Theimportant resource for the maintenance of life for ages is by natural products.The plants are the important resources for preparing life-saving drugs. Theenduring importance of natural products 25% of prescribed pharmaceuticals arederived from plants. In between 1983 Cragg et al., 1997 reported thatthe FDA approved 78% of the new drugs which were derived from unmodifiednatural products or from natural sources.

In total the half of the globalbiodiversity is considered with the large-scale screening of marine species indevelopment of new drugs (Xu et al., 2004). Marine species are a rich instructurally novel and biologically active metabolites (Borowitzka andBorowitzka, 1992; Ely et al., 2004).

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The primary or secondarymetabolites produced by these organisms are potentially bioactive compoundswhich are used in the pharmaceutical industry. There are13 genera and 58 species of seagrasses available all over the world. Of these,six genera are mostly restricted to temperate seas (Amphibolis, Phyllospadi,Heterzostera, Pseudalthenia, Posidonia and Zostera) and the remaining sevengenera (Cymodocea, Halodule, Enhalus, Halophila, Thalassia, Syringodium andThalassodendeon) are distributed in tropical seas. In India, few reportsare available regarding seagrasses (Lakshmanan, 1985, Velusamy and Kannan,1985, Den Hartog and Yang, 1988., Lakshmanan et al.

, 1988, Parthasarathyet al., 1988a., Parthasarathy et al., 1988b; Den Hartog, 1989.

,Ramamurthy et al., 1989, Ravikumar and Ganesan, 1990). Jagtap,1991 surveyed the composition of seagrasses and marine algae from India,Lakshadweep, and Andaman Islands. The seagrass ecosystems from other regions ofthe world are described by Hartog, 1970; Hartog, 1977; Jacobs, 1982; Menez etal.

, 1983; Brouns, 1986; Larkum and Den Hartog, 1989. Balasubramanian,1974 screened the three species of seagrasses against bacterial pathogens. Theresult indicated that the lipid and water-soluble phenolic extracts of bothroot-rhizome and leaf fractions of H. pinifolia showed bestantibacterial activity against all the nine pathogens tested.Bernardand Pesando, 1988 studied and reported that the aqueous and lipid extracts fromthe rhizomes of P. oceanica were found to be active against selectedGram positive and Gram negative bacteria, dermatophytes, and the yeasts alongwith the antileishmanial activity                 (Orhan et al.

, 2006).  Devi et al., (1997) reported that theseagrass H. pinifolia showed nil inhibitory activity against any of thebacterial strains tested. The moderate to high activity of acetone, ethanol andmethanol extracts of Euglena viridis was examined against virulentpathogens like A. hydrophila, V.

anguillarum, P. florescence and E.coli Das et al., (2008). Numerous studies have been reportedover the last 50 years, on the antimicrobial activities of extracts from marineplants against biologically important microorganisms. Kim and Lee, (2008)showed strong antibacterial activities of methanolic extracts of Esienabicyclis (B32) and Sargassum spp. (B36) againstMethicillin-resistant Staphylococcus aureus (MRSA), Vibrioparahemolyticus and Edward tarda.

Howayda etal., (2007) reported that the ethanolic extract of R. cirrhosa whichinhibited all of the tested bacterial isolates except E.coli.  Kolanjinathan and Stella (2009) indicatedthat acetone was the best solution for extracting effective antimicrobialmaterials from Sargassum myricystum, Hypnea musiformis, Turbinariaconoides, Halimedia gracilis and Gracilaria edulis whereas,Karthikaidevi et al., (2009) used seven different solvents such as ethylacetate and methanol for extraction of antibacterial substances from Ulvareticulate, Codium adherens and Halimeda tuna. Earlier reportsshowed that marine plants showed antimicrobial activity against fish pathogens(Dhayanithi et al., 2010; Anitha, 2006).

The MIC of Zostera marina extractswere ranged from 161mg to 8 mg/ml was active against all three human skinpathogens Han Gil et al., (2009). They also reported the MIC of then-butanol and ethyl acetate fractions were the same with 1 mg/ml against C.albicans and S.

aureus. Umamaheshwariet al., (2009) and Ragupathi et al., (2010b) reported theseagrass Halodule pinifolia showed stong antibacterial activity againstpathogens. Sundaram et al., (2011a) demonstrated that antibacterialactivity of seagrass S. isoetifolium root extracts against fishpathogens.

The broad spectrum of six fractions from acetone extract and onefraction from hexane root extracts ofC. serrulata exhibitedantibacterial activity (Sundaram et al., 2011b). The methanol extract ofH. ovalis collected from the Chunnambar estuary in Pondicherry exhibitedstong antibacterial activity of 17nm against B. cereus followed by 14nmagainst A.

baumannii Yuvaraj et al., (2011).The crudeethyl acetate extract of E. acorodies collected from the seagrass bed inMerambung Island inhibited the growth of all test bacteria with the largestzone of inhibition displayed by P. aeruginosa which was followed by MSSA,MRSA and S. epidermidis(Mohd et al., 2012).

Aswathi et al., (2012b)reported that the ethanol extraction of the sea grasses C. rotundata showedbetter zone of inhibition than other tested extracts against bacterialpathogens. Seagrass are marine plants found nearshore waters. They are one of the most highly productive tropical ecosystemsand act as shelter and food for near shore fisheries, marine reptiles and manmals.

They have a widespread distribution that encompasses every continent, exceptAntarctica but the highest diversity of seagrass species is centred in thetropical Indo-Pacific (Waycott et al., 2004). Seagrass habitatsare an important component of the marine environment, providing vital servicessuch as nutrient cycling, food provision, and climate change mitigation (Orth etal., 2006; Waycott et al., 2009).

Similar to other marine ecosystems, seagrass habitats are under threat fromhuman activities, such as coastal development and increased nutrient input.These impacts have brought about accelerated decline in seagrass habitatsglobally (Waycott et al., 2009).Seagrasses are marine flowering plants thathave the ability to complete their life cycle while fully submerged in marineenvironment constraints. Short et al., (2001) reported thatthese marine plants cover large geographic area worldwide. Seagrasses provideservices and goods for their ecological conmunity, for example, waveprotection, reduced water flow, fishing ground, and oxygen production (Bail, 2005) and (Aziz et al.

, 2006).Ahmad-Kamil etal. (2013) studied and reported that seagrasspercentage increased during the El-Nino period, due to some naturaldisturbances.

Evolution of land usage and measurements of other waterphysicochemical parameters (such as heavy metal, pesticides and nutrients)should be considered, to assess the health of seagrass ecosystem at the studyarea. The ethnotaxa ‘Saethu pasi’ (H. ovalis) is found in dark water zonesthat have a muddy substratum. Fishermen find these muddy or ‘saethu’ zones veryinteresting because they provide habitat for numerous crabs, shrimps and fish.Although it is difficult for a trained taxonomist to distinguish H. ovata (‘Pottal pasi’) from H.

ovalis (‘Saethu pasi’) without usingmacroscopic characters (e.g., seed number and cross venation pattern), localpeople can easily distinguish the two species based on the different habitatsthey grow in predominant species are Cymodocearotundata, Cymodocea serrulata, Halodule pinifolia, Halophila ovalis, Syringodium isoetifolium and Thalassia hemprichii.

These seagrasses provide critical habitat fordiverse marine fauna such as cuttlefish, dugongs, sea horses, eels, rays andscorpion fish, sea cucumbers and sea snakes, among others. He alsoreported theimportance of seagrasses to local coastal economies in the state of Tamil Naduis apparent by the engagement of the conmunity in collecting and sortingseagrasses. In fact, trading depots are established along the coast wherepeople trade or buy local produce including seagrasses (Newmaster et al., 2011).