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Serpentines of Andaman: A Unique Ecosystem from India
Arundhati Pal and A. K. Paul
Microbiology Laboratory, Department of Botany
University of Calcutta, Kolkata 700 019
Email :amalk_paul@yahoo.co.in


Serpentine or serpentinite is a mineral producing dry, nutrient-poor soil deadly to plants not specially adapted to its unusual chemistry. In folklore, the name "serpentine" is attributed to the soil's resemblance to a mottled greenish-brown snake dwelling on similar soils. The greenish soil color comes from fragments of the underlying bedrock containing magnesium silicate.

Geology of Serpentines

Serpentines are igneous rocks, containing ferromagnesium minerals mixed with silica, but iron and magnesium are unusually high in these substrates. Soil scientists call this condition ultramafic ("ma" stands for magnesium and "f" for ferrum or iron). The term serpentine is applied to three polymorphic minerals: lizardite, antigorite and chrystotile. Serpentinization or metamorphic hydration of ferromagnesian silicate [(Mg, Fe)2SiO4] leads to the formation of serpentine soils at relatively low pressure, temperature between 100-300°C and a pH greater than 10. These soils are normally acidic near the surface, but less so in deeper layers. As wind and water erode the soil, non-acidic layers are exposed, creating varied habitat for plants.

Serpentine Soil Chemistry

Soil profile development is generally slow and poor, with average pH between 6.0-7.5. Soil moisture holding capacity is low but cation exchange capacity is typically high. The soil lacks essential plant nutrients. Calcium is scarce and its ratio to magnesium is unusually low. Scientists believe this ratio is a key factor in determining plant survivorship. Both elements are positive ions that compete for the same uptake sites in plant roots. When the concentration of magnesium is high, these uptake sites become saturated, making it harder for plants to obtain calcium. Other important nutrients lacking in serpentine include potassium and phosphorous, which are both rapidly lost through weathering and leaching. Nitrogen is also scarce due to limited plant growth.

Serpentine soils often have pockets of naturally occurring heavy metals toxic to plants, such as chromium, cobalt and nickel. While serpentine outcrops differ in their concentrations of these metals, many sites contain levels that are toxic to many plant species. Because soil development is poor and slopes are steep, ultramafic soils are not stable. This affects the establishment of pioneering vegetation and, as a consequence, further slows soil development.

Serpentine Vegetation

Serpentine outcrops have been referred to as barrens since they are often sparsely vegetated. Plants growing on serpentine soil experiences stressed eco-physiological condition and heavy metal exposure, which are collectively referred to as ‘serpentine syndrome’ or ‘serpentine factor’. The "syndrome" influences the overall biomass (amount of living material) on serpentine, which is low compared to other plant communities. Nonetheless, many species, growing under such stressed ecosystem, are equipped with unique physiologies to withstand the stress and thrive on these soils.

Plants restricted to serpentine areas and endemic thereto are called serpentinophytes. Serpentine substrates house a large number of species that are found only on serpentine and have highly restricted ranges. Serpentine species are generally dwarfed than their non-serpentine counterparts and can be classified into two categories: tolerators and avoiders. The avoiders germinate from seed or sprout from underground storage structures and complete their life cycle while water is available. The tolerators are the dominant vegetation and include primarily low-growing shrubs and occasional large trees with tough leaves that look silvery or flat gray due to hairs designed to reflect light. Such species develop certain physiological properties by means of which they are able to withstand such high concentration of heavy metals. Flowering plants like Rinorea bengalensis (Wall.) O. Kuntze, Dichapetalum gelenioides ssp. andamanicum (King) Leenh., different species of Strychnos, Orophea hexandra, Anoxogorea luzoniensis, etc. are endemic to Andaman Islands, hyperaccumulate nickel and other metals in their leaves, and are termed as metallophytes. These species are important as metal indicators and could be potentially exploited in phytoremediation practices like soil reclamation, phytomining, phytoextraction of metals, etc.

Microbial Ecology

Microbial ecology of serpentine soil is poorly understood, although these soils represent an ideal system to study the microbiology of naturally metal-percolated stressed environment and represent a powerful agent for the evolution of colonizing microorganisms. Microflora living in serpentine ecosystems represents an interesting model for the evolution of metal-resistant communities completely different from that of artificially contaminated sites. These soils could provide new strains and new genetic determinants for metal resistance, which could be exploited in bioremediation purposes.

Microbial density as well as microbial activity of this soil is low as compared to normal agricultural soil. Bacterial population dominates the soil in the rhizosphere of nickel-hyperaccumulating plants and soil litter, whereas, actinomycetes strains are predominant in bare soil with scanty vegetation. Fungal population is moderate and includes common genera like Penicillium, Aspergillus, Mortierella, Pythium etc. Owing to the presence of very high concentration of heavy metals like Ni, Co and Cr, the natural microbiota of serpentine soil shows wide degree of metal resistance. They are “fond of” or “love” metals and are termed as metallophiles. The degree of nickel and chromium-resistance appears to be high among bacteria, whereas, serpentine fungi show moderate resistance towards nickel and cobalt but are highly sensitive to chromium. These metal-resistant organisms also show resistance to other heavy metals and antibiotics. The mechanism of metal-resistance appears to be diverse among the strains. Majority of nickel-resistant isolates shows nickel ion uptake by washed cells in aqueous solution. The chromium-resistant organisms shows chromate reduction phenomenon as the possible detoxification mechanism. These serpentine isolates could be utilized as heavy metal biosensor or as metal-indicators in metal-polluted ecosystems and may act as possible biotechnological tools towards metal amelioration.

Fig 1. Serpentine soil ecosystem in saddle Hill, North Andaman

Fig 2. Serpentine Vegetation of saddle peak (734 m above sea level, highest point of Andaman Islands).


Brooks, R. R. (1987) Serpentine and its vegetation, a multidisciplinary approach. Croom Helm, London.

Pal, Arundhati, Dutta S., Mukherjee, P. K. and Paul, A. K. (2005) Occurrence of heavy metal-resistance in microflora from serpentine soil of Andaman. Journal of Basic Microbiology 45: 207-218.

Proctor, J. and Woodell, S. R. J. (1975) The ecology of serpentine soils. Advances in Ecological Research 9: 255-366.

Schlegel, H. G., Cosson, J. P. and Baker, J. M. (1991) Nickel hyperaccumulating plants provide a niche for nickel resistant bacteria. Botanical Acta, 104, 18-25.

ENVIS CENTRE Newsletter Vol.7,Issue 1 Jan 2009 Back 
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