Physiological and morphological responses of Lead or Cadmium exposed Chlorella sorokiniana 211-8K (Chlorophyceae)

Abstract

The heavy metal pollution in soils and aquatic environments is a serious ecological problem. In the green-microalga Chlorella sorokiniana 211-8K (Chlorophyceae) exposed to ions of lead (Pb) and cadmium (Cd) we studied the metabolic responses to the toxicity of these two heavy metals. Our data indicate that both the pollutants alter the alga cell ultrastructure and its physiological characteristics (growth, photosynthesis, respiration, enzyme activities). The toxic effects of the two metals resulted time-dependent to the exposure. After 24 h of treatment with 250 μM Pb or Cd, photosynthesis was inhibited until to 77 and 86%, however respiration was strongly enhanced up to 300 and 350%, respectively. In the algal cells Pb or Cd exposure induced a reduction in the content of the total chlorophylls and a decrease of the soluble protein levels, significantly compromising the growth, particularly in cultures cadmium-treated. We report data on ultrastructural changes induced by the two heavy metals; they affected overall chloroplast ultrastructure of the alga. Most importantly, the O-acetyl-L-serine(thiol)lyase (OASTL) activity was appreciably increased after only 2 h of Cd exposure, indicating the existence of a link between the metal contamination and cysteine synthesis. Then, Chlorella sorokiniana cells seem to better tolerate high concentrations of Pb while appear to be more sensitive to Cd ions. These results provide some additional information that can lead to better understand consequences of heavy metal poisoning in microalgae.

Keywords: Chlorella sorokiniana; Heavy metals; OASTL activity; Photosynthesis; Respiration; TEM.

Figure 1

Chlorella sorokinianagrowth, expressed as packed cell volume, in control cells (−●-), in Pb-treated (−□-) or in Cd-treated cells (−Δ-). Data are means ± SE (n = 3). In the algal culture, Pb and Cd were supplied as 250 μM (particulars in Materials and Methods). Asterisks (*) indicate that mean values are significantly different between the treatments and the corresponding control (P < 0.05, ANOVA, Tukey multiple comparison).

Figure 2

Photosynthetic rate ofChlorella sorokinianaup to 24 h exposure to Pb or Cd. Data are means ± SE (n = 3). In the algal culture, Pb and Cd were supplied as 250 μM (particulars in Materials and Methods). Columns labeled with different letters indicate statistically significant differences (P < 0.05, ANOVA, Tukey multiple comparison).

Figure 3

Respiratory rate ofChlorella sorokinianaup to 24 h exposure to Pb or Cd. Data are means ± SE (n = 3). In the algal culture, Pb and Cd were supplied as 250 μM (particulars in Materials and Methods). Columns labeled with different letters indicate statistically significant differences (P < 0.05, ANOVA, Tukey multiple comparison).

Figure 4

Effect of Pb or Cd on total protein content (mg mL^-1PCV) inChlorella sorokinianacells after 2 and 24 h. Data are means ± SE (n = 3). In the algal culture, Pb and Cd were supplied as 250 μM (particulars in Materials and Methods). Columns labeled with different letters indicate statistically significant differences (P < 0.05, ANOVA, Tukey multiple comparison).

Figure 5

OASTL activity (U mg^-1protein) inChlorella sorokinianacells exposed to Pb or Cd for 2 and 24 h. Data are means ± SE (n = 6). In the algal culture, Pb and Cd were supplied as 250 μM (particulars in Materials and Methods). Columns labeled with different letters indicate statistically significant differences (P < 0.05, ANOVA, Tukey multiple comparison).

Figure 6

Chlorella sorokinianaTEM micrographs: control (a), Pb-treated (b, c) and Cd-treated (d, e) culture.a. A cell with a cup-shaped chloroplast containing thylakoids, a central pyrenoid, and electron clear starch grains. b. A colony of algal cells with misshaped chloroplasts, a nucleus, and cytoplasm lipid droplets. c. The typical appearance of a Pb-treated culture with cells organized as colonies. d. A cell with a shape-altered chloroplast still containing thylakoids and starch grains. e. A multivesicular body. Scale bars: 2 μ (c); 500 nm (b, d); 300 nm (a, e).