Utilization of minor red-shifted chlorophyll a for oxygenic photosynthesis under far-red light in green algae

Several eukaryotic alga species can perform oxygenic photosynthesis under far-red (FR) light using light-harvesting complexes that absorb this light. We previously isolated a green alga, Neochloris sp. Biwa 5-2, which can grow under FR LED light. However, the mechanisms that support photosynthesis and enable growth under such conditions remain unclear. Here, we performed spectroscopic analyses to investigate light harvesting and excitation-energy transfer in Neochloris sp. Biwa 5-2 under FR light. Two types of minor red-shifted chlorophyll a (Chl a), Chl 706 and Chl 718, were identified as pigments that can absorb light and contribute to oxygenic photosynthesis under FR conditions. Time-resolved fluorescence spectroscopy confirmed that these pigments were involved in excitation-energy transfer via an uphill energy transfer process. During this process, Neochloris sp. Biwa 5-2 uses constitutively present minor red-shifted Chl a to harvest FR light and sustain photosynthesis. This study appears to be the first report of minor Chl a functioning as a primary light-harvesting pigment that enables growth under FR light.