The researchers studied how spontaneous mutations affect the fitness of marine green algae. They conducted experiments on four species of algae and found that cell division rates decreased over time in one species, but not in the others. When exposed to stressful conditions, like changes in osmolarity or herbicides, mutations had both positive and negative effects on fitness. This shows that the impact of mutations depends on the environment. The minimum mutation rate in these algae species was estimated to be 0.0037 per genome per generation.