Evidence for a robust photosystem II in the photosynthetic amoeba Paulinella.

Arwa Gabr, Apostolos Zournas, Timothy G Stephens, G Charles Dismukes, Debashish Bhattacharya

The New phytologist 2022 May

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Paulinella represents the only known case of an independent primary plastid endosymbiosis, outside Archaeplastida, that occurred c. 120 (million years ago) Ma. These photoautotrophs grow very slowly in replete culture medium with a doubling time of 6-7 d at optimal low light, and are highly sensitive to photodamage under moderate light levels. We used genomic and biophysical methods to investigate the extreme slow growth rate and light sensitivity of Paulinella, which are key to photosymbiont integration. All photosystem II (PSII) genes except psb28-2 and all cytochrome b6 f complex genes except petM and petL are present in Paulinella micropora KR01 (hereafter, KR01). Biophysical measurements of the water oxidation complex, variable chlorophyll fluorescence, and photosynthesis-irradiance curves show no obvious evidence of PSII impairment. Analysis of photoacclimation under high-light suggests that although KR01 can perform charge separation, it lacks photoprotection mechanisms present in cyanobacteria. We hypothesize that Paulinella species are restricted to low light environments because they are deficient in mitigating the formation of reactive oxygen species formed within the photosystems under peak solar intensities. The finding that many photoprotection genes have been lost or transferred to the host-genome during endosymbiont genome reduction, and may lack light-regulation, is consistent with this hypothesis. © 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation.

Citation

Arwa Gabr, Apostolos Zournas, Timothy G Stephens, G Charles Dismukes, Debashish Bhattacharya. Evidence for a robust photosystem II in the photosynthetic amoeba Paulinella. The New phytologist. 2022 May;234(3):934-945