si did not change the distribution of Cd between compartments in the leaves, but it increased the proportion of Cd in the cell walls and reduced the proportion of Cd in the symplast of the root tips.

Silicon alleviates cadmium toxicity in Avicennia marina (Forsk) Vierh seedlings in relation to root anatomy and radial oxygen loss

The effects of Si on growth, the anatomy of the roots, radial oxygen loss (ROL) and Fe/Mn plaque on the root surface were investigated in Avicennia marina (Forsk.) Vierh. seedlings under Cd stress. Si prompted the growth of seedlings and reduced the Cd concentration in the root, stem and leaf of A. marina. Si prompted the development of the apoplastic barrier in the roots, which may be related to the reduction of Cd uptake. The higher amount of ROL and Mn plaque on the root surface due to Si were also related to the promotion of Cd tolerance in A. marina seedlings. Therefore, it is concluded that the alteration of the anatomy of the roots, the increase of ROL and Mn plaque of A. marina seedlings play an important role in alleviation of Cd toxicity due to Si.

Research on the nitrogen cycle in rhizosphere of Kandelia obovata under ammonium and nitrate addition.

The present study investigated nitrogen process in rhizosphere of Kandelia obovata under nitrogen input. Results showed that nitrogen additions significantly increased 4 kinds of enzyme activities (Urease, Nitrate reductase, Nitrite reductase and hydroxylamine reductase). The pH value increased to 7.1 under ammonium addition, but decreased to 6.9 under nitrate addition. Potential Nitrification Intensity (PNI) increased 200-1500% compared with control under ammonium addition, and Potential Denitrification Intensity (PDI) increased more than 200% under nitrate addition. Ten types of organic acids were detected from root exudates, which mainly included oxalic acid, tartaric acid, formic acid, acetic acid, and citric acid. The abundance of 5 kinds of microbial functional groups (nifH, AOA, AOB, nirS, nirK) responded differently. Total nitrogen in organs of K. obovata increased more than 200%. This indicated that nitrogen additions accelerated the transformation of nitrogen directly and stimulated the exudation of root exudates and 5 kinds of genes indirectly.

Optimisation for assay of fluorescein diacetate hydrolytic activity as a sensitive tool to evaluate impacts of pollutants and nutrients on microbial activity in coastal sediments.

Fluorescein diacetate (FDA) assay has been widely applied in coastal research to quantify microbial activity in sediments. However, the present FDA assay procedures embodied in sediment studies potentially include operational errors since the protocol was established for studies of terrestrial soil. In the present study, we optimised the procedure of FDA assay using sandy and cohesive sediments to improve experiential sensitivity and reproducibility. The optimised method describes quantitative measurement of the fluorescein produced when 1.0g of fresh sediment is incubated with 50mM phosphate buffer solution (pH: 7.3) and glass beads (2g) at 35°C for 1h under a rotation of 50rpm. The covariation coefficient of the optimised method ranged from 1.9% to 3.8% and the method sensitivity ranged from 0.25 to 1.57. The improved protocol provides a more reliable measurement of the FDA hydrolysis rate over a wide range of sediments compared to the original method.

Effect of enhanced reactive nitrogen availability on plant-sediment mediated degradation of polycyclic aromatic hydrocarbons in contaminated mangrove sediment

As land-ocean interaction zones, mangrove systems receive substantial polycyclic aromatic hydrocarbons (PAHs) from sewage and combustion of fossil fuel. In this study, we investigated the relationship between dissolved inorganic nitrogen (DIN) availability and degradation rate of phenanthrene, a typical PAH compound, in mangrove plant-sediment systems, using Avicennia marina as a model plant. After 50 day incubation, phenanthrene removal ratios in sediments ranged from 53.8% to 97.2%. In non-rhizosphere sediment, increasing DIN accessibility increased microbial biomass and total microbial activity, while enhancements in population size of phenanthrene degradation bacteria (PDB) and phenanthrene degradation rates were insignificant. In contrast, the presence of excessive DIN in rhizosphere sediment resulted in a significantly large number of PDB, leading to a rapid dissipation rate of phenanthrene. The differences in degradation rates and abundances of degrader in sediment may be explained by the enhanced root activity due to the elevation in DIN accessibility.

Effect of external phosphate addition on solid-phase iron distribution and iron accumulation in Mangrove Kandelia obovata (S. L.)

In this study, a pot experiment was conducted to evaluate the effect of phosphate (PO43−) addition on iron (Fe) cycling in mangrove ecosystem. Kandelia obovata (S. L.), one of the dominant mangrove species in the southeast of China, was cultivated in rhizoboxes under three different levels of P concentrations. Results showed the solid-phase Fe distribution and Fe(II)/Fe(III) values in both the root zone (rhizosphere) and bulk soil (non-rhizosphere) were comparable among all P levels (p > 0.05); P addition significantly decreased the pore water Fe content both in the rhizosphere and non-rhizosphere zone (p < 0.05); higher amount of reactive Fe was found in rhizosphere sediments, while in the non-rhizosphere sediments, higher concentration of crystalline Fe was determined; P significantly increased iron plaque formation and iron accumulation in K. obovata (S. L.) tissues (p < 0.05); P addition increased K. obovata (S. L.) biomass and chlorophyll content. It was suggested that P is implicated in the Fe cycling in mangrove plants; more reactive iron, higher abundance of root Fe-reducing bacteria (FeRB) and Fe-oxidizing bacteria (FeOB), and together with higher amount of K. obovata (S. L.) root organic acids exudation result in a rapid Fe cycling in rhizosphere, which contribute to comparable solid-phase iron distribution among different P levels.

Response of phenolic metabolism to cadmium and phenanthrene and its influence on pollutant translocations in the mangrove plant Aegiceras corniculatum (L.) Blanco (Ac)

Polyphenolic compounds are abundant in mangrove plants, playing a pivotal role in the detoxification of pollutants extruded from surrounding environments into plant tissues. The present study aimed to examine the variations of phenolic compounds, namely total polyphenolics, soluble tannins, condensed tannins and lignin, in the mangrove plant Aegiceras corniculatum (L.) due to the presence of exogenous cadmium and phenanthrene and to explore the influence of phenolic metabolism on biological translocation of these pollutants from roots to leaves. After a 6-week exposure to cadmium and phenanthrene, significant accumulations of both pollutants were observed. All determined phenolic compounds in both leaves and roots at high dosage levels were enhanced compared to the uncontaminated plant. Elevations of polyphenols in both treatments are possibly a result of stimulation in the activity of phenylalanine ammonia-lyase (PAL) and the enrichment of soluble sugar. Additionally, a significantly positive dosage relationship between polyphenolic metabolism intensity and phenanthrene contamination levels was found, while the trend observed in cadmium treatment was weak since cadmium at high levels inhibited phenolic production. The enrichment of polyphenols led to a decline in the biological translocation of these pollutants from roots to leaves. The immobilization of pollutants in the plant roots is possibly linked to the adsorption potential of polyphenols. These results will improve the understanding of the tolerance of mangrove plants to exogenous pollutants and will guide the selection of plants in phytoremediation because of the variability of polyphenol concentrations among species.

Influence of polycyclic aromatic hydrocarbons on nitrate reduction capability in mangrove sediments

In the present study, we investigated the influence of phenanthrene (PHE), a three-ring polycyclic aromatic hydrocarbon (PAH) compound, on nitrate (NO3-) reduction processes in mangrove sediments using microcosms. After 10days, nitrate/nitrite reductase activity and abundance of narG and nirS significantly decreased in the bulk sediment at both 10/50mgPHEkg-1 contamination groups. In the rhizosphere, abundance of narG, nirS and nirK markedly declined at PHE treated sediments, while the drop in reductase activity at 10mgkg-1 PHE treatment was insignificant. After 50days, apart from 10mgPhekg-1 treated bulk sediment, abundance of denitrifiers and reductase activity in all PHE spiked sediment samples significantly dropped. Therefore, the influence of PAHs on NO3- reduction capability in mangrove sediments is dependent on spiked concentration, temporal scale of exposure and interaction with roots. Generally, PAHs play an inhibitor role, slowing NO3- turnover rates, which warrant attention from coastal managers.