Chapter 2: Screening for heavy metal tolerant fern species 53 2 Screening for heavy metal tolerance in common Australian fern species * ABSTRACT The effects of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) on the growth and uptake of 10 fern species was investigated under a controlled environment in order to evaluate their suitability for phytoremediation. Fern species included Adiantum aethiopicum, Blechnum cartilagineum, Blechnum nudum, Calochlaena dubia, Dennstaedtia davallioides, Doodia aspera, Hypolepis muelleri, Nephrolepis cordifolia, Pellaea falcata and the arsenic (As) hyperaccumulating Pteris vittata. Ferns were exposed to four levels of heavy metals at concentrations of 0, 50, 100 and 500 mg kg –1 for a period of 20 weeks. In general, heavy metal translocation was limited, with the majority of heavy metals held in roots, suggesting an exclusion mechanism as part of the ferns’ tolerance to the applied heavy metals. Similar heavy metal-accumulation patterns were observed for all species in that accumulation generally increased with increasing levels of heavy metals applied. In most cases a sharp increase was observed between 100 and 500 mg kg –1 treatment levels, suggesting a breakdown in tolerance mechanisms and unrestricted heavy metals transport. This was corroborated by enhanced visual toxicity symptoms and a reduction in survival rates of ferns when exposed to 500 mg kg –1 treatment levels; and to a lesser extent 100 mg kg –1 treatment levels. Nephrolepis cordifolia and H. muelleri were identified as possible candidates in phytostabilisation of Cu-, Pb-, Ni- or Zn-contaminated soils; similarly D. davallioides appeared favourable for use in phytostabilisation of Cu- and Zn- contaminated soils. These species had high survival rates and accumulated high levels of the aforementioned heavy metals relative to the other ferns investigated. Ferns belonging to the family Blechnaceae (B. nudum, B. cartilagineum and D. aspera) and C. dubia (Family Dicksoniaceae) were least tolerant to most heavy metals, had a low survival rate and were classified as being unsuitable for phytoremediation purposes. Heavy metal tolerance was also observed in P. vittata when exposed to Cd, Cr and Cu; however, no hyperaccumulation was observed. * This chapter has been published in the Australia Journal of Botany, 300: 207–219 (2007) under the title ‘Heavy metal tolerance in common fern species’. Authors are Anthony G Kachenko, Balwant Singh and Naveen P Bhatia.
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Chapter 2: Screening for heavy metal tolerant fern species
53
2 Screening for heavy metal tolerance in common Australian
fern species*
ABSTRACT
The effects of cadmium (Cd), chromium (Cr), copper (Cu),
nickel (Ni), lead (Pb) and zinc (Zn) on the growth and uptake of
10 fern species was investigated under a controlled environment
in order to evaluate their suitability for phytoremediation. Fern
Nephrolepis cordifolia, Pellaea falcata and the arsenic (As) hyperaccumulating Pteris
vittata. Ferns were exposed to four levels of heavy metals at concentrations of 0, 50, 100
and 500 mg kg–1
for a period of 20 weeks. In general, heavy metal translocation was
limited, with the majority of heavy metals held in roots, suggesting an exclusion
mechanism as part of the ferns’ tolerance to the applied heavy metals. Similar heavy
metal-accumulation patterns were observed for all species in that accumulation
generally increased with increasing levels of heavy metals applied. In most cases a sharp
increase was observed between 100 and 500 mg kg–1
treatment levels, suggesting a
breakdown in tolerance mechanisms and unrestricted heavy metals transport. This was
corroborated by enhanced visual toxicity symptoms and a reduction in survival rates of
ferns when exposed to 500 mg kg–1
treatment levels; and to a lesser extent 100 mg kg–1
treatment levels. Nephrolepis cordifolia and H. muelleri were identified as possible
candidates in phytostabilisation of Cu-, Pb-, Ni- or Zn-contaminated soils; similarly D.
davallioides appeared favourable for use in phytostabilisation of Cu- and Zn-
contaminated soils. These species had high survival rates and accumulated high levels of
the aforementioned heavy metals relative to the other ferns investigated. Ferns
belonging to the family Blechnaceae (B. nudum, B. cartilagineum and D. aspera) and C.
dubia (Family Dicksoniaceae) were least tolerant to most heavy metals, had a low
survival rate and were classified as being unsuitable for phytoremediation purposes.
Heavy metal tolerance was also observed in P. vittata when exposed to Cd, Cr and Cu;
however, no hyperaccumulation was observed.
*This chapter has been published in the Australia Journal of Botany, 300: 207–219 (2007) under the title ‘Heavy metal tolerance in common fern species’. Authors are Anthony G Kachenko, Balwant Singh and Naveen P Bhatia.
A. G. Kachenko
54
2.1 INTRODUCTION
Soil pollution of heavy metals is a global problem with ramifications to human, animal and
environmental health. An increase in heavy metal pollution has long been associated with
population growth, fuelled by industrial advancement. Activities that have caused heavy metal
contamination include mining and smelting, industrial and manufacturing emissions and
application of fertilisers and sewage sludge contaminated with heavy metals (Singh 2001).
Industrialised countries and developing countries are equally affected; however, in the latter
there is an increased pressure to use contaminated soils for food production (Patel et al. 2005).
Cadmium and lead (Pb) contamination have been the focus of numerous studies and are
considered environmentally toxic metallic pollutants (Koeppe 1977; Sanchez-Camazano et al.
1994; Liu 2003). Heavy metals including copper (Cu), chromium (Cr), nickel (Ni) and zinc
(Zn) are also considered environmental pollutants and at high concentrations are often
associated with the expression of phytotoxicities in plants (Chatterjee and Chatterjee 2000;
Cuypers et al. 2002; Denkhaus and Salnikow 2002). Copper and Zn are essential
micronutrients and are required by plants at low concentrations; beyond the critical toxicity
levels of Cu > 20–30 mg kg–1 and Zn > 100–300 mg kg–1 phytotoxicity symptoms are often
pronounced (Marschner 1997). There have been few studies addressing Cr toxicity in plants,
however, it is known that Cr toxicity depends on its valence state, with CrVI being more toxic
and mobile compared to CrIII (Shanker et al. 2005).
In response to contaminated soils, a variety of physico-chemical remediation methods have
been adopted, including solidification, electrokinetics and encapsulation (Mulligan et al.
2001). In many cases, these strategies have resulted in criticisms due to their high cost, energy
intensiveness, site destructiveness, associated logistical problems and growing degree of
public dissatisfaction (Rulkens et al. 1998). The implementation of alternative strategies that
address these concerns is critical in effectively removing metallic pollutants from soil. In
recent years significant progress has been made with the implementation of phytoremediation
techniques as alternative technologies capable of remediation and restoring heavy metal
contaminated soils.
An approach to phytoremediation is phytostabilisation, an important in situ site stabilisation
technique that uses plants as a preventative barrier. The end result is a two-way barrier that
Chapter 2: Screening for heavy metal tolerant fern species
55
prevents the leaching of heavy metals throughout the soil profile, while minimising erosion
and ecological exposure to the heavy metals. Phytostabilisation is a versatile technique and
has been successfully applied in the containment of heavy metals in mine spoils, metalliferous
waste and smelters (Smith and Bradshaw 1979; Pierzynski et al. 2002; Stoltz and Greger
2002).
Plants suitable for phytostabilisation need to express a degree of tolerance that enables them
to survive in contaminated soils. Three mechanisms have been suggested by which plants
respond to heavy metals. They relate to the heavy metal concentrations found in plants and
the substrates in which they grow (Baker 1981). Plants termed ‘accumulators’ concentrate
heavy metals in aboveground biomass regardless of the heavy metal concentration in which
they are growing. Such properties have been found in ferns, for example P. vittata and
Pytyrogramma calomelanos for As (Ma et al. 2001; Francesconi et al. 2002). Conversely,
plants termed ‘excluders’ maintain low concentrations of heavy metals in aboveground
biomass, compared with their substrate, up to a certain threshold before the mechanism breaks
down. Last, plants termed ‘indicators’ are those in which the heavy metal concentration in the
aboveground biomass reflects the substrate concentration, and are often used in mineral
prospecting (Nkoane et al. 2005).
Ferns have often been associated with contaminated soils, particularly those associated with
mining operations. A notable example is the Chinese brake fern (P. vittata) that was identified
growing on an arsenical mine dump in Rhodesia (Wild 1974a); however, it was not until 2001
that it earned its classification as the first As hyperaccumulator (Ma et al. 2001). The silver
back fern (P. calomelanos) is the only fern outside the Pteris genus that has been identified as
an As hyperaccumulator (Francesconi et al. 2002). Lepp (2001) indicated that ferns are
regarded as the fourth most abundant group (in terms of species richness) of plants associated
with Cu-enriched substrates and also reported their dominant presence on serpentine and other
ultramafic bodies. Terrestrial ferns have received less attention than vascular plants in relation
to heavy metal tolerance and accumulation, and consequently there are few systematic
investigations that have considered the interactions between ferns and heavy metal substrates.
Large stretches of land across Australia have been contaminated by a variety of heavy metals,
mainly because of intensive agriculture and mining activities (Sproal et al. 2002; Archer and
A. G. Kachenko
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Caldwell 2004; Pietrzak and McPhail 2004; Cooper 2005). Owing to strict quarantine
regulations on import of planting material suitable for phytoremediation, a number of native
Australian ferns were screened for their tolerance to a variety of heavy metals (Cd, Cr, Cu, Ni,
Pb and Zn). None of these Australian species have been studied for heavy metal tolerance and
accumulation. The primary objective of this study was to determine the degree of
accumulation and the biomass produced when nine of the most common Australian fern
species along with P. vittata were exposed to elevated levels of heavy metals under controlled
conditions. This information could be used to select suitable species available for
phytoremediation of heavy metal(s)-contaminated soils within Australia and elsewhere.
2.2 MATERIALS AND METHODS
2.2.1 Selection of fern material
Nine Australian native fern species along with a known As hyperaccumulator P. vittata were
screened for their potential to tolerate Cd, Cr, Cu, Ni, Pb and Zn. Species were chosen based
on their hardiness, vigour and distribution pattern across Australia (McCarthy 1998), and