Abstract
Light-harvesting complex II (LHCII) from the marine green macroalga Bryopsis corticulans is spectroscopically characterized to understand the structural and functional changes resulting from adaptation to intertidal environment. LHCII is homologous to its counterpart in land plants but has a different carotenoid and chlorophyll (Chl) composition. This is reflected in the steady-state absorption, fluorescence, linear dichroism, circular dichroism and anisotropic circular dichroism spectra. Time-resolved fluorescence and two-dimensional electronic spectroscopy were used to investigate the consequences of this adaptive change in the pigment composition on the excited-state dynamics. The complex contains additional Chl b spectral forms – absorbing at around 650 nm and 658 nm – and lacks the red-most Chl a forms compared with higher-plant LHCII. Similar to plant LHCII, energy transfer between Chls occurs on timescales from under hundred fs (mainly from Chl b to Chl a) to several picoseconds (mainly between Chl a pools). However, the presence of long-lived, weakly coupled Chl b and Chl a states leads to slower exciton equilibration in LHCII from B. corticulans. The finding demonstrates a trade-off between the enhanced absorption of blue-green light and the excitation migration time. However, the adaptive change does not result in a significant drop in the overall photochemical efficiency of Photosystem II. These results show that LHCII is a robust adaptable system whose spectral properties can be tuned to the environment for optimal light harvesting.
Highlights
Bryopsis corticulans (B. corticulans) is a marine green macroalga, growing in the rocky intertidal shores in the form of a single-cell thallus with tubular filament siphons
B. corticulans is more distant to plants than other green algae such as well-studied Chlamydomonas reinhardtii, its Lhcbm apoprotein has >70% sequence identity with the major light-harvesting complex II (LHCII) (Lhcb1–3) in higher plants and is expected to possess a similar structure with three transmembrane α-helical regions [4,5,6]
The peak positions and their amplitudes slightly differ between B. corticulans and P. sativum owing to the different pigment composition of the two species
Summary
Bryopsis corticulans (B. corticulans) is a marine green macroalga, growing in the rocky intertidal shores in the form of a single-cell thallus with tubular filament siphons. The spectral adaptation involves changes in the pigment composition of light-harvesting complex II (LHCII) – the membrane-intrinsic chlorophyll (Chl) a/b-binding peripheral antenna of Photosystem II (PSII). B. corticulans is more distant to plants than other green algae such as well-studied Chlamydomonas reinhardtii, its Lhcbm apoprotein has >70% sequence identity with the major LHCII (Lhcb1–3) in higher plants and is expected to possess a similar structure with three transmembrane α-helical regions [4,5,6]. Instead of the xanthophylls lutein and violaxanthin present in higher plants, B. corticulans contains siphonaxanthin and BBA - Bioenergetics 1861 (2020) 148191 siphonein – unique keto-carotenoids [5,6,7] which are more reminiscent of the carotenoids found in the light-harvesting antenna complexes of red algae and diatoms. The amino acid sequence of the Lhcbm proteins in B. corticulans is altered to accommodate the large hydroxyl group of siphonaxanthin and the large ester group of siphoneins [4,5,6]
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