Nostoc flagelliforme (Berk. & Curtis) Bomet & Flab. is an edible terrestrial cyanobacterium and Microcystis aeruginosa Kutz. is one species responsible for harmful freshwater blooms. On the basis of reviewing previous researches on inorganic carbon utilization of cyanobacteria, plants and increasing atmospheric CO_2, biology of N. flagelliforme and freshwater blooms, the photosynthetic responses of N. flagelliforme to environmental factors during the cycle of dry→wet→dry, and the responses of M. aeruginosa to elevated CO, were investigated in this paper. Main results were as follows:Photosynthetic recovery in dried field populations ofN. flagelliforme during rehydration was a light-dependent process. Addition of K~+ resulted in significant enhancements of P_n and PSII photochemical efficiency, but other nutrients (Fe~(3+), Mg~(2+), Na~+, PO_4~(3-), NO_3~-, Cl~-) and trace metals (A_5) showed little effect. Potassium content in N. flagelliforme affected the extent of photosynthetic recovery. Once it had absorbed sufficient potassium, N. flagelliforme no longer required exogenous addition for the photosynthetic recovery. Net photosynthesis increased to reach a maximum at about 30% water loss, then decreased, while dark respiration always decreased with the progress of desiccation. Photosynthetic efficiency was maximal at the beginning of desiccation and then reduced with increased water loss. Desiccation markedly affected the light requirement of photosynthesis but its effect on light compensation points was insignificant. Elevated CO_2 enhanced the photosynthetic activity, narrowed the scope of optimum temperature, promoted the light utilization efficiency, decreased the apparent photosynthetic affinity to CO_2 and dark respiratory activity during desiccation. Photorespiratory CO_2 release could not be detected during the course of desiccation. Furthermore, daily production of high-CO_2-grown samples with twice daily watering in high light zone was the maximum. Elevated CO_2 enhanced the growth of M. aeruginosa by 52 - 77% in the late exponential and stationary phase. Pigment composition did not alter with the increase of CO_2 concentration, but the content of chlorophyll a per cell decreased. Dark respiration and photosynthetic efficiency normalized with chlorophyll a content seemed to increase at elevated CO_2, while light-saturated photosynthesis per cell and I_k values seemed to decrease. M. Aeruginosa showed high apparent photosynthetic affinity for DIC, and DIC saturated photosynthesis and the K_(0.5)(DIC) values were little affected by the increase of CO_2 concentration. The photosynthetic characteristics of N. flagelliforme were different from M. Aeruginosa, the former could be sensitive to elevated CO_2. Meanwhile, their responses to light and temperature regimes were distinct. These reflected their interspecific difference and adaptability to their habitats.