Morphological, molecular and ecological insights on Kryptoperidinium foliaceum strains from two Mediterranean lagoons (Sardinia, Italy)

Kryptoperidinium foliaceum (Stein) Lindemann is a harmful dinoflagellate categorized as high biomass bloom forming species. Its blooms have been associated with massive water discolorations, but also with fish mortality events. The species is widely distributed with a great tolerance range to salinity, being reported from almost freshwater to transitional ecosystems (i.e. coastal lagoons, marshes and estuaries), but also from hypersaline tidal creeks. Despite its wide distribution, information on the morphology, genetic and ecology of K. foliaceum is scarce. Recurrent blooms of K. foliaceum have been detected in Calich Lagoon (north west coast of Sardinia, Italy) in the last decade. The species was also seldom observed in Santa Giusta Lagoon (central west coast of Sardinia, Italy), where bloom events have not been registered in recent years. The aim of the present study was to investigate morphological, molecular and ecological features of K. foliaceum from the two lagoons. Firstly, the long-term ecological data from the field (2008-2015 for Calich, and 1990-2002 and 2010-2018 for Santa Giusta) were analyzed to characterize the two lagoons and the temporal dynamic of the species. Therefore, two cultured K. foliaceum strains from Calich and Santa Giusta lagoons were characterized for morphology and genetic. In addition, the growth rates of the two strains were determined at three different temperatures (11°C, 20°C and 30°C) and six salinities (5, 10, 20, 30, 40, 50). Field data showed the constant presence of K. foliaceum in Calich along the series, with highest cell densities mainly observed in spring (maximum of 4 x 106 cell L-1 in April 2015). On the contrary, the species was sporadically observed in Santa Giusta in the last decade, with cell densities usually rather low (<104 cell L-1) and the maximum observed in July 2013 (6 x 105 cell L-1). Vegetative cells from both strains were almost oval in shape and extremely flattened dorsoventrally, sharing the same plate formula 3', 2a, 7'', Xc, ?s, 5''', 2'''' and the same plate arrangement (e.g. very large 1'). The two strains also showed identical LSU rDNA and ITS sequences. Morphological and molecular results are in agreement with information available for other ‘European’ strains of K. foliaceum. The highest growth rates were detected at 20°C for both strains, and at different salinity depending on the strain. The results of this study combining laboratory and field data add new information on the scarcely investigated ecology of K. foliaceum. Further analyses will help elucidate the different behaviour of the species in the considered lagoons.