Mangroves

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Introduction

An above and below water view of the edge of a mangrove

Mangroves are highly productive ecosystems which cover roughly 60-75% of the world’s tropical coastlines. Distributed over more than 112 countries with a total area near 181,000km2, mangals are a widespread ecosystems. Since mangroves are located on the coast and have highly developed root systems. They are a deterent to shoreline erosion and can lessen the damaging effects of tidal waves such as tsunamis and storm surges. Sediment microorganisms play important roles in the mangrove ecosystem and make essential contributions to its productivity. Mangrove litter decomposition and the nutrient release which accompanies it is a vital part of the function of adjacent coastal ecosystems.

Physical environment

Vegetation

The vegetation of the mangrove swamps is likely their most indicative feature. Of the recognized 110 mangrove species, only about 54 species in 20 genera from 16 families constitute the "true mangroves", species that occur almost exclusively in mangrove habitats. These mangrove species have adapted in many different ways, such as limiting salt uptake, surviving in low oxygen conditions, limiting water loss, and even increasing the survival rate of their offspring.

Sediment

In the sediment of most mangrove swamps there is a distinction between two primary layers. The majority of the sediment is anaerobic where there is a lack of oxygen. On top of that is a thin aerobic layer where oxygen is available.

Hydrology

Since mangroves are located in coastal tidal areas, they are usually inundated with water twice a day. Also because they are associated with coastal zones the water they are inundated with has a relatively high salt content.

Biological interactions

The mangrove ecosystem includes organisms such as plants, bacteria, fungi, microalgae, invertebrates, birds, and mammals. This ecosystem provides habitat for marine organisms to breed, grow, and feed. Young fish and shrimp use the vegetation as protective cover and food until they are able to migrate to the ocean.

Bacteria create mutualistic relationships with the mangrove trees. The bacteria provide services such as N-fixation while the mangroves trees provide root exudates, stimulating microbial growth activity. Fungi show similar relationships with the mangrove trees. Plants also supply oxygen to these organisms. There is also competition among the microorganisms because of the limited amount of nutrients available in mangroves. These competetive relationships can even be between a mangrove tree and a colony of bacteria. All of these things togeather make mangroves highly efficient nutrient cyclers.

Decomposition of mangrove vegetation is carried out by organisms such as crabs, fungi, bacteria, protozoa, and microalgae. Crabs relocate and macerate the fallen leaves while the other microorganisms decompose the leftover material through the use of enzymes such as cellulase, pectinase, protease, and amylase. Detritus feeders such as crustaceans, mollusks, and insect larvae consume organic particle matter as well.

Predation is another imporatnt biological interation in mangroves. Many detrivores are eaten by economically important fish and bird species as well as other carnivores.

Mangroves also supply coastal waters with a large supply of organic matter.

Microbial processes

Nitrogen Fixation

Nitrogen fixation is the process of conversion of gaseous forms of Nitrogen [N2] into combined forms such as ammonia or organic nitrogen. This process is generally carried out by some bacteria or cyanobacteria. N-fixation has had increased rates associated with dead and decomposing leaves, but in mangrove sediment is likely to be hindered by a limited supply of energy sources.

Sulfate Reduction

Generally a mangrove sediment consists of anaerobic conditions with an overlying aerobic zone. In the aerobic zone decomposition of organic matter usually follows the pathway of aerobic respiration, however in the underlying anaerobic zone decomposition occurs mainly through sulfate reduction. Sulfate reduction accounts for nearly 100% of the total CO2 emissions from the mangrove sediment.

Phosphate Solubilization

Phosphate usually precipitates in mangroves because of the large amount of cations in the interstitial water of mangrove sediments, which makes phosphorous largely inaccessible to plants. The phosphate solubilizing bacteria are a great advantage for mangrove plants because they are a source of soluble phosphorous. There is little research done on this particular group of bacteria in the marine environment; however it is speculated that the mechanism responsible for phosphate solubilization probably involves the production of organic acids, which could act as chelators displacing metals from phosphate complexes.

Key Microorganisms

Nitrogen Fixing Microbes

Most N-fixing microbes are Cyanobacteria. In the rhizosphere of mangrove trees two specific microbes have been found: Listonella anguillarum and Vibrio campbellii. They are diazotrophs. Although they are symbiotic to the mangrove trees they can be pathogenic bacteria in fish and shellfish.

Sulfate-Reducing Microbes

Sulfate-Reducing Bacteria:Generally sulfate-reducing bacteria are of the class Proteobacteria. An example of one specifically is Desulfosarcina. Sulfate-reducing bacteria (SRB) are also important in aerobic environments if they can proliferate in anaerobic zones. For example, in marine sediments and in aerobic wastewater treatment systems, sulfate reduction accounts for up to 50% of the mineralization of organic matter.

Phosphate-Solubilizing Microbes

In one study thirteen phosphate-solubilizing species of bacteria were isolated from rhizosphere of black and white mangrove roots. These species included: Bacillus amyloliquefaciens, Bacillus licheniformis, Bacillus atrophaeus, Paenibacillus macerans, Vibrio proteolyticus, Xanthobacter agilis, Enterobacter aerogenes, Enterobacter taylorae, Enterobacter asburiae, Kluyvera cryocrescens, Pseudomonas stutzeri, and Chryseomonas luteola.

Methanogenic Microbes

These microbes carry out the process of Methanogenesis, or the production of methane gas. Microbes in this category are known as Methanogens and are generally of the domain Archaea. A specific example of this category of microbes is Methanoccoides methylutens--a methanotrophic methanogen that utilizes methylamines (rather than competing w/ sulfate-reducing bacteria).

Current Research

"Polycyclic Aromatic Hydrocarbon-Induced Structural Shift of Bacterial Communities in Mangroves Sediment"

This study showed that microbial diversity in mangroves is reduced by contaminations by polycyclic aromatic hydrocarbons(PAHs). organic poolution can cause divergent effects on microbial communities, and in this case, concentrations of PAHs as low as 60mgkg-1 caused toxic effects. The detriment to microbial diversity increased with increasing concentration of PAHs in this study.[6]

"Bacterial Contribution to Mitigation of Iron and Manganese in Mangrove Sediments"

This study showed that mangrove sediments are more resilient to metal contamination because nitrifiers regulate metal concentration and decontamination of mangrove sediments through the use of TEAs and respiration. Both autochthonous autotrophs and heterotrophs mitigate iron and manganese in sediments. Bacterial flora in mangroves adjoining the Mandovi estuary in India self-regulate metal concentrations.[7]

"Diversity and Functional Responses of Nitrogen-Fixing Microbes to Three Wetland Invasions"

This study exposed three wetlands to invasive species, mangroves being one of them, to determine the impact of exotic species on microbial function and diversity. The result of the mangroves invading into a salt marsh was a negative impact on microbial activity and ecosystem function. Despite high microbial diversity, somewhat low functional redundancy results in a lack of immunity to invasive species. Habitat modification is a likely factor in the negative effect on nitrogen fixation and features such as redox, physical substrate, and light availability. Other anthropogenic factors may increase these detrimental features of invasives on microbial diversity and function. This study shows that mangroves can be bad for ecosystems as well.[8]

References


[1] Sahoo, K. & N.K. Dhal, 2009."Potential microbial diversity in mangrove ecosystems:a review". Indian Journal of Marine Sciences. 38(2):249-256
[2] Holguin, G., P. Vazquez, Y. Bashan, 2001."The role of sediment microorganisms in the productivity, conservation, and rehabilitation of mangrove ecosystems:an overview". Biology and Fertility of Soils. 33:265-278
[3] Liang, J., Y. Chen, C. Lan, N. Tam, Q. Zan, L. Huang, 2007."Recovery of novel bacterial diversity from mangrove sediment". Marine Biology. 150:739-747
[4] Zhang, Y, J. Dong, Z. Yang, S. Zhang, Y. Wang, 2008."Phylogenetic diversity of nitrogen-fixing bacteria in mangrove sediments assessed by PCR-denaturing gradient gel electrophoresis". Archives Microbiology. 190:19-28
[5] Alongi, D.M., 1988."Bacterial Productivity and Microbial Biomass in Tropical Mangrove Sediments". Microbial Ecology. 15:59-79
[6] Zhou H.W., A.H.Y. Wong, R. Yu, Y.D. Park, Y.S. Wong, N. Tam, 2009."Polycyclic Aromatic Hydrocarbon-Induced Structural Shift of Bacterial Communities in Mangroves Sediment". Microbial Biology. 58:153-160
[7] Krishnan, K.P., S.O. Fernandes, G.S. Chandan, P.A. Loka Bharathi, 2007."Bacterial contribution to mitigation of iron and manganese in mangrove sediments". Marine Pollution Bulletin. 54:1427-1433
[8] Moseman, S.M., R. Zhang, P.Y. Qian, L.A. Levin, 2009."Diversity and functional responses of nitrogen-fixing microbes to three wetland invasions". Biological Invasions. 11(2):225-239
[9] Rajankar P.N., D.H. Tambekar, and S.R. Wate, 2007. "Study of Phosphate Solubilization Efficiencies of Fungi and Bacteria Isolated From Saline belt of Purna river basin". Journal of Agriculture and Biological Sciences. 3(6): 701-703
[10] Mohanraju, R, B.S. Rajagopal, L. Daniels. 1997. Isolation and charcterization of a methanogenic bacterium from mangrove sediments. Journal of Marine Biotechnology 5(2-3):147-152
[11] Holguin, G., M. A. Guzman, and Y. Bashan, 1992. "Two new nitrogen-fixing bacteria from the rhizophere of mangrove trees: Their isolation, identification, and in vitro interaction with rhizoshpere Staphyloccus sp." FEMS Microbiology 101(207-216)
[12] Vazquez P.,G. Holguin, M.E. Puente, A. Lopez-Cortes, Y. Bashan, 2000. "Phosphate-solubilizing microorganisms associated with the rhizosphere of mangroves in a semiarid coastal lagoon". Biology and Fertility of Soils. 30:460–468


Edited by Gregory C. Johnston, a student of Angela Kent at the University of Illinois at Urbana-Champaign.