Studies On The CO₂ Concentrating Mechanism And Ecophysiology Of The Edible Cyanobacterium Ge-Xian-Mi

Ge-Xian-Mi (Nostoc) is an edible cyanobacterium. It occupies a prestigious position in Chinese human social behavior and has been used as a food delicacy or ingredient of Chinese medicine since the Eastern Jin Dynasty. This species has been found in mountain paddy fields of Lingqiu (Shanxi Province), Hefeng (Hubei Province), Fangxian (Hubei Province), Xiangfan (Hubei Province), Shennongjia Forest District (Hubei Province), Beiliu (Gunagxi Province) and Renhua (Guangdong Province). The distribution and ecology of Ge-Xian-Mi in rice fields of Hefeng County in China were investigated. Furthermore, the CO2 concentrating mechanism (CCM) in cyanobacterium was reviewed and the utilization of inorganic carbon in Ge-Xian-Mi in addition to its role in alleviating the photoinhibition were studied in the present paper.Hefeng is a montane county in the western parts of Hubei province near to Hunan. There are 796 ha of rice fields suitable to its growth in the County of Hefeng and the maximum annual yield has reached to 25 t. However, the annual yield has been reduced to 0.5 t at present. Ge-Xian-Mi grows in paddy fields around the town of Zouma Hefeng County and its habitats are located (2938'N, 11038'E) at about 960 m above sea level. The annual mean temperature is about 12.2C, and the annual rainfall is 1934 mm with mean relative humidity of 78-87%. The distribution of Ge-Xian-Mi was associated with the source of water and the pH values of water samples suitable to its growth were 6.2-6.3. Soils in its habitats were enriched with phosphorus and contained more microbes than those without its distribution. With changing agricultural techniques most of the habitats were endangered or already extinct. As a consequence, Ge-Xian-Mi has become vulnerable, rare or extinct in various parts of China where it has been known in the past. The widespread use of herbicides, pesticides and fertilizers containing chlorine had been suggested to be an important factor limiting its distribution. Meanwhile, the taxonomic identity of Ge-Xian-Mi was discussed in this paper.Ge-Xian-Mi used HCO3- in addition to CO2 for its photosynthesis and oxygen evolution was greater than the theoretical rates of CO2 production derived from uncatalyzed dehydration of HCO3-. Multiple transporters for CO2 and HCO3-operated in air-grown Ge-Xian-Mi. Na+-dependent HCO3- transport was the primary mode of active inorganic carbon (Ci) uptake and contributed to 52.8% -62.8% of net photosynthetic activity at 250 umol-L-1 KHCO3 and pH 8.0. However, the CO2-uptake systems and Na+-independent HCO3- transport played minor roles in Ge-Xian-Mi and supported respectively 39.4% and 7.6% of net photosynthetic activity. The steady-state fluorescence decreased and the photochemical quenching increased in response to the transport-mediated accumulation of intracellular Cj. Inorganic carbon transport was a major factor in facilitating quenching during the initial stage and the initial rate of fluorescence quenching in the presence of iodoacetamide, an inhibitor of CO2 fixation, was 87.9% of control. Both the initial rate and extent of fluorescence quenching increased with increasing external dissolved inorganic carbon (DIC) and saturated at higher than 200 utnol L-1 HCO3-. The operation of CCM in Ge-Xian-Mi served as a means of diminishing photodynamic damage by dissipating excess light energy and higher external DIC in the range of 100-10000 umol-L-1 KHCO3 was associated with more severe photoinhibition under strong irradiance.