Study of akinetes in the sediments of large European lakes, and hatching experiments to infer long-term colonization dynamics by cyanobacteria

Distinct cyanobacteria belonging to Nostocales have the ability to form resting cells, which provide a mean to persist in harsh environmental conditions. Previous studies carried out in long-sediment cores, showed that akinetes could survive for long-time (up to over 60 years). Studying akinetes in lake sediments may provide a more comprehensive knowledge of past phytoplankton assemblages. The aim of this contribution is to study the colonization history of Nostocales by examining the vertical distribution of akinetes in the sediments of 4 European lakes included in the project EULAKES, namely lakes Garda (oligo-mesotrophic; Italy), Neusiedler (meso-eutrophic; Austria), Balaton (meso-eutrophic; Hungary), and Charzykowskie (eutrophic; Poland). Viability of akinetes was investigated by putting sediment taken from several sections of the core in ASM-1 culture medium at 18° for 16-21 days. The subsequent microscopic examination showed a very low diversity in Lake Garda, with the development of Dolichospermum lemmermannii in the first 10 cm (which correspond to the period since the 1980s). Lake Balaton showed a more diversified group of Nostocales, which included Aphanizomenon cf. gracile, followed by Aphanizomenon issatschenkoi, Aphanizomenon cf. capricorni, and Cylindrospermopsis raciborskii. A. cf. gracile was observed down to ca. 50 cm (>100 yr), whereas A. issatschenkoi and A. cf. capricorni were reported in the more recent sediments and more discontinuously. Anabaena viguieri was a typical representative in Lake Neusiedler down to 50 cm of core sediment (corresponding to the last ca. 60 years), along with Aphanizomenon gracile (in the first 10 cm, around 2000) and some unidentified Anabaena (between 20-60 cm, since early 1900s). In Lake Charzykowskie, the only Nostocales were Anabaena solitaria down to 60 cm of core sediment (late 1940s) along with an unidentified Anabaena sp. The observed differences provide an important platform to interpret the distribution of cyanobacteria in European lakes, which are characterized by different physiographic, climatic and trophic characteristics