6.8 Microbial biodiversity of salt pans

Salt pans are rectangular soil-based basins, protected by mud bunds. They are used to collect and evaporate water from the sea or estuaries to make brine and eventually crude crystalline salt. Common salt is not pure sodium chloride; it contains varying amounts of sodium, potassium, calcium and magnesium

Ecology and seasonal cycle

Salt pans in Goa are inundated by tidal waters and monsoon runoff. They undergo three seasonal phases: submerged (June to November), shallow (December to February), and drought (March to June).

The light intensity, temperature, pH and oxygen content of the water and sediments change from season to season. Such extreme physicochemi cal changes are detrimental to normal life, but allow microorganis ms capable of adapting to survive.

Salt pans in Goa

Goa has about 200 salt pans along its 105 km coastline.

Salt-loving microbes

Salt-loving microorganis ms are called "halophiles". They include fungi, diatoms, bacteria and cyanobacteria, which occur as free forms or in associations called "mats". Two types, halophilic archaebacteria and cyanobacteria are of special interest due to their significance to the ecosystem and economic potential.

Halophilic archaebacteria

This unique group of bacteria is one of the oldest forms of life on earth. They can adapt to major changes in temperature, oxygen, salinity and acidity, unlike true bacteria, plants and animals.
Some archacbacteria have photosensory pigments that enable them to use the sun's energy to grow and reproduce.

Salt lovers

· Slightly halophilic: tolerate 5-7% NaCI (salt) content.
· Moderately halophilic: tolerate 8-14% salt content.
· Extremely halophilic: tolerate 14-35% salt content.

As evaporation increases the salinity of a salt pan, only extreme halophiles called halophilic archaebacteria grow.

Types of halophilic archaebacteria

Halobacteria:
Rods, disks, cups, squares, rectangles and triangles.

Halococcus:
Spheres.

Haloarcula:
Change in shape as they grow.

Types of halophilic archaebacteria

In 1980, these remarkable organisms were assigned to an exclusive biological kingdom, "Archaebacteria".

Cyanobacteria

Cyanobacteria have pigments which harness light energy and fix carbon. Some also fix nitrogen and increase soil fertility.

These microorganisms occur in a wide range of forms: spheres, hemispheres, cylinders, elliptical, filaments-branched and unbranched.

Halophilic cyanobacteria:

· Fix atmospheric nitrogen and enrich the nitrogen economy of the neighbouring khazan (saline) paddies.

· Fix atmospheric carbon as biomass which is then available as feed to shrimp cultures of khazan land.

· Take in carbon dioxide and give out oxygen.

Economic importance

Both halophilic archaebacteria and Cyanobacteria could be harnessed for agricultural and industrial use, for instance, to produce biogas. With their ability to adapt to toxic levels of chemicals and physical stress, they could also be used to remove heavy metals, crude oil and suspended matter from polluted water.

Potential uses of halophilic micro-organisms

Salt pans under threat

Goa's salt pans are becoming extinct due to use of land for housing, roads, railways and aquaculture. Pollution (such as oil and grease) from barges and other boats, monsoon runoff from mining areas, and industrial effluents in estuarine waters put a heavy stress on even the tolerant halophilic microorganisms. These pollutants may also contaminate the salt produced in the pans.

Salt pans under threat

Conservation

Salt pans are an important ecological niche for microorganisms involved in the cycling and fixing of oxygen, sulphur, carbon and nitrogen.

· Salt pans should be protected from encroachment and pollution.

· Research is needed to isolate these unique microflora and conserve them for biotechnology research and industrial use.

Prepared by Dr. I. Furtado