Interest in the systems supplying dissolved forms of iron to the sea from upland forests and wetlands has increased because iron is abundant on land but has low bioavailability in seawater. This can be a limiting factor for the growth of marine phytoplankton. Organic complex iron, a typical form of iron dissolved in seawater, is supplied to the ocean through rivers from forest and wetland soils. As a related study, we focus on mangrove ecosystems located at the boundary between the land and sea and on polyphenols present in leaves as ligands for the formation of iron complexes. When mangrove leaf litterfalls on the wet forest floor, phenolic compounds leach out from the leaves and might solubilize insoluble iron in the sediments (i.e., iron complexation). However, the reaction mechanism is not simple in the field, and it might be made more complex by tidal currents and intervention by crabs and snails, which consume mangrove leaf litter. In the present study, we focused on a detritivorous snail, Terebralia palustris, as a facilitator of iron solubilization associated with phenolic compounds, and examined how the snail contribute to iron solubilization processes. Our results indicated that the amounts of phenolic compounds in mangrove sediments are strongly related to iron solubilization. Furthermore, the average dissolved iron and phenolic contents in sediments from areas inhabited by the snail were significantly higher than those of sediments where the snail was not present. We additionally report that the solubilization of iron was promoted when snail feces were added to mangrove sediments. In conclusion, we propose that iron solubilization in mangrove sediments is promoted by the interaction between i) iron in the sediment, ii) phenolic compounds derived from mangroves, and iii) the consumption of leaves and the deposition of feces by the snail.

Research Highlights: Mangrove forests are absorbing anthropogenically produced excess nitrogen under moderate to intensive human interaction in the study sites, further indicating the degree of deviation from the natural ecosystem condition. Background and Objectives: Mangrove species, when directly connected to anthropogenic activities such as sewage disposal, agricultural inputs, and receiving of animal manure, absorb excess nutrients from the systems and act as ecological indicators of long-term natural changes. However, there is a paucity of examples of how the mangroves respond to a land-use gradient comparing to the non-mangrove plants under indirect anthropogenic impacts. Materials and Methods: In this investigation, foliar total nitrogen (N), carbon to nitrogen (C/N) ratio, and δ¹⁵N of mangrove and non-mangrove species collected from 15 watersheds on three islands in Okinawa, Japan, have been compared. The land-use areas in the study watersheds were delineated by ArcGIS software, and the correlation between the foliar traits and the human-affected area ratios were examined. Results: Foliar δ¹⁵N of the mangroves, which was significantly different from those of the non-mangroves on each island, showed significantly higher values (5‰ to 14‰) in human-affected forests, whereas the values were up to 3‰ in pristine forests. Furthermore, the significant positive relationship between foliar nitrogenous traits and the human-affected area ratios suggested that the anthropogenic N might be regulating foliar N content and δ¹⁵N signature on the sites. Conclusion: Different degrees of foliar isotopic fractionation with the land-use gradient have clarified that mangroves can be a powerful tool for monitoring ecosystem conditions under anthropogenic disturbances.

Herbal remedies by using mangrove plants have been recognized in several countries, and the plant materials are often consumed as a tea, which are infusions of dried plant parts steeped in boiling water. These infusions from mangrove plants are believed to be effective agents for treating and/or prevent infections, ailments, and diseases, and some mangrove plants have attracted attention as functional food resources in recent years. To reveal the beneficial properties of mangrove plants, clarification of the chemical ingredient and its characteristics of mangrove plants is required. This study focused on amino acids, which were functional ingredients (i.e., bioactive compounds) because amino acids are expected to function as ‘functional ingredients’ in addition to conventional ‘nutrients’ in recent years. To demonstrate the further possibilities of mangrove tea, we firstly evaluated the content of total amino acids in the leaf of several dominant mangrove species with commercial teas as references. Next, we clarified the composition of amino acids by using LC-MS analysis. As the results, free amino acid content (FAC) in leaf sample of Rhizophora stylosa, Bruguira gymnorrhiza, Kandelia obovata, Avicennia marina, Sonneratia alba, and Lumnitzera racemosa was 747, 1056, 946, 829, 623, and 896 mg/100 g DW, respectively. The FAC in all mangrove leaf samples were higher than those in green tea of low price, black tea, oolong tea, barley tea, and mate tea. Moreover, it was found that GABA accounts for over 10% of the total amino acid in the mangrove leaf samples. Especially in mature leaves of K. obovata, the ratio was very high levels 35%. In the case of K. obovata, the contents were higher than GABA tea. Based on this, we concluded that several mangrove plants, especially K. obovata has the potential of a functional food which is GABA-rich.

This study evaluated the freshwater use of two mangrove species (R. mucronate: Rhizophora mucronata and A. marina: Avicennia marina) growing in a coastal area of Djibouti City and on Moucha Island in Djibouti. We accomplished this by comparing hydrogen and oxygen stable isotope ratios (δ²H and δ¹⁸O) in the body water with those of creek water, soil water, groundwater, and rainwater samples. Line-conditioned excess (lc-excess) was calculated for the δ²H and δ¹⁸O values of the groundwater and rainwater samples. Although two regression lines were drawn using the δ²H and δ¹⁸O values of the plant body water as well as the environmental water, no significant differences were found, suggesting that the effects of isotope fractionation due to evaporation in all the samples were almost similar. Comparison of lc-excess and δ¹⁸O suggested that the body water from the two mangrove species growing on Moucha Is. were strongly influenced by seawater, but contained some freshwater sources other than the seawater, the soil water, and the creek water. It was also found that Avicennia marina growing in the coastal area was strongly influenced by groundwater. The origin of the freshwater absorbed by the mangroves on the island was not clear, but it could be groundwater recharged in a faraway place.