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IMDEA Water (Instituto Madrileño de Estudios Avanzados, IMDEA AGUA), Alcalá de Henares, Spain

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Applied Environmental Science, Stockholm University, Stockholm, Sweden

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A peculiar feature of the Baltic Sea is the massive summer blooms of cyanobacteria. Various environmental, economic and sanitation repercussions of these blooms have attracted considerable attention among the scientific community, water management agencies and general public. Of particular concern is the increase in frequencies and amplitude of the hepatotoxic blooms of Nodularia spumigena. This is a planktonic, toxic, filamentous nitrogen-fixing cyanobacterium capable of forming massive blooms when environmental conditions are suitable. The toxicity of N. spumigena is mainly attributed to the production of the hepatotoxin nodularin, which is also a tumor promoter. Furthermore, through its nitrogen fixation activity, N. spumigena contributes significantly to the total annual nitrogen load in the Baltic Sea. Here, we highlight the physiological peculiarity of N. spumigena related to nitrogen fixation and heterocyst formation and how this may be regulating its ability to form blooms. We also discuss some key molecular and physiological aspects of the toxin production. Furthermore, we highlight the interactions between N. spumigena and its grazers in the food webs and potential effects of climate-related factors on these interactions. All these aspects are important to consider if we want to predict consequences of the eutrophication and global change for bloom proliferation and toxin production by N. spumigena.

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The Baltic Sea is the largest brackish water ecosystem in the world. Approximately, 16 million people live in nine countries surrounding the Sea and an approximately 85 million people live in the 14 countries in its catchment.

The Sea is composed of several basins and is connected to the North Sea through the narrow Skagerrak/Kattegat. Due to its semi-enclosed nature, water exchange is limited and there is a salinity gradient stretching from nearly fresh water (< 2) in the Bothnian Bay to > 20 in the Skagerrak. In the Baltic Proper, there is a permanent halocline between 30 m and 80 m that prevents vertical water mixing [1]. The deep water of the Baltic is displaced to a significant degree through non-frequent large inflows from the North Sea, while small but more frequent inflows are generally of minor importance. These peculiarities in the Baltic Sea morphometry and hydrology result in limited water exchange, long residence times, retaining of nutrients and organic matter within the Sea [2,3]. Sedimenting organic matter together with poor mixing generate anoxic bottom areas that have been reported to increase over the last century due to eutrophication and resulted in a severe decrease of macrobenthic communities below the halocline [4].

Starting from the mid-19th century, human population in the region expanded accompanied by expansion of industrial and agricultural activities. The Sea ecosystem changed as a result, with the most prominent anthropogenic impact being eutrophication caused by discharges of nutrients, especially of nitrogen and phosphorus.

Cyanobacterial blooms that occur every summer in the Baltic proper, and the Gulfs of Finland and Riga are one of the main problems caused by the Baltic eutrophication. The blooms affect recreation industry and tourism when blown ashore due to nuisance odors and scums forming under calm conditions. Moreover, the blooms can also be toxic and incidences of animal death and human illness have been reported.

The frequency and intensity of the blooms has increased [5,6], which is of public concern. On the other hand, there is evidence that such blooms have been a natural feature of the Baltic Sea since it became a brackish sea, about 7000 years ago [7].

In addition, the blooms fix nitrogen and add several hundred thousand tonnes of nitrogen to the Baltic Sea each summer, thus adding to its eutrophication [8] and facilitating bottom hypoxia [9]. Therefore, cyanobacteria are of high biogeochemical importance for this region. In general phytoplankton growth in the Baltic proper and the Gulf of Finland are nitrogen- limited except for the nitrogen (N2)-fixing cyanobacteria [8,10,11], which are nitrogen

sufficient through their growth period. During the peak of their seasonal development, N2-

fixing cyanobacteria suffer phosphorus limitation and slow down their growth to nearly zero [8]. During autumn, the joint-effects of phosphorus limitation, changes in some abiotic factors, such as light intensity, temperature and availability of nutrients and cyanophage attacks cause blooms to collapse [12,13,14].

The filamentous cyanobacteria Nodularia spumigena, Aphanizomenon flos-aquae and

Anabaena spp. are important constituents of the cyanobacterial summer blooms in the Baltic

Sea [15, 16]. Together, they can form kilometers-large blooms that are considered to be among the largest bloom-formations in the world. While Aphanizomenon is present in the water column during the whole year, Nodularia and Anabaena are present only during the warm summer period [17, 18].

All species are heterocystous nitrogen-fixers and contribute significantly to the nitrogen input into the Baltic Sea. Phylogenetic studies on planktonic Nodularia strains isolated from the Baltic Sea revealed that they all belong to the toxin-producing species N. spumigena [19]. To date, N. spumigena is the most studied filamentous cyanobacteria in the Baltic. To understand relative importance of various environmental factors on nitrogen fixation and toxin production, these studies have been facilitated by relatively easy maintenance of

Nodularia strains in laboratory conditions. In comparison to Nodularia, the studies on Aphanizomenon and Anabaena spp. have been limited, which is in part due to the difficulty of

isolating their strains.

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