Detection of the bloom-forming cold-water dinoflagellate Biecheleria baltica in the Baltic Sea using LSU rRNA probes

Cold-water dinoflagellates frequently dominate the spring phytoplankton community of the northern Baltic Sea and contribute substantially to the spring primary production. These dinoflagellate communities are largely composed of 3 different species (Biecheleria baltica, Scrippsiella hangoei, Gymnodinium corollarium) that cannot be unambiguously separated by conventional light microscopy. In this study, a fluorescence in situ hybridization (FISH) assay was developed for the detection of B. baltica in field samples. The probe-based method was tested and optimized in vitro, and potential effects of the B. baltica life-cycle transitions were evaluated by examining the labeling quality of the probes on different life-cycle stages before the assay was applied in a field study. The BbRNA4 probe had the highest specificity and was chosen for downstream applications. The life-cycle experiment showed significant differences in labeling efficiency between cultured cells from different growth phases, particularly for the nutrient-limited treatment compared to the control. Tests with spiked field samples revealed that cell recovery in the FISH assay was low (30%), resulting in a considerable underestimation of in situ abundances. However, a strong relationship between FISH and Utermohl counts of field samples suggests that trends of the bloom dynamics can be followed reliably by this method. By applying the FISH detection method in a field survey, we found that the dynamics of the B. baltica bloom follows that established earlier for the Scrippsiella/Biecheleria/Gymnodinium complex, with 2 peaks throughout the season. When corrected for processing losses, FISH-based abundance estimates suggest that B. baltica comprised the major fraction of the total dinoflagellate complex during the spring bloom at the southwest coast of Finland.