Page 145 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 133

accumulation which caused Minamata Desease. The accumulation was first
observed in fishes consumed by Minamata community (Steinnes, 1990). Baker
(1990) also showed that Cu accumulation may cause a heredity desease called
Wilson desease. In 2010, a similar tragedy also occurred in Buyat Bay, Minahasa,
North Celebes, Indonesia.
The toxicity levels of heavy metals are various (Lagerwerff, 1982; Bohn et al.,
1985; Tiller, 1989; Alloway, 1990d; Steinnes, 1990; Widowati et al., 2008). Among
heavy metals with high toxicities are Hg, Cd, Cu, and Zn; those with moderate
toxicities are Cr, Ni, and Co; and those with low toxicities are Mn and Fe. However,
there is a fact that different living things shows different tolerance to toxic heavy
metals; particular living things may show higher or lower tolerance. The toxicities
of some heavy metals on some living things are listed in Table 9.6. Heavy metals
on the right side are less toxic than those on the left side; therefore, the heavy
metals on the left side are toxic at lower concentrations than those located on the
right side. This means that for algae, for example, Hg is more toxic than are Cr and
Hg, which are toxic at lower concentrations than Cr.

Table 9.6. Heavy metal toxicities.

No. Organisms Order of Toxicity

1 Algae Hg > Cu > Cd > Fe > Cr > Zn > Co > Mn
2 Flower Plants Hg > Pb > Cu > Cd > Cr > Ni > Zn
3 Fungi Ag > Hg > Cu > Cd > Cr > Ni > Pb > Co > Zn >Fe
4 Phytoplankton Hg > Cu > Cd > Zn > Pb
*Adapted from Sposito (1989)

9.2 Direct Effects of Heavy Metals on Soil Enzymes

Heavy metals may give direct or indirect effect on the soil enzymatic
activities. Heavy metals may directly interact with enzyme molecules and, hence,
inactivate the active sites of enzyme molecules through the formation of
complexes between heavy metal cations and enzyme molecules (Tate III, 1987).
Heavy metals may also affect the soil enzymes indirectly through their effects of
enzyme producers, i.e. soil microorganisms, soil macroorganisms, and plant root

Abdul Kadir Salam - 2014