Physiological characterization and light response of the CO2-concentrating mechanism in the filamentous cyanobacterium Leptolyngbya sp. CPCC 696
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We studied the interactions of the CO2-concentrating mechanism and variable light in the filamentous cyanobacterium Leptolyngbya sp. CPCC 696 acclimated to low light (15 μmol m−2 s−1 PPFD) and low inorganic carbon (50 μM Ci). Mass spectrometric and polarographic analysis revealed that mediated CO2 uptake along with both active Na+-independent and Na+-dependent HCO3 − transport, likely through Na+/HCO3 − symport, were employed to concentrate Ci internally. Combined transport of CO2 and HCO3 − required about 30 kJ mol−1 of energy from photosynthetic electron transport to support an intracellular Ci accumulation 550-fold greater than the external Ci. Initially, Leptolyngbya rapidly induced oxygen evolution and Ci transport to reach 40–50% of maximum values by 50 μmol m−2 s−1 PPFD. Thereafter, photosynthesis and Ci transport increased gradually to saturation around 1,800 μmol m−2 s−1 PPFD. Leptolyngbya showed a low intrinsic susceptibility to photoinhibition of oxygen evolution up to PPFD of 3,000 μmol m−2 s−1. Intracellular Ci accumulation showed a lag under low light but then peaked at about 500 μmol photons m−2 s−1 and remained high thereafter. Ci influx was accompanied by a simultaneous, light-dependent, outward flux of CO2 and by internal CO2/HCO3 − cycling. The high-affinity and high-capacity CCM of Leptolyngbya responded dynamically to fluctuating PPFD and used excitation energy in excess of the needs of CO2 fixation by increasing Ci transport, accumulation and Ci cycling. This capacity may allow Leptolyngbya to tolerate periodic exposure to excess high light by consuming electron equivalents and keeping PSII open.
KeywordsCi pump/leak cycle CO2-concentrating mechanism CO2 uptake Cyanobacteria Leptolyngbya Na+-dependent HCO3− transport Light response Photoprotection Photosynthesis
This work is supported through Discovery grants from the Natural Sciences and Engineering Councils of Canada (NSERC) to Steven M. Short, Douglas A. Campbell and George S. Espie. Elvin D. de Araujo is the recipient of an NSERC-USRA grant and Charlotte de Araujo is the recipient of a NSERC—Canada Graduate Scholarship.
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