Different functions of Lhcx isoforms in photoprotective mechanism in the marine diatom Thalassiosira pseudonana

Photosynthesis needs light energy, but that exceeding the maximal capacity of photosynthesis enhances formation of reactive oxygen species, which potentially causes photodamages. Therefore, light-harvesting complexes (Lhc) in phototrophs harbor various proteins and pigments to function in both light capture and energy dissipation. Diatom Lhcx proteins are reported to be a critical component for thermal dissipation of excess light energy, but the molecular mechanism of photoprotection is still not fully understood and the functions of each Lhcx isoform are not yet differentiated. Here, we focused on two types of Lhcx isoforms in Thalassiosira pseudonana : TpLhcx1/2, putative major components for energy-dependent fluorescence quenching (qE); and TpLhcx6_1, functionally unknown isoform uniquely conserved in Thalassiosirales. TpLhcx1/2 proteins accumulated more under high light than under low light, while the TpLhcx6_1 protein level was constitutive irrespective of light intensities and CO2 concentrations. High-light induced photodamage of photosystem II was increased in the genome-editing transformants of these Lhcx isoforms relative to the wild-type. Transformants lacking TpLhcx1/2 showed significantly lowered qE capacities, strongly suggesting that these proteins are important for the fast thermal energy dissipation. While in contrast, genome-editing transformants lacking the TpLhcx6_1 protein rather increased the qE capacity. TpLhcx6_1 transformants were further evaluated by the low-temperature time-resolved chlorophyll fluorescence measurement, showing the longer fluorescence lifetime in transformants than that in the wild type cells even at the dark-acclimated state of these cells. These results suggest that TpLhcx6_1 functions in photoprotection through non-photochemical energy dissipation in the different way from qE.