Use it or lose it? Loss of grazing defenses during laboratory culture of the digestion-resistant green alga Oocystis

A strain of the green alga Oocystis isolated from a lake showed a decline in digestion defenses and an increase in growth rate over 3 years of culture. Changes over time were corroborated by comparisons between a sub-strain grown continuously in light and two sub-strains kept refrigerated in the dark most of the time. Continuous culture in light leads to a sharper decline in grazer defenses and an increase in growth rate. The decline in digestive defenses was evidenced by increases in carbon assimilation efficiency and juvenile growth rate of Daphnia feeding on the alga as well as decreases in the incidence and thickness of the protective gelatinous sheath of the alga. Moreover, two strains of Oocystis from culture collections isolated decades earlier showed no evidence of grazer defenses in comparison with a high quality control alga. At high algal concentrations, Daphnia juvenile growth rate increased from 0.10 to 0.38 day(-1) for field-isolated Oocystis over time in culture and ranged from 0.57 to 0.61 day(-1) for the three undefended algae. Our experiments suggest that grazing favors the evolution of digestion defenses in Oocystis at the cost of slower growth and show that caution is needed when using cultured algae in food chain experiments.