Autunmal sedimentation of Microcystis spp. as result of an increase in carbohydrate ballast at reduced temperature

Autumnal sedimentation of the Microcystis population was studiedin Lake Nieuwe Meer (Amsterdam, The Netherlands). In summer,Microcystis formed a high percentage of the total phytoplanktonin the water column, but a low percentage in sedimentation traps.The reverse was found during September and October, with a highpercentage in the sedimentation traps, but a low percentagein the water column. The decrease in the numbers of Microcystiscolonies coincided with a decrease in waler temperature. Inexperiment with a strain of Mirocystis, isolated from Lake NieuweMeer, the percentage of total colonies that were sinking increasedin a few days to 100% after a shift from 20C to 15.3, 13.0or 10.5C. The gas vesicle volume in the cells remained constantduring the incubations. Sinking of the colonies resulted froman increased glycogen content. Calculation of carbon (C) flowsduring the first 2 days of the incubation at reduced temperatureshowed that the glycogen accumulation was the result of a muchlower rate of protein synthesis during the light period at thelower temperatures. Although the photosynthetic rate itselfdecreased at reduced temperature, it resulted in more fixedCO₂ being stored as glucose. Because the respiratory rate alsodecreased (with an almost similar decrease to that of photosynthesis),glycogen accumulated at lower temperatures. It was calculatedthat after an incubation period of-I week at reduced temperature,the rate of photosynthesis had decreased by 10.1% of the valueat 20C per 1C, while the rate of respiration had decreasedonly 1.8%. It is proposed that there is a feedback mechanismin which an increasing concentration of glycogen inhibits photosynthesisand stimulates respiration.