A truncated antenna mutant of the unicellular green alga Coccomyxa sp. strain Obi shows better biomass productivity than the wild-type strain under higher irradiance, at higher cell density, and in greater depth of culture

Abstract

Mutants with reduced chlorophyll contents were isolated from the industrial green alga Coccomyxa sp. strain Obi after chemical mutagenesis. The growth of most of the isolates was slower than that of the parental strain with one exception, which was named strain 5P. Chlorophyll a (Chl a) content in rapidly growing cells of strain 5P was >30 % lower than the wild-type level, while its chlorophyll b (Chl b) content was below 20 %. Strain 5P did not have trimeric light-harvesting antenna complexes (LHCs) for photosystem (PS) II, but its reaction centers for PSI and PSII were intact. Whole-genome resequencing of strain 5P identified many mutations, including a nonsynonymous mutation in the gene encoding chlorophyllide a oxygenase (CAO) involved in the biosynthesis of Chl b. Chl b is present only in LHCs, and the reduction of Chl b confers the reduction of LHCs that accompanies the reduction of Chl a. Genetic complementation of strain 5P with the genomic CAO gene (CAO) resulted in the full recovery of Chl a and Chl b contents. Thus, the low-chlorophyll phenotype of strain 5P was caused by the CAO mutation. The growth yields of strain 5P were superior to those of the wild-type strain in deeper cultures, and in nutrient-rich culture medium that supported cell growth to higher densities. The growth advantage over the wild-type strain was not observed in the CAO-complemented strains. We concluded that the mutant possessing a smaller antenna size exhibits a growth advantage under limited growth conditions.