Characteristic Features of Laser-Induced Fluorescence Parameters in Alexandrium catenella and Their Dependence on Temperature

Harmful algal blooms (HABs) pose a serious threat to public health, aquaculture, and coastal ecosystems, making the development of tools for their rapid and specific detection a high priority. Laser-induced fluorescence (LIF) spectroscopy enables the assessment of characteristic photosynthetic pigments, offering a pathway to automated, high-throughput monitoring systems. Here, we investigate the temperature dependency of LIF spectra in the range of 20–80 °C to establish stable fluorescence fingerprints for the harmful microalgae Alexandrium catenella. Critically, we demonstrate that the relationship between temperature and both fluorescence intensity and spectral position remains consistent over 35 days of cultivation, independent of culture age. We performed complementary flow cytometric and pigment analyses (HPLC) to characterize the culture’s physiological state. Over the 35-day period, cell concentration increased 20-fold, while cell size, granularity, and fluorescence spectra remained stable. A transient decrease in fluorescence intensity observed on day 10 coincided with a drop in peridinin concentration, confirming the link between the spectral signal and pigment composition. Obtained results validate the use of this fluorescence fingerprint for the reliable identification of A. catenella without prior knowledge of the culture’s age—a key advantage for field applications. Furthermore, these fingerprints remained clearly distinguishable even when the culture was diluted with seawater to just 3% of its original volume, underscoring the potential sensitivity of this approach for early warning systems.