Rhizosphere is a composite system defining a crucial interface between soil and plant root together [1,2] . The soil layering around the plant root system exhibited occurrence of various microbial communities.  The beneficial role played by microbes includes nitrogen fixation, nutrient solubilization, mobilization & uptake, production of growth stimulant, thereby establish, maintain and improve the whole soil–plant association [3-7]. Such type of interactive plant microbe relationship depends upon molecular, biochemical and metabolic fluxes exerted in intra and inter environmental conditions and coordinate the sustainability of living system and environment [8].

Rhizospheric interactions (soil and plant) built up and promote variety of microorganisms that put forth the advantageous impact on plant growth and development. The  microbes thrive in the vicinity of soil as well as host root and play an important role in different ecosystem including wetlands and mangrove ecosystems as well [9] .  It play a vital role in regulating nutrient cycling and metabolism through biomineralization assisting in nutrient mobilization and uptake. These microbes are plant growth promoters  and enhance plant growth and development through various mechanisms like nitrogen fixation, phosphate solubilization, siderophore formation, synthesis and production of phytohormones like auxins, gibberellic acid , cytokinins , abscisic acid ; ACC deaminase which regulate the ethylene level. These mechanisms are direct process through which microbes upgrade the growth and development of host plants. Indirectly, they act through inhibition of phytopathogens as biocontrol agent, production of antimicrobial substances and antibiotics, lytic enzymes, hydrogen cyanide, competitive behavior for the available nutrients [10-12].

The role of microorganisms in growth and development of plants is imperative. They do have capability to colonize various plants and help them directly or indirectly through biodegradation, mineral acquisition and nutrient cycling. They are potential tool to protect plants from different stress viz, diseases, cold, temperature, drought, metal, saline, acidic and alkaline conditions. Plant growth promoting microbes do have various factors like enzyme synthesis, phytohormone production, bioconversion  of toxic forms, synthesis and secretion of bioactive compounds, bioleaching and mineral solubilisation, secondary metabolites and siderophore production Hence, Plant growth promoting microbes are important for production, protection and preservation ( conservation ) of plant system [13,14] . The plant growth promoting microbes are mostly categorized into different  groups – Biostimulants or phytostimulators which promote plant growth through production of phytohormones such as  auxins, gibberellic acid, cytokinines;  Biofertilizers which enable nutrient  acquisition and availability through nitrogen fixation and phosphate solubilisation;   bioprotectants which provide protection to plants against phytopathogens via antibiotics, siderophores ; induced systemic resistance  and rhizoremediants (degrade organic pollutants) [15-19].

Ecological and Microbiological Importance of Mangrove Ecosystems

Mangrove ecosystem is compacted coastline vegetation developed in saline environments of inter tidal zones of river deltas in tropical and subtropical regions [20,21]. These are coastal wetland ecosystem found at the intertidal zones of estuaries, creeks, lagoons, deltas, backwaters, marshes and mudflats of latitudes of subtropical and tropical regions. The degree of inundation, seasonal rainfall, salinity gradient, soil characters and silt-clay-sand combination effect the biodiversity and compositional richness. Thereby, mangrove ecosystem are highly productive which create a micro habitat for different microorganisms that endowed remarkable morphological, physiological and metabolic adaptive traits to flourish.  The microbial diversity in mangrove has also been linked to biodegradation and mineralization of different biotic and abiotic components of food web of the tropical marine ecosystem [22]. In spite of biologically diversified, mangrove ecosystems are less studied as far as microbial diversity is concerned. However, a number of studies have recently reported the occurrence of bacteria and fungi from mangrove ecosystem of Asia-Pacific region [23-38].  Occurrence of many bacterial and fungal species have been reported from diverse mangrove habitats of Egypt, Mai Po mangroves in Hong Kong,  Mauritius, Maldives, Mexico, Japan , Taiwan besides India [39-57].  Various nitrogen fixing, purple photosynthetic , salt tolerant, chemoheterotrophs, methanogenes, organic solvent degraders , plant growth hormone producers, biomineralizers, halotolerant, enzyme producers of mangrove origin have been described well [58-65] . Hence, the microflora are an important component of the mangrove ecosystem and play vital role in biogeochemical cycles and ultimate transformation of nutritional and organic matter [66,67].  Their contribution towards biodegradation, production of secondary metabolites, metabolic and biochemical interactive phenomenon with the host plants and surrounding environment helps in specific ecological sustainability [66-70].    Mangrove environment and its other biological components faces the periodic tidal flooding turn into variability in salinity and nutrient availability. Many fungi like Aspergillus niger, Penicillium nigricans, Fusarium, Helminthosporium, Cladobotrytis, Paecilomyces, Alternaria occurred in mangrove soil 71-73. Many rhizobacteria like Vibrio, Listonella, Phyllobacterium, Bacillus, Paenibacillus, Xanthobacter, Vibrio, Desulfovibrio, Desulfotomaculum, Desulfosarcina and Desulfococcus are present in the mangrove rhizosphere and proved to be important for plant growth and development, and protection against salinity and other negative biological factors [1,74]. Mangrove sediments are anaerobic and good environment for the incidence, growth and survival of purple sulfur bacteria, green and purple non sulphur bacteria.  Many phototrophic anaerobic bacteria like Chloronema, Chromatium, Beggiatoa, Thiopedia, Rhodobacter and Rhodopseudomonas and Leucothio   have been isolated from different mangroves [75-77].

Microorganisms are cosmopolitan and resides in natural environment.   However, mangrove habitat is a unique ecological niche differ from terrestrial counter part of the ecosystem due to salinity gradient and their floral and faunal residents. Salinity is the condition when soluble salts get accumulated in the solution (soil or water) to a level that has a negative impact on the growth and development of inhabitant plants [78]. Salinity   flocculate the soil which causes good aeration and root growth but high salinity adversely affect the plant growth and yield.  This include seed germination, reproductive development , phytohormone production and their regulatory mechanisms, irregular nutritional uptake and utilization, enzymatic synthesis and metabolism, cellular integrity and tissue necrosis, accumulation of osmolytes , water absorption [79-81]. Microorganisms of these extreme environment adapt essential and inductive metabolic system to survive along with abiotic and biotic stress. Many of their physiological and metabolic activity extend indirect help towards plant growth and development in adverse conditions. Several microorganisms especially growth promoting bacteria and fungi have been reported from various mangrove ecosystems and exhibited plant growth promoting properties due to their presence in rhizosphere , rhizoplane and /or endophytic environment. The stress tolerance capacity of these microbes depend upon genetic makeup, physiological and metabolic mechanisms, biofilm of metabolites, and salt tolerant enzymes. Mangrove ecosystem has also been reported as good source of various enzymes like lipases, asparaginases, cellulases, xylanses, pectinases, amylases etc [82-84]. They are able to survive in saline condition due to extracellular metabolites and exopolysaccharide production. This metabolite also promote their colonization in roots as well rhizospheric soil resultant in establishment and improvement in plant growth [85, 86].

Growth Promotion through Phosphate Solubilizing Microbes

Phosphorus is an essential element next only to nitrogen which is necessary for plant growth [87,88] . Unlike nitrogen, this element is not acquired through biochemical fixation but comes from other sources which include animal manures, chemical fertilizers, and plant residues including domestic wastes, green manure, human and industrial wastes and native compounds of phosphorus, both organic and inorganic already present in soil to meet plant requirements [89,90]. As reported, dissolved inorganic phosphate exists in the sea mainly as ionic forms of orthophosphoric acid. Because of the negative charge of phosphate ions, they are quickly absorbed after weathering of clays or detritus particles, forming insoluble forms of aluminum, calcium, or iron phosphates, all unavailable to mangroves. In nature, Phosphorus present in bound form and its mobilization depend upon the mineralization through metabolite pool develop by the microbes in the soil.

Plant growth promoting microbes do have capacity to converts insoluble forms of phosphorus to a soluble form and make it available for uptake of host plants [72,91,92].  Such phosphate solubilizing microbes increases the phosphate uptake and mobilization in soil and accessible by host plant root. The phosphate solubilizing microbes are also categorized in plant growth promoters (PGPR) which has been considered as one of the promising substitute for inorganic phosphate fertilizers for improving the plant growth and development. Plant growth promoting rhizobacteria solubilize   insoluble inorganic phosphate compounds, such as tricalcium phosphate, dicalcium phosphate, hydroxyapatite, and rock phosphate and use one or more indirect mechanism to increase the plant growth and yield.

Mangrove ecosystem is known to be highly rich due to high amount of dissolved and particulate organic matter due to various microbial activities. Microbes from these area play an important role in biodegradation of plant material through their various enzymatic and metabolic activities. Mangrove sediments act as a sink for phosphorus with high retention capacity. The rhizospheric and pneumatophoric zones endowed specific microbial community comparatively.  A large number of heterotrophic and autotrophic soil microorganism now known to have the capacity to solubilize inorganic phosphate through their metabolic activities directly or indirectly. The phosphorus deficiency has not been exhibited in mangrove sediments may be due the presence of phosphate solubilising microbe which can mediate the supply of soluble phosphorus possibly due to phosphatase enzyme and /or organic acids which are able to mineralize organic phosphates into inorganic form that eventually become available to the plants [93-96].

The occurrence of phosphate solubilising bacteria from different mangrove ecosystem have been reported well. Phosphate solubilising bacteria of the rhizosphere of the mangrove ecosystem of Great Nicobar Island and southeast coast of India [97,98]. Sankaralingam et al studied the   phosphate solubilizing bacteria from mangrove soil collected from the Manakudi estuary, Kanyakumari, Tamilnadu, India [99]. The phosphate solubilizing potantial of Pseudomonas plecoglossicida isolated from rhizosphere of Avicennia germinus and Laguncularia racemosa (L) clearly exhibited under in vitro condition [73]. Various factors were tested for pH, temperature, carbon and nitrogen sources, tricalcium phosphate influenced bacterial growth and phosphate activity in static and shaking conditions and thus characterized.  Tam and Diep studied the bacteria of mangrove rhizosphere in the Mekong Delta, Vietnam [100]. Ramya et al studied the occurrence and abundance of phosphatase-producing bacteria (PPB) in mangrove ecosystems [101]. The study detailed the isolation and characterization of PPB from rhizospheric, pneumatophoric and bulk sediments of Avicennia marina along Mumbai coast, India and also estimated alkaline phosphatase activity in sediment in addition to the phosphatase enzymatic activity of isolates. Many microbial cultures isolated from Bhitarkanika mangroves of Odisha were found to be mineral solubilisers [102,103]. Evaluation of in vitro solubilization potential of phosphate solubilising Streptomyces isolated from phyllosphere of Heritiera fomes (mangrove) has also been reported well [91,92]. A good account of phosphate solubilising microbes resides in mangrove has been documented variously indicate their useful potential for the growth and development of mangrove plants grow in saline and different abiotic stress environment.

Beneficial Role of Mangrove Endophytes

Microorganisms inhabit and grow with in the growing plant tissue with no negative effects are known as endophytes. These are good source of exploitable natural source of different product important in agriculture, health and industrial sectors. Reports are cited on endophytes as an important tool for plant growth promotion as they improve the plant nutrition and act as biocontrol agent. They help plant in acquiring nutrients through biomineralization and uptake, nitrogen fixation, preventing pathogen through antimicrobials, producing phytohormones, and developing systemic resistance. Diversity and biotechnological potential of endophytic microbes resides in mangrove forests of the different world wide saline and oceanic ecosystem has been described well [64,104]. Literature reveal the occurrence of different endophytic microbes sheltered with in the host tissues of different mangrove plants. Mangrove trees have remarkable adaptation and grow abundantly in saline coastal sediment. It has been proved that the endophytic colonization has played a major role in the ecological adaptation of the host and increased their survival under adverse conditions [105]. Although fungal endophytes from mangrove are well-documented, there have been limited reports on bacterial endophytes of mangrove and its application [106]. They open an avenue towards discovery of potential bioinoculants, mineral solubilizers, antitumor agents, insecticides, enzymes, vitamins, immune suppressants, immune modulators, and other natural products [75,107-109].

Role of Plant Growth Regulators of Microbial Origin

Plant growth regulators are the substances that influence plant metabolism and physiology at very low concentration. Several microorganisms including plant growth promoting rhizobacteria produces indole acetic acid (IAA) is one of the major physiologically active auxins. Acinetobactor, Bacillus, Enterobacter, Klebsiella, Pseudomonas, Serratia have ability to synthesize auxin [80,110]. Production of IAA is needed for root elongation, enhancement in root hairs and laterals which results into better nutrient uptake [111]. Its role in regulation of plant response towards environmental conditions as well as interaction with host plant has also been well recorded especially the production of phyohormone production, biocontrol properties and stress tolerance [112-116] .The study indicate the richness of mangrove environment with such microbes endowed high metabolic potential which indirectly help mangrove in saline environment to sustain in high saline conditions.

Mangrove Restoration through Microbial Applications

Gradual increase in soil dynamisms especially salinity caused drastic effect on growth and productivity of various plant groups. Besides improvement in soil fertility, appropriate strategies are required to overcome salt stress too.  Microbes can survive in different environment and endowed potential to survive in stress condition as well as help other biological associates like plants to survive in salt stress conditions. Bacteria and fungi which can grow   under salt conditions are better capable to colonize rhizosphere and endophytically under saline conditions. Hence, indirectly help host plants through beneficial metabolic and biochemical activities like water and nutrients uptake. Extracellular secondary metabolites, volatile organic compounds, antioxidants, and osmolytes from microbes help plants to overcome salt stress by removing toxic substances, maintaining water level, and increasing photosynthesis, germination, root growth, and fresh and dry weight. Such type of microbial strains from saline and mangrove environment are helpful for the other economically important agriculture, horticulture and forestry plants besides mangrove species.  The mangrove ecosystem is considered as diversified, productive and involve in multifarious biological and environmental activities, they are threatened and degraded due to natural calamity and /or human induced stresses [117]. Mangroves in the tropics can regenerate themselves or be restored using low-technology propagule planting, arid mangroves (areas having limited or no access to fresh water) can seldom regenerate, and if they do, it happens very slowly. However, plant microbe interaction and their resultant in the form of nutrient cycling and ultimate completion of food chain involving microbes and mangroves are evident now. The role of rhizosphere bacteria from mangrove origin or from terrestrial counterpart can be used as a key factor to improve the seedling development, establishment and productivity [17]. Now it is very clearly understood that mangrove rhizosphere bacteria can be used as a tool to enhance reforestation of mangrove seedlings. This can be done by inoculating seedlings with plant growth-promoting bacteria participating in one or more of the microbial cycles of the ecosystem. Kathiresan and Selvam described the potential of beneficial bacteria in promoting mangrove seedling growth Forty-eight bacterial strains were isolated from rhizosphere soil of a mangrove species (Rhizophora mucronata) [31]. Two strains which doubled the growth of mangrove seedlings were identified as Azotobacter vinelandii and Bacillus megaterium. This raises the possibility of using the bacterial species to enhance the growth of mangrove seedlings.

Deivanai et al examined the compatible association of bacterial endophytes from Rhizophora apiculata, a red mangrove of the Merbok brackish river in Semeling, Kedah, Malaysia, in colonizing rice tissues and discussed the likelihood of utilizing the symbiotic association of these endophytic bacterial isolates to increase the fitness of rice seedlings [118]. The gene sequences of the isolates were closely related to two genera, namely, Bacillus and Pantoea. These endophytic bacterial strains were isolated from the twig and petiole tissues of the mangrove and identified based on their 16S ribosomal ribonucleic acid (rRNA) gene sequence homology. A detailed study was carried out on  the effect of two strains Pseudomonas fluroescens and Enterobacter sp. which had IAA production, phosphate solubilizaion and nitrogen fixation activity in in vivo assays in Acacia polyphylla, an usual reforestation tree on degraded areas and plant growth endophyte increased the dried mass of A. polyphylla shoots and roots, suggesting that the presence of this strain has important benefits that promote the growth and fitness of this plant, thereby promoting the seedling production of this tree and improving plant development [106,116].

Organically, mangroves are very rich though deficient in nitrogen and phosphorus [73]. Microbes responsible for nutritional transformation in such ecosystem play an important role and maintain the ecological balance, hence, plants can survive in such extreme physical, biological and environmental stress.  The stimulation of root exudation through root bacteria in turn receives nutrition for self.  Such phenomenon of the occurrence of microbial community in mangrove sediments has also been reported variously [6]. Castro et al isolated bacteria from Rhizophora mangle, Laguncularia racemosa and Avicennia nitida and evaluated for nitrogen fixation, phosphate solubilization and IAA production [116]. The mangrove microbial strains were used for the inoculation in Acacia polyphylla reforestation programe. This inoculation increases Acacia polyphylla shoot dry mass, demonstrating that this strain effectively promotes the plant’s growth and fitness, which can be used in the seedling production of this tree. Microbes from mangrove origin can be used for the reforestation programe of mangrove also as its population and forest is gradually degrading day by day.

Synergism with Bacteria and Mangrove Growth

Mangrove productivity usually depend upon the sediment having various bio-geo-chemicals endowed specific pH, total dissolved solids and organic content along with the  rhizosphere microorganisms which are the major biological components [22,117,118].   These factors especially microbial association is being suggested by many researchers for the reforestation of degraded mangroves [119,120]. Not as much of information on plant growth promoting bacteria regarding their interactive mechanisms with mangrove host plants is available. Numerous studies have been reported on incidence of nitrogen fixing bacteria like Azospirillum, Azotobacter, Rhizobium, Clostridium and Klebsiella,Vibrio campbelli, Listonella anguillarum, Vibrio aestuarianus and Phyllobacterium in decomposing leaves, pneumatophores, the  rhizosphere, tree bark, and sediments, mechanism of nitrogen fixation and their applications in mangrove environment [28,121,122]. Rani and Newton evaluated the effect of diazotrophic filamentous cyanobacterium, Microcoleus chthonoplastes on black mangrove seedlings [123]. The impact of  mucilaginous sheath which exhibited the presence of beneficial microbes, on nitrogen fixation activity and total nitrogen concentration in inoculated seedlings that was significantly higher than in uninoculated plants.  More nitrogen assimilation in plant leaves than the other plant tissues indicated the biological interaction among microbe and mangrove host.  The outcome of this study also suggested the role of bioinoculation for reforestation programe, confirm the nitrogen fixation phenomenon in mangrove sediments and the rhizosphere [78,124]. In vitro colonization and increase in nitrogen fixation of seedling roots of black mangrove inoculated by cyanobacteria has also been reported in detail [57]. Role of diazotrophic filamentous cyanobacteria in nitrogen fixation in black mangroves has been reported variously. Vazquez et al studied the phosphate-solubilizing microorganisms associated with the rhizosphere of mangroves growing in a semiarid coastal lagoon [73]. The phosphate solubilizers exhibited the growth enhancement of seaweed Salicornia bigelovii a mangrove associate [125]. Monocultre as well as dual culture inoculation also promote growth enhancement of host plants as observed by Khammas and Kaiser for pectin decomposition by Bacillus sp and nitrogen fixation by Azospirillum.  Inoculation of such type of beneficial microbial strain jointly provide dual benefit to the host plants [126]. Many reports indicated the dual benefit of mangrove; A. brasilense for nitrogen fixation in presence of Staphylococcus which produces organic acid into the growth medium.

Mangroves are commercially and ecological valuable for mankind. Due to urbanization, this has been threatened for contaminant input of heavy metals. Hence, mangrove is now considered as sink for heavy metals. Many endophytes from mangrove have been studied widely for their diversity and usefulness [127- 129]. In fact, many endophytes inhabit the roots of plants and their collaboration may play a key role in growing in the heavy metals contaminated environment. Hence, it is necessary to exploit the function of endophytes for the tolerance of mangrove plants to heavy metals.  Endophytic fungus Purpureocillium sp isolated from Kandelia candel protected the plants under Cu stress [130].  It reduces the Cu level in plant tissues and enhanced the Cu content in soil. This study is significant and illustrates a promising potential use endophytes for environmental remediation and also may partially explain the large capacity of mangrove ecosystems in retaining heavy metals as excessive uptake of heavy metals especially copper by mangrove plants causes phytotoxic damage [131-133].

Bioconversion and Bio-Utilization of Organic Matter

It involves the usage of plant growth promoters in conversion of toxic metals to less toxic forms so that it is accessible by plant roots. Plant associated rhizobacteria and mycorrhiza increase the bioavailability of various heavy metal ions for their uptake by plants by catalyzing redox transformation leading to changes in heavy metal bioavailability [134]. Survival of plants under metal stress is possible due to phyto-accumulation, phyto-extraction, rhizo-filtration and phyto-volatization where PGPR are involved. A study was also made on the microbiological processes of production and destruction of organic matter in mangroves. It was reported that significant amount of organic matter and high temperature promote activity of aerobic and nonaerobic bacteria. Benka and Olumagin studied the effects of waste drilling fluid on bacterial isolate from a mangrove swamp oilfield located in Nigeria [2]. 

Microorganisms occurred in mangroves have been reported for the degradation of pollutants. Methylobacteriaum from Brazilian mangroves have been found with heavy metal tolerance especially cadmium, lead, and arsenic suggested bioremediation potential for oil spills via immobilization of heavy metals. In similar way, some mangrove bacteria have been endowed with degradation potential for malachite green and phenol. This could be important biomarker for pollution monitoring around coastal environment [134-136]. Cabral et al studied the anthropogenic impact on mangrove sediments which triggers differential responses in the heavy metals and antibiotic resistomes of microbial communities [68].

Siderophores are iron chelating compounds produced by microbes which can chelate iron present in the surrounding medium. This phenomenon helps in preventing phytopathogen by depleting iron in the surrounding environment. Simultaneously elucidating iron coverage to the plant system through nutrient uptake and ultimately promote plant growth.  Presence of iron in the form of Fe in soil gets limited due to accumulation of iron in the form of oxids and hydroxides. However, microbes have developed mechanisms of acquiring iron by production of iron carriers chelating agents called siderophores. It is observed that majority of PGPM facilitate uptake of iron from environment and promote plant growth [137,138]. Kharangate and Bhosle Studied the siderophore and pigment produced by an adhered bacterial strain Halobacillus trueperi from the mangrove ecosystem of Goa, India [69].

Growth Promoting Impact Of AM Fungi

A wide spectrum of studies on arbuscular mycorrhizal (AM) fungi have been carried out on their ecology and distribution, their effects on host physiology, biochemistry and genetics by different group of researchers. This group of fungi are important as they act as transporter and carrier of various minerals like P, Zn, Mn, Mg, Cu, and Al etc from soil to the host plants, thereby improving growth and productivity of host plant. Their biofertilising potential has been very well documented with respect to various agricultural, horticultural and forest plants [25, 139]. Environmental and edaphic factors as well as zonal plant diversity certainly have impact on the associated microflora. Mangrove ecosystem are saline habitat which have also been studied for the occurrence of various fungi and bacteria including AM fungi.  Several mangrove plants species have been found to be mycorrhzal in non-saline and saline conditions [140,141]. The association of AM fungi with mangrove has also been reported from Pichavaram forest, Bhitarkanika mangroves and ganges river estuary. Bencherif studied the influence of native arbuscular mycorrhizal fungi and Pseudomonas fluorescens on Tamarix, a salt tolerant shrubs under different salinity levels and   efficiency of native co-inoculation (AMF/P fluorescens) in improving plant growth under saline conditions was demonstrated [141]. The results revealed the potential use of native microbial inoculants for improving plantation of such salt tolerant species in disturbed soil.

Edaphic factors especially pH, minerals, dissolved solids and salinity, acidity or alkalinity affect the plant diversity and productivity. Mangrove environment has different zones of salinity as it is the intermediate between terrestrial and marine environment. Though, mangrove has adaptation towards salt stress,  increasing concentration may affect the absorption potential, conductivity, stomata conductance, net assimilation rate, photosynthetic  rate,  intercellular carbon dioxide concentration [142]. Under such metabolic and physiological stress, as reported, arbuscular mycorrhizal fungi and bacterial application plays a vital role to alleviate such saline stress and help host plants to improve growth and health. Arbuscular mycorrhizal fungi promote plant growth through improvement in mineral uptake, hormone biosynthesis and consequently plant metabolism. Bacteria like Pseudomonas induce enhancement in growth of host plants through growth hormone production, reduction of ethylene production, nitrogen fixation and phosphate solubilization, helpful in establishing mycorrhizal symbiosis, spore germination and mycelia extension.  Such beneficial microbes have also been reported from mangrove environment and these can be exploited as a tool to increase plant growth and production through bio inoculation in order to aid the required functions.

Protection and Indirect Growth of Mangrove through Antimicrobial Activity      

Biological control through microorganisms causes inhibitory activity against phytopathogen through indirect mechanisms like competition for host space, antagonism, localized and systemic resistance [143,144]. A good account on antifungal properties of microbes associated with mangrove environment has also been cited variously Mangrove bacterial endophytes Serratia, Bacillus, Pseudomonas, Micrococcus, and Enterobacter exhibited the broad spectrum antimicrobial activity against plant pathogens [145-147]. Antifungal activity of mangrove fungi against phyllosphere pathogen of rose is also reported 148. Selection and evaluation of antifungal metabolite from mangrove bacteria active against Verticillium albo-atrum was done.  Ravikumar et al isolated 14 endophytic bacteria from the leaves of 11 different halophytic plant species collected from Pichavaram mangrove forest of Tamil Nadu, India [64]. Among these endophytic isolates, two isolates, identified as Bacillus thuringiensis and B pumilus using 16S rDNA sequencing, showed antimicrobial activity against shrimp pathogens. These authors concluded that the endophytic bacterial strains isolated from the leaves of mangrove plants have promising antibacterial activity against shrimp pathogenic bacteria Pseudomonas aeuroginosa isolated from healthy leaves, stem, and root samples of the mangrove species Rhozophora mucronata exhibited inhibition against Sclerotinia sclerotiorum, Xanthomonas oryzae, and Rhizoctonia solani and Fusarium oxysporum . Though this bacterial strain exhibited antibacterial and antifungal activity against plant pathogens of agriculture importance, their potential may be exploited for the mass production and plantation programe of mangroves as its population is gradually degrading due to natural calamity and human interference 

Mangrove bacteria and fungi are important source many antimicrobials and showed potential as health care agent for plant and human as well [149]. Ravikumar et al  reported a endophytic actinomycetes isolated mangrove plants viz, Avicennia marina, Bruguiera cylindrica, Rhozophora mucronata exhibited inhibitory activity against some bacterial pathogens, such as Acinetobacter sp, Enterobacter sp, Escherichiacoli, Klebsiella sp, pneumonia, Proteus morganii, Pseudomonas aeruginosa [64]. The endophytic fungi of mangroves can produce many types of metabolites with great potential for antimicrobial use (Diaporthe, Phomopsis) has been described as one of the most dominant endophytic fungi in mangrove forests [149-155].

The vitality of mangrove ecosystem is complemented with the soil micro flora which sustained the salinity gradient and associated with different resident macro and micro flora and fauna 156, 157.  The mangrove microorganisms endowed the useful extracellular activity viz, production of secondary metabolites like organic acid, plant growth hormones, polysaccharides, antimicrobials; enzymes, mineral solubilizes and ion chelators. Together, their metabolism and physiological activity depend upon the edaphic environment that results into biodegradation of organic matter, solubilization of bound phosphates, iron chelation, production of antifungal and antibacterials which provides nutrient for growth and development and protection against abiotic and biotic stress. These microbes are associated with rhizosphere of plant system, sometimes endophytic and symbiotic in nature, hence known as plant growth promoters and protectors. As literature says mangrove ecosystem is goldmine of such beneficial microbes put forth growth benefit directly and or indirectly in natural conditions. The plant growth promoting microbes have adaptive traits towards stress management that also helps growth and establishment of mangrove plants in stress conditions. These are nothing but a potent tool to protect plants from adverse abiotic and biotic stress.  They play a vital role in plant growth enhancement, establishment and long-lasting sustainability of mangroves in different salinity gradient. A detailed survey of different mangrove ecosystems of Indian coastal regions regarding incidence of plant growth promoting microbes and their exploitation towards development of bioinoculants to achieve better establishment , growth and productivity of mangrove plantations as well as other agriculture, forestry and horticultural crops stressed by saline soil. Hence, such plant growth promoting microbes from mangrove ecosystems may contribute towards concept of production, protection and preservation (conservation).

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