Mechanism of far-red adaptation in alveolate algae studied by two-dimensional electronic spectroscopy

There is a growing interest in understanding the diverse and still largely unknown mechanisms of far-red light adaptation used by photosynthetic systems, as this knowledge can have significant implications for enhancing photosynthetic yields. In this study, we employ two-dimensional electronic spectroscopy to investigate the primary light-harvesting complex from Chromera velia (CLH) and its far-red adapted oligomeric analog (FR-CLH), both of which lack resolved structural information. Our analysis reveals rapid relaxation in the far-red states of FR-CLH, typical of excitonic aggregates of chlorophylls. Following red band excitation, both samples display energy equilibration in 200 fs, followed by energy transfer to the far-red chlorophyll states in FR-CLH in 874 fs. By integrating these data with existing knowledge, we propose that excitonic interactions from oligomerization create far-red chlorophyll states in FR-CLH and discuss hypotheses regarding the pigment arrangement within the two antennas.