Mielke et al. 2013
, , R.E. Blankenship, and D. Mauzerall, 2013: Photosystem trap energies and spectrally-dependent energy-storage efficiencies in the Chl d-utilizing cyanobacterium, Acaryochloris marina. Biochim. Biophys. Acta Bioenerg., 1827, 255-265, doi:10.1016/j.bbabio.2012.11.002.
Acaryochloris marina is the only species known to utilize chlorophyll (Chl) d as a principal photopigment. The peak absorption wavelength of Chl d is redshifted — 40 nm in vivo relative to Chl a, enabling this cyanobacterium to perform oxygenic phototrophy in niche environments enhanced in far-red light. We present measurements of the in vivo energy-storage (E-S) efficiency of photosynthesis in A. marina, obtained using pulsed photoacoustics (PA) over a 90-nm range of excitation wavelengths in the red and far-red. Together with modeling results, these measurements provide the first direct observation of the trap energies of PSI and PSII, and also the photosystem-specific contributions to the total E-S efficiency. We find the maximum observed efficiency in A. marina (40±1% at 735 nm) is higher than in the Chl a cyanobacterium Synechococcus leopoliensis (35±1% at 690 nm). The efficiency at peak absorption wavelength is also higher in A. marina (36±1% at 710 nm vs. 3± 1% at 670 nm). In both species, the trap efficiencies are — 40% (PSI) and — 30% (PSII). The PSI trap in A. marina is found to lie at 740±5 nm, in agreement with the value inferred from spectroscopic methods. The best fit of the model to the PA data identifies the PSII trap at 723±3 nm, supporting the view that the primary electron-donor is Chl d, probably at the accessory (ChlD1) site. These results demonstrate that the E-S efficiency in A. marina is not thermodynamically limited, suggesting that oxygenic photosynthesis is viable in even redder light environments.