Mangroves in India

1. They are evergreen forests with salt tolerant trees (halophytes). They are adapted to low oxygen (anoxic) conditions of waterlogged mud.
2. The richest Mangrove communities occur in tropical and subtropical areas i.e.
   1. Between 30 degree North and South latitudes.
   2. Where Water temperature is > 24 degrees in the warmest month.
   3. Where Annual rainfall is > 125 cm.
   4. Where Mountain ranges > 700 m are found close to the coast.
3. Mangroves are found almost in all continents except Europe, the Arctic and the Antarctic. The best Mangroves are found in Asia, especially in India and Bangladesh.
4. India has 7% of the World's total area of Mangroves. Of the total Mangroves
   1. 80% are found along the East coast - Sundarbans, Bhitarkanika and Andaman and Nicobar.
   2. 20% of Mangroves are scattered on the west coast of Kutch to Kerala.
   3. Mangroves of Sundarbans
      1. They are the largest single block of tidal halophytic mangroves in the world.
      2. The major species include Herritiera fames, Rhizophora spp, Bruguiera spp, Ceriops decandra, Sonnerata spp and Avicennia spp.
      3. Sundarbans is the largest mangrove forest in the World both in size(it is tall) as well as Biodiversity.
   4. Mangroves of Bhittarkanika are the 2nd largest in the Indian subcontinent.
   5. The largest stretch is from West Bengal to AP and in the Western area, it is in creeks and estuaries.
   6. On the west coast, the largest mangroves are in Gujarat (Gulf of Kachch and Kori creek). Kori creek is the paleodelta of the Indus River.
   7. Kerala has minimum Mangroves. Mangroves of Pichavaram and Vedaranyam are degraded mainly due to the construction of aquaculture ponds and salt pans.
   8. In size, the Gulf of Kachch Mangroves is dwarf while taller Mangroves are found in Sundarbans. It is present in A & N but not in Lakshadweep.
5. Adaptation of Mangroves
   1. Mangrove plants live in hostile environmental conditions like high salinity, hypoxic (oxygen deficient) waterlogged area, tidal pressures, strong winds and sea waves.
   2. Mangroves can grow in saltwater as well as freshwater. The presence of salt is not necessary for its growth. Only 1 advantage is the lack of competition in brackish water.
      1. Species of Mangroves like Rhizophora, Ceriops, Bruguera are all salt excluders i.e. they filter out salt at root level from entering the plant.
      2. Leaves have many special salt glands for salt secretion. Examples are Avicennia, Sonneratia and Acanthus.
      3. Concentrate salt in the bark or in older leaves.
   3. Mangroves have specialized root systems
      1. Prop roots: Ex. Rhizophora spp. White Mangroves have cable roots that radiate out from the central trunk.
      2. Breathing roots are special vertical roots called Pnuematophores that form from lateral roots in the mud which allow some oxygen to reach the roots. Pneumatophores are green and contain chlorophyll.
      3. Stilt roots are the main organs for breathing especially during high tide (Rhizophora and Avicennia). The stilt roots of Rhizophora mucronata extend > 1 m above the soil surface and contain many small pores (lenticles) which at low tide allow oxygen to enter roots through aerenchyma (open passages). Aeration also occurs through lenticels in the bark of mangrove species.
6. Reproduction of Mangroves: They reproduce through 2 strategies: Dispersal by means of water and vivipary. In Vivipary, the embryo develops continuously while attached to the parent tree and during dispersal).
7. Mangrove fauna can be classified as Aquatic, Semi-aquatic and Terrestrial.
   1. It hosts as a nursery habitat for fish whose adults occupy other habitats like Coral reefs and seagrass beds. Butterflies and moths are commonly found in the Mangrove ecosystem.
   2. Estuarine or Saltwater crocodile is the largest crocodile found in the World. (The other 2 are freshwater crocodiles i.e. Marsh Crocodile and Gharial).
   3. Olive Ridley Sea Turtles, Green Turtle, Hawksbill Sea Turtle, Bibron's soft shell turtle, Batagur turtle and Leatherback turtle are found in Mangroves. The last 2 are Endangered.
   4. Water Monitor Lizard is one of the largest lizards in the World. In India, it is found in association with the Saltwater crocodiles. They are a major predator of crocodile and turtle eggs. It is endangered in India.
   5. Royal Bengal Tiger (Endangered), Dugong/ Sea Cow, Otters, Crab eating macaque (found only in A&N islands) etc. are found in Mangrove habitat.
8. Role of Mangroves
   1. Mangroves protect coastal lands from tsunami, hurricanes and floods.
   2. They enhance the natural recycling of nutrients. They support flora, avifauna and wildlife.
   3. They enhance the deposition of sediment in areas, stabilize the coastal shores and provide a breeding grounds for fishes.
   4. It provides Ecosystem services like woods, firewoods, and medicinal plants.
   5. Fine anoxic sediments deposited under mangroves act as sinks for a variety of heavy (trace) metals which are scavenged from overlaying seawater by colloidal particles in the sediments.
   6. They take in CO2, store carbon in their roots, leaves and branches and release Oxygen and a little methane gas in the atmosphere.
9. Threats to Mangroves:
   1. Mangroves are being cleared for Agricultural use, use of chemical pesticides, fertilizers and industrial purposes.
   2. Mangroves of Pichavaram and Vedaranyam are degraded mainly due to the construction of aquaculture ponds and salt pans.

What is the news?

• Scientists at the DBT-Institute of Life Sciences, Bhubaneswar and SRM-DBT Partnership Platform for Advanced Life Sciences Technologies, SRM Institute of Science and Technology,Tamil Nadu have reported for the first time a reference-grade whole genome sequence of a highly salt-tolerantand salt-secreting true-mangrove species,Avicennia marina.
• Mangroves are a unique group of species found in marshy intertidal estuarine regions and survive a high degree of salinity through several adaptive mechanisms.
• Mangroves are important resources for the coastal region and are of great ecological and economic value.
• They form a link between marine and terrestrial ecosystems, protect shorelines, provide habitat for a diverse array of terrestrial organisms.
• Avicennia marina is one of the most prominent mangrove species found in all mangrove formations in India.
• It is a salt-secreting and extraordinarily salt-tolerant mangrove species that grows optimally in 75% seawater and tolerates >250% seawater.It is among the rare plant species, which can excrete 40% of the salt through the salt glands in the leaves, besides its extraordinary capacity to exclude salt entry to the roots.
• This study published in the recent issue of the Nature Communications Biology reports the assemblage of a 456.6 Mb of the estimated 462.7 Mb A. marina genome (98.7% genome coverage) in 31 chromosomes derived from 88 scaffolds and 252 contigs. The percentage of genomes in gaps was 0.26%, thereby proving it to be a high-level assembly.
• The A. marina genome assembled in this study is nearly complete and can be considered as a reference-grade genome reported so far for any mangrove species globally and the first report from India”.
• This study employed the latest genome sequencing and assembling technologies and identified 31,477 protein-coding genes and a “salinome” consisting of 3246 salinity-responsive genes and homologs of 614 experimentally validated salinity tolerance genes.
• The study reported identi?cation of 614 genes, including 159 transcription factors, which are homologous to the genes that were functionally validated for salinity tolerance in transgenic systems.
• This study assumes significance as agriculture productivity globally is affected due to abiotic stress factors such as limited water availability and salinization of soil and water.
• Availability of water is a significant challenge to crop production in dryland areas, accounting for ~40 per cent of the world’s total land area. Salinity is prevalent in ~900 million hectares globally (with an estimated 6.73 million ha in India), and it is estimated to cause an annual loss of 27 billion USD.
• The genomic resources generated in the study will pave the way for researchers to study the potential of the identified genes for developing drought and salinity-tolerant varieties of important crop species of the coastal region that is significant for India with 7,500m of coastline and two major island systems.