Responses of Landoltia punctata to cobalt and nickel: Removal, growth, photosynthesis, antioxidant system and starch metabolism

Landoltia punctata has been considered as a potential bioenergy crop due to its high biomass and starch yields in different cultivations. Cobalt and nickel are known to induce starch accumulation in duckweed. We monitored the growth rate, net photosynthesis rate, total chlorophyll content, Rubisco activity, Co2+ and Ni2+ contents, activity of antioxidant enzymes, starch content and activity of related enzymes under various concentrations of cobalt and nickel. The results indicate that Co2+ and Ni2+ (<= 0.5 mg L-1) can facilitate growth in the beginning. Although the growth rate, net photosynthesis rate, chlorophyll content and Rubisco activity were significantly inhibited at higher concentrations (5 mg L-1), the starch content increased sharply up to 53.3% dry weight (DW) in L. punctata. These results were attributed to the increase in adenosine diphosphate-glucose pyrophosphorylase (AGPase) and soluble starch synthase (SSS) activities and the decrease in alpha-amylase activity upon exposure to excess Co2+ and Ni2+. In addition, a substantial increase in the antioxidant enzyme activities and high flavonoid contents in L. punctata may have largely resulted in the metal tolerance. Furthermore, the high Co2+ and Ni2+ contents (2012.9 +/- 18.8 and 1997.7 +/- 29.2 mg kg(-1) DW) in the tissue indicate that L. punctata is a hyper accumulator. Thus, L. punctata can be considered as a potential candidate for the simultaneous bioremediation of Co2+- and Ni2+-polluted water and high-quality biomass production.