Introduction
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).
References:
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.
