Stabilization and modulation of the phycobilisome by calcium in the calciphilic freshwater red alga Bangia atropurpurea

The bangiophycean filamentous red alga Bangia atropurpurea is distributed in freshwater habitats such as littoral and splash zones of lakes or rapid currents distant from the sea. In these habitats, the distribution and growth of this alga appear to be related to hard water rich in calcium ions. To characterize the eco-physiological properties of this calciphilic red alga, we examined the effects of long-term and short-term Ca2+ depletion on photosynthetic growth of the thallus and on the phycobilisome. Long-term culture experiments suggested that higher Ca2+ concentrations (>50 mg L-1) were required to sustain thallus growth and pigmentation of cells. In short-term Ca2+-depletion treatments, fluorescence derived from phycoerythrin (PE) fluctuated, although the absorption spectra of the thalli did not change. After 30 min of Ca2+ depletion, the fluorescence lifetime of PE became markedly longer, indicating that the energy transfer from PE to phycocyanin (PC) was suppressed. The fluorescence lifetime of PE returned to its original value within a short time after 4 h of Ca2+ depletion, however, energy transfer from PE to PC was still suppressed. This suggested that the excitation energy absorbed by PE was quenched during prolonged Ca2+ depletion. The efficient energy transfer from PC and allophycocyanin were unchanged during these treatments. (C) 2011 Elsevier B.V. All rights reserved.