Nodularia spumigena is a filamentous diazotrophic cyanobacterium that dominates the annual late summer cyanobacterial blooms in the Baltic Sea. But N. spumigena also is common in brackish water bodies worldwide, suggesting special adaptation allowing it to thrive at moderate salinities. A draft genome analysis of N. spumigena sp. CCY9414 yielded a single scaffold of 5,462,271 nucleotides in length on which genes for 5,294 proteins were annotated. A subsequent strand-specific transcriptome analysis identified more than 6,000 putative transcriptional start sites (TSS). Orphan TSSs located in intergenic regions led us to predict 764 non-coding RNAs, among them 70 copies of a possible retrotransposon and several potential RNA regulators, some of which are also present in other N2-fixing cyanobacteria. Approximately 4% of the total coding capacity is devoted to the production of secondary metabolites, among them the potent hepatotoxin nodularin, the linear spumigin and the cyclic nodulapeptin. The transcriptional complexity associated with genes involved in nitrogen fixation and heterocyst differentiation is considerably smaller compared to other Nostocales. In contrast, sophisticated systems exist for the uptake and assimilation of iron and phosphorus compounds, for the synthesis of compatible solutes, and for the formation of gas vesicles, required for the active control of buoyancy. Hence, the annotation and interpretation of this sequence provides a vast array of clues into the genomic underpinnings of the physiology of this cyanobacterium and indicates in particular a competitive edge of N. spumigena in nutrient-limited brackish water ecosystems.

Massive blooms of toxic cyanobacteria frequently occur in the central Baltic Sea during the summer. In the surface scum, cyanobacterial cells are exposed to high light (HL) intensity, high oxygen partial pressure and other stresses. To mimic these conditions, cultures of Nodularia spumigena CCY9414, which is a strain isolated from a cyanobacterial summer bloom in the Baltic Sea, were incubated at a HL intensity of 1200 lmol photons m À 2 s À 1 or a combination of HL and increased oxygen partial pressure. Using differential RNA sequencing, we compared the global primary transcriptomes of control and stressed cells. The combination of oxidative and light stresses induced the expression of twofold more genes compared with HL stress alone. In addition to the induction of known stressresponsive genes, such as psbA, ocp and sodB, Nodularia cells activated the expression of genes coding for many previously unknown light-and oxidative stress-related proteins. In addition, the expression of non-protein-coding RNAs was found to be stimulated by these stresses. Among them was an antisense RNA to the phycocyanin-encoding mRNA cpcBAC and the trans-encoded regulator of photosystem I, PsrR1. The large genome capacity allowed Nodularia to harbor more copies of stress-relevant genes such as psbA and small chlorophyll-binding protein genes, combined with the coordinated induction of these and many additional genes for stress acclimation. Our data provide a first insight on how N. spumigena became adapted to conditions relevant for a cyanobacterial bloom in the Baltic Sea.

The toxic cyanobacterium Nodularia spumigena regularly forms large surface blooms in the central Baltic Sea. The Baltic Sea is characterized by a salinity gradient. We analyzed the salt acclimation of the strain N. spumigena CCY9414, the only Nodularia strain with a known genome sequence. N. spumigena CCY9414 showed a rather low salt tolerance range, displaying a growth optimum at 12.5 g NaCl l −1. Sucrose was identified as the major compatible solute. The expression of the sucrose-phosphate synthase gene was salt-stimulated, which indicates that the salt-induced sucrose accumulation could be regulated at the transcriptional level. Potassium ions and glutamate were also accumulated in Nodularia cells, especially at high salinities when sucrose levels were rather low. Our results indicate that N. spumigena CCY9414 represents a truly brackish-water-adapted cyanobacterial strain. KEY WORDS: Compatible solute · Glutamate · Potassium · SucroseResale or republication not permitted without written consent of the publisher Aquat Microb Ecol 70: 207-214, 2013 west connection to the North Sea and almost freshwater conditions in the Northern and Eastern edges (Samuelsson 1996). The distribution of Nodularia spumigena could depend on the external salinity because blooms occur mostly in the central Baltic Sea characterized by brackish waters around 8 practical salinity units (PSU) (Feistel et al. 2010). To adjust the internal osmotic potential to changing salinities, cyanobacteria are known to accumulate specific classes of compatible solutes (Hagemann 2011). Members of the group Nostocales, such as the model strain Anabaena sp. PCC 7119, usually show optimal growth under fresh water conditions and can acclimate to brackish water salinities by the accumulation of sucrose, which is catalyzed by sucrose-phosphate synthase (SPS) (Porchia & Salerno 1996).Compared to freshwater Anabaena strains, Nodularia spumigena seems to prefer brackish water conditions. To get deeper insight into the basic saltacclimation strategy of this ecologically important cyanobacterial genus, we aimed to investigate the salt response of N. spumigena under defined laboratory conditions. For this purpose, we used the Baltic Sea isolate N. spumigena CCY9414 (hereafter Nodularia CCY9414), taking advantage of its known genome sequence (GenBank accession no. AAVW 00000000; Voß et al. 2013). Genome searches revealed that Nodularia CCY9414 is potentially able to synthesize sucrose as well as trehalose as compatible solutes. Our results suggest that Nodularia CCY9414 is a brackish-water-adapted cyanobacterium because it grew optimally at salinities around 10 g NaCl l −1 and accumulated sucrose as the main compatible solute under these conditions. MATERIALS AND METHODS Strain and cultivationNodularia spumigena CCY9414 was obtained from the Culture Collection Yerseke (CCY) at the Netherlands Institute of Sea Research (NIOZ). This Baltic Sea strain was isolated from surface waters of ~9 PSU east of the island of Bornholm. It was taken into the strain ...

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