Neglected thermal effects of color-temperature on algal growth in aquatic environment under light

Light and temperature critically regulate algal blooms; however, laboratory studies often fix temperature, neglecting light's thermal effects (endogenous photothermal effects, ETP) or equating them to external heating (exogenous thermal input, ETI). Color-temperature determines photon distribution, influencing thermal intensity in aquatic systems. In this study, Microcystis aeruginosa was exposed to five color-temperatures (1001-10,000 K) under thermostatic versus photothermal conditions, revealing that growth kinetics within optimal temperature ranges deviate from Arrhenius models, with color-temperature dictating thermal synergism or antagonism. Focusing on 1968 K, physiological and transcriptomic analyses demonstrated that dynamic thermal gradients (ΔT=4.8 °C) accelerated state transitions and sustained an efficient electron transport chain. Compared to thermostatic conditions, ETP and ETI resulted in 48 % and 24 % increases in electron transfer efficiency, respectively. ETP also synergistically upregulated carbon metabolism, optimizing energy conversion. In contrast, ETI obstructed electron transfer at the cytochrome b₆f complex, triggering photoprotection and reducing growth rate by 42 % and cell density by 19 % compared to ETP. Field data confirmed that photothermal fluctuations govern the timing of diurnal bloom peaks. These findings highlight light's dual role in photosynthesis and thermal feedback, urging reassessment of light-temperature synergies and advancing bloom mitigation strategies.