Induction and characterization of pigment mutants in the red microalga Porphyridium sp.(Rhodophyceae)

Several pigment mutants of the red microalga Porphyridium sp. were isolated after induced mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine (NNG). The mutants were screened on agar plates incubated at 150 μmol photons m⁻² s⁻¹ for clones that did not show red pigmentation. After two weeks of incubation, three green or greenish-yellow mutants, C-12, 0-10 and F-11, were chosen for further characterization. The mutation frequency of these mutants ranged between 5 × 10⁻⁶ and 8 × 10−6, their phenotypic pigmentation was stable over many consecutive transfers, and no reversion to the wild phenotype was observed. The reversion frequency was estimated to be less than 3 × 10⁻⁸. The most striking feature of the mutants was their low phycoerythrin content. When the mutants were cultured under conditions of high photon flux density (300 μmol photons m−2 s−1) no phycoerythrin was detected in cells of mutants D-10 and F-11, while mutant C-12 produced about 25% of the phycoerythrin found in the wild type cells. However, when cultured at a low photon flux density (40 μmol photons m⁻² s⁻¹) all mutants produced phycoerythrin, although at a level that ranged between 5 and 30% of that of the wild type. Similarly, during growth at high photon flux density, the chlorophyll content of the mutants was lower than that of the wild type. Under conditions of low photon flux density the chlorophyll values in mutant cells were higher than those found at high photon flux density and were comparable with those of the wild type. No significant difference was found between the phycocyanin content of the mutants and that of the wild type. The growth rate of all mutants was found to be similar to that of the wild type. However, when the mutants were cultured at high photon flux density, the cell biomass of two of them (D-10 and C-12) was lower than that of the wild type. This could be partially attributed to the reduced amount of cell wall polysaccharide in these mutants. Two of the mutants (C-12 and F-11) showed similar or slightly higher photosynthetic performance compared with the wild type, on a per cell basis, while all three had higher Pmax values and mutant F-11 had a higher α value when normalized to the chlorophyll α level.