Impacts Of Solar Radiation On Morphology, Photosynthesis And Growth Of The Economic Cyanobacterium Arthrospira (Spirulina) Platensis

The relationship of morphological characteristics and metabolisms of filamentous Arthrospira (Spirulina) platensis with environmental changes has been the focus of this economically important cyanobacterium. Spiral structure of A. platensis is known to alter according to environmental changes, however, little has been documented on the mechanisms about the effects of solar radiation changes on its morphology. In addition, effects of solar ultraviolet radiation (UVR) on its photosynthesis and growth also need to be further studied. Therefore, impacts of photosynthetic active (PAR) and UV radiations as well as their combined effects with temperature and DIC on its morphology, growth and photosynthesis were investigated in this study. The main results are as following:The spiral structure was affected by PAR and UVR, but such effects of PAR and UVR were dependant on temperature. At temperature levels lower than 20 oC, the helix pitch of A. platensis (D-0083) became smaller due to the interactive effect of PAR and UVR, however, at temperature levels (25-35 oC) suitable for its growth, irradiation with PAR alone resulted compressed spirals. Although change of the compressed helix pitch happened faster in existence of UVR, UVR alone did not tighten the helix. With the technique of SDS-PAGE for separation of the proteins washed off from the cellular membrane, it was found that a protein of 52.0 kDa was responsible for the compression of the spirals. The tightened spiral structure played a protective role against high levels of UVR or PAR by increasing shelf-shading among the cells, alleviating the photo-damage of PSII and reducing the breakage of trichomes. The compression of the helix pitch did not depend on specific wavebands of PAR, nevertheless, the blue (400-500 nm) and red light (610-700 nm) were the most effective components. The infrared radiation (>700 nm) had no effect on its morphology at all. Furthermore, different wavebands of PAR showed discrepant effects on the growth and photosynthesis, blue and red light was more effective for its growth too. The efficiency of blue and red light on the spiral compression might be due to its higher efficiency for driving higher rate of photosynthesis, which runs the biochemical machinery for generation of proteins.The spiral filaments of A. platensis broke when irradiated with high levels of PAR and/or UVR. This was proved to link to the accumulation of reactive oxygen species (ROS) in the cells. High PAR and UVR suppressed the activity of superoxide dismutase (SOD) and catalase (CAT), leading to an increase of ROS in the cells. The accumulated ROS damaged chlorophyll a, phycobilisome and PSII, decreased the photosynthetic electrode transfer rate (ETR) and accelerated the oxidation of cell membrane (with an increased production of malondialchehyche, MDA), which was thought to result the spiral breakage.The limitation of dissolved inorganic carbon (DIC) could change the filaments'morphology too. When the DIC concentration in the culture medium was reduced to 0.3-4.0 mmol/L, the spiral filaments broke even under moderate levels of PAR and the trichomes with much smaller size appeared. DIC limitation decreased the contents of phycocyanin (PC) and allophycocyanin (APC) in the cells, however, the contents of carotenoid (CAR) increased. Compared with filaments cultured with normal Zarrouk medium, the maximal photosynthetic rate of adapted cells in medium with much lower DIC concentration decreased by 25% and the apparent affinity for DIC (K1/2(DIC)) increased about 14 times.Buoyancy provided by gas vesicles has been suggested to play important roles in regulating vertical distribution and nutrient acquisition in cyanobacteria. However, little is known about how PAR (400-700 nm) as well as UV radiation (UVR, 280-400 nm) which change with day time and depth would affect the buoyancy. In this study, it was demonstrated that the floatation activity of A. platensis decreased with increased photosynthetic rates associated with increased PAR, but it decreased less in the presence of UVR that resulted inhibitory effects. When the cells were grown under isoenergetic levels of solar PAR or UVR alone, they migrated downward under the PAR but maintained buoyant under the UVR. The buoyancy regulation of A. platensis depended on the exposed levels of PAR as well as UVR, which affected photosynthesis and growth in an antagonistic way. The buoyancey of A. platensis in water columns is much likely to be dependant on diurnal photosynthetic performance regulated by solar radiation, and can hardly be considered as an active strategy to gain more energy during sunrise/sunset or to escape from harmful irradiations during noon period.