|Other Abstract||There is a rather common point of view that desertification is becoming increasingly severe in the world. Emphasis on methods and means in control of desertification are widely given. Implanting biological crust with man-made measures at arid sandy zones is an important avenue to control desertification and worthy of being studied. Technology of fixing-sand algal crust is a new method to control desertification, and it adopts ecological, physiological principles on algae and theory of biological crusts. Desert algae were inoculated onto shifting sand for forming fixing-sand algal crust. Fixing-sand algal crust can provide fast, effective and long fixing-sand effects and has been applied to control desertification step by step. In this study, researches on field cultivation and developmental characteristics of man-made algal crusts were conducted, and influences of environmental factors on desert algae and algal crusts were also investigated. The main research results were provided as follows:
1. Morphologic properties, development and succession as well as ultra- structure of man-made algal crusts were examined by means of engineering culture of desert algae, inoculation of algae and cultivation of algal crusts. Based on the above researches, the roles of man-made algal crusts in fixing-sand, improvements of soil characteristics and feasibility of antidesertification were discussed. The results showed that artificial algal crusts came into being after desert cyanobacteria were inoculated onto surface of dunes. The crusts were 1.0-5.4mm in thickness and were grayish, sage green and highly black, and coverage of the crusts over 70% along with coverage of vegetation for 60-90% when artificial algal crusts acquired growth and development during one year and three years. It was showed from observation to ultra-microstructures of algal crusts that artificial algal crust was found to own abundant filamentous forms that could band and felt soil particles by their mechanical force, and simultaneously they exuded polysaccharide to hasten soil aggregation and cement soil particles together, which caused the crusts to bear high compressive strength and maybe contributed to cementation process of artificial algal crusts. All the above research results revealed that man-made algal crusts could not only fix shifting sand dunes, reduced soil erosion by wind or water but also had such ecological functions as changing the physiochemical properties of soils (water contents, pH values and electric conductivity, etc) and affecting soil nutrient circulation (i.e., contents of C, N, P and organic matter), as well as increasing soil biological activities such as soil enzyme activities and amounts of soil microorganisms.
2. The cyanobacteria, Microcoleus vaginatus Gom. and Scytonema javanicum (Kütz.) Born et Flah., were begun with stock cultures and sequential mass cultivations, and then the field experiment was performed by inoculating the inocula onto shifting sand for forming cyanobacterial crust during late summer and autumn of 2007 in Hopq Desert, northwest China. Measurements of dew amount and Chlorophyll a content were carried out in order to evaluate the changes in crust biomass following dew. Also, we determined the activity of photosystemⅡ (PSⅡ) within the crust in the laboratory by simulating the desiccation/rehydration process due to dew. Results showed that the average daily dew amount as measured by the cloth-plate method (CPM) was 0.154 mm during fifty-three days and that the crust biomass fluctuated from initial inoculation of 4.3 μg Chlorophyll a cm-2 sand to 5.8–7.3 μg Chlorophyll a cm-2 crust when dew acted as the sole water source, and reached a peak value of approximately 8.2 μg Chlorophyll a cm-2 crust owing to rainfalls. It indicated that there was a highly significant correlation between dew amounts and crust moistures (r=0.897 or r=0.882, all P<0.0001), but not a significant correlation between dew and the biomass (r=0.246 or r=0.257, all P>0.05), and thus concluded that dew might only play a relatively limited role in regulating the crust biomass. Correspondingly, we found that rains significantly facilitated biomass increase of the cyanobacterial crust. Results from the simulative experiment upon rehydration showed that approximately 80% of PSⅡ activity could be achieved within about 50 min after rehydration in the dark and at 5 ◦C, and only about 20% of the activity was light-temperature dependent. This might mean that dew was crucial for cyanobacterial crust to rapidly activate photosynthetic activity during desiccation and rehydration despite low temperatures and weak light before dawn. It also showed in this study that the cyanobacterial crusts could receive and retain more dew than sand, which depended on microclimatic characteristics and soil properties of the crusts. It may be necessary for us to fully understanding the influence of dew on regulating the growth and activity of cyanobacterial crust, and to soundly evaluate the crust’s potential application in fighting desertification because of the available water due to dew.
3. Sand burial is a noticeable environmental disturbance commonly experienced by various types of microbial crusts in the field. The cyanobacterial crusts, existing at early developmental stage of the biotic crusts, are exceedingly susceptible to burial by sand. Therefore, a study campaign was conducted in fields to evaluate the effects of sand burial (burial depth of 0, 0.5, 1, 3 and 5cm) on the crusts in Hopq desert, China. The results showed that sand burial imposed severe stress on the cyanobacteria crusts, such as the reduction of biomass, the restraint of scytonemin synthesis and the damage to PSⅡactivity, as well as the decrease of EPS excretion, etc. Furthermore, in this study the possible mechanisms by which cyanobacterial crusts tolerate burial were fully discussed, and a positive role of rain in the recovery from burial for cyanobacterial crusts was verified. It is necessary for the artificial cyanobacterial crusts to allay burial disturbances by assistants of sand-fixing means in order to ensure their better contribution to desert reclamation.
4. It indicated in this study that chlorophyll a contents and PS Ⅱ activity of S. javanicum were significantly affected by low temperature stress, and MDA content and soluble protein content of the cyanobacterium were distinctly increased due to low temperature. It also found that exterior saccharide played roles in relaxing salt stress to some extent when the saccharide was added into the cultures of S. javanicum. When the temperature was suddenly declined to 2℃, S. javanicum was found to have some tolerance to low temperature after adaptation at the temperature of 10℃. On this condition, the declines of chlorophyll a and PS Ⅱ activity of S. javanicum exhibited slower, and MDA content and soluble protein content increased slowly. At the same time, the osmosis of membrane decreased and the production of EPS increased. This acclimation to low temperature may be an ecological strategy for desert algae to tolerate cold stress. Results from the study showed that tolerance to low temperature was advanced when algal crust was subjected to supplement from nutrition salt. It also indicated that algal crust have better growth and photosystem activity under dark and low temperature, which might mean that the condition both low temperature and dark played some roles in resisting low-light inhibition.
5. Two desert algae, S. javanicum and Palmellococcus sp. were respectively subjected to desiccation under four relative humidity of 100%、75%、43% and 0 based on differences of air humidity owned by different saturated solutions. Results from this study indicated that PSⅡ activity decreased significantly due to the desiccation stress, and MDA content and SOD activity greatly increased along with the fluctuation of CAT activity when the desert alga were exposed to desiccation, which might signify some damages to membrane system and antioxidant system when the desert algae subjected to desiccation stress. It also indicated that the contents of soluble protein and soluble saccharide were increased when the algae were exposed to desiccation, which might represent a safeguard reaction to desiccation. PSⅡ activity obtained great recover when the desert algae were rehydrated by BG-110 medium, and recover in PSⅡ activity were also acquired when rehydrated by means of the solutions of saccharid, NAC and Pro. While PSⅡ activity was evidently inhibited by additions of DCMU and CMP during rehydration.
6. It indicated in the study that Palmellococcus sp. was damaged in physiological activities when exposed to salt stress, such as declines of PS Ⅱ activity, increases of MDA content and SOD activity, as well as fluctuation of CAT activity. Physiological activities of the green algae were recovered due to the additions of exterior sucrose and EPS when the algae were subjected to salt stress. It may be conclude that there are two ways of resisting salt stress for the species of green algae within biotic crusts. On the one hand, it is endogenesis regulation such as metabolic intensification of saccharide, emergent reactions from membrane system and antioxidant system. On the other hand, green algae in crusts may increase their resistance to salt stress by means of excretion of EPS and other saccharide from cyanobacteria within biotic crusts.
7. It also showed that the similar effects of salt stress on Scytonema javanicum were found when the cyanobacterium was subjected to salt stress, such as declines of PS Ⅱ activity, increases of MDA content and SOD activity, as well as fluctuation of CAT activity. Moreover, the damages of ultrastructure and the increase of EPS production in Scytonema javanicum were observed due to salt stress. Results indicated in the study that the lanthanon, Ce, was found to promote growth and activity of Scytonema javanicum in low concentration of Ce, while inhibition effects occurred in high concentration of Ce, and moderate concentration of Ce availed to relax the salt stress effects on Scytonema javanicum.|