A mutant of the red microalga Porphyridium sp. (Rhodophyceae) resistant to DCMU and atrazine

A mutant (DC-2) of the unicellular red alga Porphyridium sp. (UTEX 637) resistant to the herbicide diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea, DCMU] was isolated after induced mutagenesis with N-methyl-N′-nitro-N-nitrosoguanidine (NNG) at a frequency of 2.1 × 10⁻⁵. The minimal concentration of DCMU that completely inhibited growth of the wild type was fiftyfold lower than that required for complete inhibition of the mutant's growth. The mutant was found to be stable over several consecutive growth cycles in the absence of DCMU. When the wild type was exposed to a sub-lethal concentration of DCMU (0.2 μM),its photosynthesis was reduced by up to 72%, while the mutant was not affected by this concentration. In the presence of up to 5.0 μM DCMU the Pmax of the mutant was slightly reduced. However, these concentrations completely inhibited photosynthesis in the wild type. DCMU at a concentration of 0.2 μM decreased the cellular level of phycobiliproteins (phycoerythrin and phycocyanin) but not of chlorophyll a, while no change was recorded in the level of these pigments in the mutant culture grown with 2.5 μM DCMU. Sulphated polysaccharides responded in the opposite way to phycobiliproteins: their total content did not change at 0.2 μM DCMU in the wild type, while that of DC-2 was reduced by 38%. DC-2 mutant proved to be resistant to another photosystem II-inhibiting herbicide, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine). It grew in a medium containing up to 100-μM atrazine, whereas growth of the wild type was completely inhibited at 5.0 μM.