Cellular N:P ratio of Microcystis as an indicator of nutrient limitation—implications and applications

This study tests the hypothesis that the cellular N:P ratio of the cyanobacterium Microcystis remains at its optimum level if the N:P ratio in the external environmental remains at similar levels. To analyze the relationship between cellular and environmental N:P ratio, Microcystis was cultured in media with varying N:P mass ratios (5, 8.75, 13.3, 25 and 40) for 35 days. In all treatments, the cell density of Microcystis increased throughout the first 15 days and remained stable thereafter. The final N:P mass ratios in the culture medium were 1.0, 3.8, 36, 196 and 437, respectively. The ultimate cellular mass N:P ratios (5.1, 7.1, 11.4, 14.2 and 16.7) were reached and stabilized during the final 15 days. The stable cellular mass N:P ratio equaled the N:P ratio in both the initial and final culture medium if the N:P ratio in the initial culture medium was approximately 7 (mass ratio, known as the Redfield ratio). The Redfield ratio can be explained by the physiology of Microcystis under the culture conditions. The study suggests that cellular N:P ratio is a reliable indicator of nutrient limitation for Microcystis. In addition, the cellular N:P ratio of Microcystis occupying Lake Taihu (China) was investigated. The mass ratio always exceeded 7 during the period from July to November, indicating that Microcystis in Lake Taihu is P limited.