Effects of Cadmium on Phytoplankton Growth

3 Effects of Cadmium on Phytoplankton Growth 3.1 Beneficial Effect 

Two lines of evidence indicate that Cd is used to substitute for Zn either as a metal center in Zn proteins or in Cd-specific proteins that replace Zn proteins  

It is well known that elevated Cd concentrations are toxic to phytoplankton and that different species have different sensitivity to Cd toxicity [36,41 6]. For example, in a comparison of phytoplankton taxa. Brand and coworkers showed that cyanobacteria were the most sensitive to Cd toxicity, and diatoms the least sensitive with coccolithophores and dinoflagellates having intermediate sensitivity [41]. Another study found no systematic differences among taxa [44]. Differences in sensitivity can also be found within the same genus for example, oceanic Thalassiosira species are more resistant to Cd toxicity than coastal ones [45]. The free Cd ion concentration that causes 50% reduction in growth rate ranges from a few pmol to several hundred nmol [41,44]. 

The biochemical mechanisms of Cd toxicity in phytoplankton are in some respects similar to those in higher plants (see Chapter 13). One of the well-known effects is that Cd can compete with essential metals for uptake sites on the cell surface. High concentration of Cd inhibits the uptake of Mn and thus causes Mn deficiency in cells at low Mn concentrations [47 9]. Similarly, Cd also inhibits Fe uptake and assimilation and thus causes Fe deficiency, as evidenced by decreases in cytochrome / to chlorophyll a ratio and nitrate reductase activity 

The differences in sensitivity to Cd toxicity between phytoplankton species may be related to differences in their ability to detoxify the metal [43,44]. Cd-induced phyto-chelatin production is the most common detoxification mechanism in phytoplankton (see Section 5.1), but other thiol-containing peptides or proteins may also be involved. For example, a Cd-tolerant phytoplankton species, Isochrysis galbana, produces a metal-binding protein rich in cysteine [54] the cyanobacterium Synechococcus sp. produces a metallothionein-like protein to complex metals [55]. In addition, some species may induce efflux systems to remove intracellular Cd [47,56,57], or sequester Cd into the vacuole to reduce the cytosohc Cd concentration [58]. 

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