Abstract
The Baltic Sea is a shallow basin of brackish water in which the spatial salinity gradient is one of the most important factors contributing to species distribution. The Baltic Sea is infamous for its annual cyanobacterial blooms comprised of Nodularia spumigena, Aphanizomenon spp., and Dolichospermum spp. that cause harm, especially for recreational users. To broaden our knowledge of the cyanobacterial adaptation strategies for brackish water environments, we sequenced the entire genome of Dolichospermum sp. UHCC 0315, a species occurring not only in freshwater environments but also in brackish water. Comparative genomics analyses revealed a close association with Dolichospermum sp. UHCC 0090 isolated from a lake in Finland. The genome closure of Dolichospermum sp. UHCC 0315 unraveled a mixture of two subtypes in the original culture, and subtypes exhibited distinct buoyancy phenotypes. Salinity less than 3 g L−1 NaCl enabled proper growth of Dolichospermum sp. UHCC 0315, whereas growth was arrested at moderate salinity (6 g L−1 NaCl). The concentrations of toxins, microcystins, increased at moderate salinity, whereas RNA sequencing data implied that Dolichospermum remodeled its primary metabolism in unfavorable high salinity. Based on our results, the predicted salinity decrease in the Baltic Sea may favor toxic blooms of Dolichospermum spp.
Highlights
The Baltic Sea is one of the largest brackish water ecosystems in the world
The Baltic Sea is notorious for its annual cyanobacterial blooms composed of Nodularia spumigena, Aphanizomenon flos-aquae, and Dolichospermum spp.[9,10,11]
UHCC 0315 was obtained using Pacific Biosciences (PacBio) chemistry, and sequencing errors were corrected with Illumina Hiseq2500 reads by two rounds of corrections (Freebayes 217 errors and Pilon 8 errors)
Summary
The Baltic Sea is one of the largest brackish water ecosystems in the world It undergoes substantial spatiotemporal variation in the salinity gradient maintained by uneven saline-water inflow via the Danish Straits and varying amounts of riverine runoff. The Baltic Sea is notorious for its annual cyanobacterial blooms composed of Nodularia spumigena, Aphanizomenon flos-aquae, and Dolichospermum spp. Dolichospermum spp. is a filamentous and nitrogen-fixing cyanobacterial species that typically forms massive blooms in freshwater environments[14,15,16]. We have previously defined seven Nostocales subgroups (I a–d; II a–c), based on the comparison of average-nucleotide and amino-acid identities of 30 sequenced Nostocales strains[23] In this classification, Dolichospermum/Anabaena species were found in four subgroups, implementing a high demand for further taxonomic clarification. Comparative genomic analysis was applied to elucidate the ambiguity within the sequenced Dolichospermum/Anabaena species
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