Elvin de Araujo (Department of Chemical and Physical Sciences)

Abstract Description for Oral Presentation at 2011 CSM Conference:

Characterization of the CO2 concentrating mechanism in the Filamentous Cyanobacterium Leptolyngbya sp. CPCC 696

Elvin de Araujo¹, Rogers S.P.², de Araujo C.¹, Short S.M.¹, Campbell D.A.², Espie G.S.1

¹ University of Toronto, Mississauga
² Mount Allison University, New Brunswick

Explosive growth of toxin producing cyanobacteria is thought to be sustained by an adaptive biophysical CO₂ concentrating mechanism (CCM), which facilitates CO₂/HCO₃^- (Ci) acquisition and supports autotrophic growth in changing environments. To understand how the CCM responds to environmentally relevant fluctuations in light and Ci concentrations, we have examined the photosynthetic physiology of Leptolyngbya sp CPCC 696, a filamentous cyanobacteria isolated from an autumn bloom in the western basin of Lake Erie. Mass spectrometric and polarographic analysis of low light (15 μmol m^-2 s^-1 PPFD), and low Ci (30 μM) acclimated Leptolyngbya cells were capable of active HCO₃^- transport and NADPH mediated CO₂ uptake. These membrane transport processes lead to an intracellular accumulation of Ci that was 550 fold greater than extracellular levels and utilized up to 30 kJ/mol of energy from photosynthetic electron transport. Ci transport and accumulation occurred at photon flux densities well above the necessary levels to sustain optimal photosynthesis, suggesting there is an excess capacity for Ci accumulation over that of CO₂ fixation requirements. These processes occur without apparent photoinhibition even up to PPFD of 3000 μmol m^-2 s^-1. Ci uptake creates a substantial electrochemical gradient and was accompanied by a significant outward flux of CO₂ as well as simultaneous internal CO₂ / HCO₃^- cycling. In the presence of this high affinity, high capacity CCM, Leptolyngbya showed a low intrinsic susceptibility to photoinactivation of photosystem II (PSII) measured using the chlorophyll a fluorescence parameter, 1 ‐ qL. Beyond saturating levels of photosynthesis and Ci transport, susceptibility to photoinactivation increased, while the counteracting capacity for PSII repair dropped. The CCM of Leptolyngbya responds dynamically to fluctuating PPFD and utilizes excess light energy by increasing Ci transport, accumulation and Ci cycling. This capacity allows Leptolyngbya to tolerate periodic exposure to high light by consuming electron equivalents to keep PSII open, thereby limiting photooxidative damage. Thus, the surplus capacity of the CCM in Leptolyngbya sp. may serve a dual role both in Ci acquisition/concentration as well as in photoprotection.