Cellular characteristics and growth behavior of iron-limited Microcystis aeruginosa in nutrient-depleted and nutrient-replete chemostat systems

Cellular responses of Fe-limited Microcystis aeruginosa were investigated under nutrient-depleted and -replete conditions. Cellular growth, Fe quota and Fe uptake kinetics were examined in chemostat systems using nutrient-replete Fraquil(*) (where all nutrients except for Fe are present at sufficient level to achieve optimal growth) and nutrient-deplete Fraquil(*) (where some nutrients in addition to Fe are potentially growth-limiting factors). For both nutrient conditions, cellular Fe quota increased with increasing dilution rate in a manner consistent with Droop theory. However, the Fe quota in nutrient-deplete Fraquil(*) was determined to be lower, indicating lower cellular Fe requirement in the nutrient-depleted condition. Short-term Fe uptake assays indicated that cells acclimated in nutrient-replete conditions adjust to various degrees of Fe stress by solely increasing maximum Fe uptake rate, consistent with expected negative feedback regulation. In contrast, the maximum Fe uptake rate decreased with increasing degree of Fe limitation in the nutrient-depleted chemostat (particularly nitrate and molybdenum in this study). This non-negative feedback regulation is likely associated with lower Fe requirement for specific functions (e.g., intracellular nitrate reduction). Cellular affinity for Fe uptake and cellular size were independent of degree of Fe stress for both nutrient conditions.