Effects of Cerium on the Endurance of Salt Stress of Scytonema javanicum

Desertification is one of rigorous environmental problems faced by international society today and biological soil crusts(BSCs) have been considered a significant desert colonizer.The importance of BSCs in stabilization of sand dunes and in promotion of desert soils has been extensively recognized.BSCs are predominantly composed of desert algae(including cyanobacteria,green and brown algae,etc),moss-es,lichens,and fungi as well as bacteria.Especially,cyanobacteria make up a large component of BSCs in semiarid and arid regions.Desert algae have been proved to be able to well adapt to extreme environments and salt stress is one of important ecological factors influencing sur-vival of desert algae.Recently,rare earths are becoming a hotspot due to its broad applications in many domains,and the biological effects and ecological toxicity of rare earths on organisms have received critical attentions.In our field study area,desert algae are found to have an abroad distribution and sand soils of the area are detected to contain rich rare earths.However,very little information can be obtained regard-ing the effects of rare earths on desert algae for the moment.In order to further acquire a better insight into the roles of rare earths in physio-logical activities of desert algae under salt stress,experiments were therefore performed in laboratory.In this study,Scytonema javanicum(fil-amentous cyanobacteria)was selected for experiment material because of its typical presence in BSCs,and the influences of Ce on S.javan-icum under salt stress were investigated by measurations of physiological,biochemical properties and ultra-structures of the algae.During the process of experiments,the effect concentration of 5.0 μg.mL-1 Ce was first selected to act on S.javanicum and then 0(as control),0.05 mol.L-1 and 0.3 mol.L-1 NaCl were selected as salt stress.Synchronously,another treatment was conducted by adding exterior Ce into the algal cul-tures with 0.05 mol·L-1 and 0.3 mol·L-1 NaCl, and final concentrations of Ce in the culture media with salt were arrived at 5.0 μg·mL-(1i.e., the selected effective concentration of Ce). Biomass(expressed as chlorophyll a), PSⅡ activity(Fv/Fm), productions of water-soluble proteins, phycocyanin(PC), scytonemin, exopolysaccharides(EPS)and MDA contents as well as ultra-structure of the algal cells were determined in respective treatment. The results showed that salt stress resulted in an obvious decrease in biomass and PSⅡ activity of the treated cells com- pared to the control cells. It was indicated in the study that EPS of the algae had plentiful accumulation under salt stress, while the produc- tions of phycocyanin and scytonemin present obvious decline in salt treated cells. Furthermore, salt stress had a significantly deleterious effect on membrane lipid superoxide as assessed by measurement of MDA content, and MDA took on obvious increases as compared to the control algae. Moreover, ultra-structures of S.javanicum were found to be highly destroyed due to salt stress subjected by the algae. We discovered that S.javanicum obtained a certain recover in growth activity and cellular structures when Ce was added into algal cultures under salt treat- ments. Nevertheless, Ce was detected to destroy cellular structures of the algal cells without salt addition, and also was found not to signifi- cantly affect metabolic processes of some important substances such as phycocyanin, scytonemin and EPS in the algal cells. Therefore, the bi- ological effects of Ce on desert algae and its toxicity to desert ecological system should arouse our regards in future researches and applica- tions. The researches maybe provided us a significant finding for better understanding physiological effects of Ce on desert algae, and simulta- neously supplied a direction for applications of rare earths in desert algal crust cultivations.