The entry, establishment, and spread of non – native species to a new ecosystem, is frequently described as biological invasion (Vitousek et al., 1997; Ricciardi and Cohen, 2007). In many cases, biological invasions are caused by human – induced environmental interference and have substantial economic impact, serious ecological consequences, and human health hazards (Pimentel et al., 2001). They also may threaten global biodiversity by altering the structure and function of ecosystems and disrupting key biological interactions (Levine et al., 2003; Traveset and Richardson, 2006). The invasion of freshwater and marine ecosystems by non-native species of various taxonomic groups, mostly crustaceans, mussels, fish, and aquatic macrophytes is well documented (Meinesz et al., 2001; Richardson, 2011). The invasion of free-living microorganisms to new aquatic habitats is rather cryptic and difficult to detect therefore invasions of these “invisible invaders” have been rarely reported (Litchman, 2010). Furthermore, information on prior and current composition of the microbial communities, which is required to determine if a given microorganism is an invader, is often missing. Algae and cyanobacteria are exceptions in that sense as they have visible characteristic of spectral signature and microscopic morphological features. This property partly contributed to the increased number of reports on cyanobacteria invasion to lakes and water reservoirs across Europe, North, and South America and the Middle East that were recently published (Dyble et al., 2002; Wiedner et al., 2007; Hadas et al., submitted).

Here we summarize recently published records on the invasion of cyanobacteria to subtropical and temperate lakes and water reservoirs and try to forecast different scenarios of ecological and management consequences. The invasion of two Nostocales genera, Cylindrospermopsis and Aphanizomenon is presented and analyzed based on the main concepts of invasion ecology. The following issues are critically addressed: recent variations in geographic distribution of invasive Nostocales; How does a Nostocales species become an invader? What are the special traits of the invading Nostocales? What are the level and the role of genetic variability in the invasion of Nostocales? Is invasion of Nostocales to new location further expected and under what conditions? What are the expected ecological effects of the invasion?

Biological invasion is considered as an extension of normal colonization processes such as succession (Elton, 1958) therefore it was proposed that the term “invasive species” will be restricted to describe non-native species that expanded their geographic range, became abundant and have environmental and economic impact (Colautti and Macisaac, 2004). Because the terminology associated with biological invasion is rather confusing and sometimes misleading (Colautti and Macisaac, 2004), here we define an invading or a non-native species as a species that spreads beyond its native range or a species that enters a habitat where it had not resided before. However, it should be kept in mind that many cyanobacteria species (as well as other microorganisms) can inhabit an aquatic ecosystem unnoticed, since it may remain at a rather low biomass concentration and it does not form a conspicuous fraction of the community. Furthermore, the lack of detailed water quality monitoring program or an insufficiently comprehensive surveillance do not pick up the presence of these species (Kling et al., 2011). Typically it is only when a non-native population is widespread and abundant, and causes ecological or economic harms, that it is named “invasive.” It is possible for a species to be invasive in one ecosystem, but non-invasive in another. This can be due to a variety of factors, such as the presence of less-than-ideal habitat conditions (Lockwood et al., 2007). We further suggest evaluating the invasion of free-living microorganisms within an appropriate timeframe, since an invader may become a common resident of a dynamically changing environment. Therefore native and non-native species may be viewed as part of a continuum with respect to their time of residency and the extent of evolutionary and ecological interactions they have within a given environment.

During the last decades, the number of reports on cyanobacterial domination and persistence in freshwater and marine ecosystems significantly increased covering spatial and temporal distribution of persistent perennial blooms. Many of these bloom events are of toxic Microcystis species and strains that proliferate under current environmental alterations, including nutrient enrichment, global warming, and regional hydrologic changes (Paerl and Paul, 2011). However, not all blooms are associated with an invasion process as many species are of broad geographic distribution and rapidly respond to current environmental changes.

The most important freshwater invasive Nostocales are the tropical species C. raciborskii and to a lesser extent also Aph. ovalisporum. The map in Figure 1 shows the current global distribution of C. raciborskii and Aph. ovalisporum in their native tropical region and in subtropical and temperate areas to which they expanded in the last two decades. C. raciborskii is a tropical species which was first described from Indonesia (Java) and was subsequently discovered in India and Thailand, Japan, the Philippines, Egypt, Senegal, Brazil, Cuba, Chile, Uruguay, Australia, New Zealand, and Florida (Bouvy et al., 2000; Molica et al., 2002; Dobberfuhl, 2003; Komárek and Komárkova, 2003; Wood and Stirling, 2003; Briand et al., 2004; Chonudomkul et al., 2004; Vidal and Kruk, 2008). Recently Kaštovský et al. (2011) summarized the invasion of freshwater cyanobacteria and algae to aquatic ecosystems in the Czech Republic and portrayed the invasion of C. raciborskii from its native tropical area to subtropical and temperate zones. The expansion of C. raciborskii and Aph. ovalisporum to new geographic regions is described below.

Successful biological invasions involve complex interactions between the invading species and the physical and biological characteristics of the recipient environment. Often, invasive cyanobacteria species owe their success in colonizing new ecosystems to unique physiological characteristics (Figure 2). The invasion may be initiated with dispersion to new zones and is facilitated by human activities by migrating animals or birds or by winds. Obviously, aeolian transport is an efficient means to transfer free-living microorganisms, prokaryotes, and protists (Smith et al., 2011). But reaching a new environment is not sufficient and the invader needs a variety of traits that support its establishment and proliferation. Two distinctive traits of Nostocalesa are: (1) The ability to form dormant cells (akinetes) that may survive long and extreme dispersion routes, on one hand and serve as overwintering form that survive unfavorable conditions and assure perennial germination and proliferation, on the other hand. (2) The ability to fix atmospheric nitrogen in the absence of combined inorganic sources. This last trait opens invasion opportunities for Nostocales in oligotrophic aquatic ecosystems, besides ecosystems temporarily limited in combined nitrogen and extends the spectrum of ecosystems to which they can invade.

Invasive cyanobacteria have growth rates that are relatively high compared with those of native species (Isvánovics et al., 2000). They may have high resource use efficiency or possess traits allowing them to gain access to resources unavailable to other species, such as the capacity to fix molecular nitrogen. C. raciborskii has high competitiveness under fluctuating nitrogen availability due to efficient uptake capacity for ammonium (Figueredo et al., 2007) and it also can fix atmospheric nitrogen, which allows it to escape N limitation (Briand et al., 2004; Moisander et al., 2011). This species may also utilize other limiting resources, such as phosphorus, more efficiently than other cyanobacteria due to high affinity and P storage capacity (Isvánovics et al., 2000; Wu et al., 2011) and, thus, be a superior nutrient competitor. Padisák (1997) suggested that C. raciborskii, tends to invade lakes with very high nutrient concentrations and low phytoplankton diversity, in addition to its ability to form akinetes allowing easy dispersal and environmental resistance. An important feature of Cylindrospermopsis is its wide thermal tolerance (Briand et al., 2004), which is essential to maintain the populations during cold winters. Detailed information on the effect of temperature on Nostocales life cycle processes such as akinete formation, their survival and germination, and the recruitment of vegetative filaments is currently scarce, but Cirés (2012) indicated that akinetes production of Aph. ovalisporum is rather restricted to a relative narrow temperature range. Padisák (1997) reported that akinetes of C. raciborskii germinate at temperatures <24°C. Further elucidation of life cycle processes responding to temperature is required in order to determine the invasiveness capabilities and limitations of various Nostocales species.

In many cases, floating Nostocales populations create a thick scum that blocks light penetration through the water column and inhibits the growth of other species, thus under high cyanobacterial growth, the populations remain established for longer periods minimizing competition by other algal groups. A scum formation is a dynamic process that depends on a delicate balance between flotation, cellular ballast and water turbulence (Calandrino and Paerl, 2011). Filaments floating at the water surface gain advantage over sub-surface phytoplankton populations. They directly intercept CO₂ diffusing into the water from the atmosphere, thus minimizing inorganic carbon limitation of photosynthetic growth. Buoyant species contain UV-absorbing compounds such as mycosporine-like amino acids (MAAs) and scytonemin that ensure survival under extremely high irradiance conditions (Paerl and Paul, 2011). However, in some occasions scum formation represents a trap for the organisms that lose their dynamic buoyancy behavior and they are exposed to full sunshine (including high UV radiation in summer), as well as to nutrient deprivation. Frequently they die and remain on the water body shores with extremely low metabolic activities (i.e., C and N uptake, Oliver and Ganf, 2000 and references therein).

Another trait commonly found among Nostocales but also in other cyanobacteria (i.e., Microcystis) is the ability to synthesize a suite of secondary metabolites some of which were identified toxic to humans and animals (Stewart et al., 2011) and others may affect community structure functioning as allelo-chemicals that inhibit other phytoplankton species or deter and reduce grazing (Fastner et al., 2007; Paerl et al., 2011). Allelopathy was suggested as a beneficial trait of C. raciborskii that contributes to its stable dominance and geographic expansion (Figueredo et al., 2007) but also for other cyanobacteria species (Kaplan et al., in review). A unique feature of Aph. ovalisporum to acquire phosphate under oligotrophic conditions was recently reported by Bar-Yosef et al. (2010). Excreted chemicals including the toxin cylindrospermopsin cause cells of some other phytoplankton species to make extracellular alkaline phosphatase whose inorganic phosphate product can be used by Aph. ovalisporum due to its high affinity to phosphate, thus contributing to its proliferation and domination. Aph. ovalisporum has been also found to outcompete other cyanobacteria at high temperatures due to higher growth rates than other Nostocales under these circumstances (Mehnert et al., 2010; Cirés et al., 2011).

In an attempt to follow the invasion route of C. raciborskii to new ecosystems Neilan et al. (2003) studied genetic variations between strains isolated from freshwater rivers and reservoirs in Australia, Brazil, Germany, Hungary, Portugal, and the USA by characterizing their 16S rRNA gene nucleotide sequences and by analyzing cyanobacterium-specific short tandem repeat sequence (HIP1). Three distinct groups of C. raciborskii strains were clustered: (1) a group consisting of strains from the USA and Brazil; (2) a group comprised of European strains (Germany, Hungary, and Portugal); (3) a group of strains from Australia. A subsequent study that examined other isolates of C. raciborskii revealed the same continental cluster distribution based on the 16S–23S internally transcribed spacer (ITS1) sequences (Gugger et al., 2005). This study suggests that the current expansion of C. raciborskii in Europe and in Central – and North America did not result from recent invasion and colonization by African or Australian strains (Padisák, 1997) but rather represent local strains that maintained “cryptic” populations over time and only recently proliferated due to climate change and variations in other environmental conditions. In a recent study, Piccini et al. (2011) proposed that phenotypic and genetic variability of C. raciborskii populations is linked to the existence of different ecotypes whose success is subject to the local environmental conditions.

Nevertheless, analysis of the 16S rRNA gene nucleotide sequences revealed 99.1% similarity between the strains of C. raciborskii collected from a range of global locations and continental clustering was not supported by statistically significant bootstrap values (Neilan et al., 2003). Therefore it cannot be excluded that C. raciborskii was transferred from the Australasian region to Europe unintentionally by humans or by migrating birds, thus providing the source of this cyanobacterium which is now reported in many previously unaffected temperate and subtropical water bodies. It is further speculated that some physiological characteristics unique to C. raciborskii support its proliferation in newly invaded ecosystems as they are currently exposed to higher temperature and environmental perturbations.

In order to exploit its wide spectrum of physiological traits and to successfully outcompete native species, the conditions in a new habitat have to fit the invader’s needs. Increased water temperatures, mostly during the summer, in subtropical, and temperate zones, is considered one of the major factors supported the invasion of C. raciborskii and other Nostocales (Figure 2). (Padisák, 1997; Briand et al., 2004; Rücker et al., 2009). Both C. raciborskii and Aph. ovalisporum are known to proliferate at temperatures higher than 20°C (Hadas et al., 1999, 2002; Mehnert et al., 2010), thus moderate elevation in water temperatures may open new ecological habitats for them. The persistence of Nostocales in their new locations is further maintained by their ability to form dormant cells, akinetes, which stay dormant but viable in the sediment for extended periods of time (Hense and Beckmann, 2006; Kaplan-Levy et al., 2010). This seed bank may reside in the sediment and wait for a warm summer to germinate. Consequently, bloom events may follow local climatic patterns. It is important to note that freshly settled akinetes are more likely to germinate during the next summer relative to those settled in earlier blooms as the latter are already covered deeper in the sediments and probably have lower germination potency.

Nostocales species are recently reported to reside in, and dominate eutrophic as well as oligotrophic aquatic systems (Figure 3). Eutrophic conditions provide ample nutrients that can be easily and competitively utilized by opportunistic invaders (Piehler et al., 2009). Such eutrophic systems maintain high internal nutrient recycling processes that fortify primary production and biomass accumulation. Although N₂ fixation is commonly recorded during Nostocales blooms in eutrophic aquatic systems (mainly indicated by the presence of heterocysts, specialized cells formed under N limiting conditions), this process is responsible for up to 2% of N acquisition by the phytoplankton community (Ferber et al., 2004; Ariosa et al., 2006). The ability of many Nostocales to efficiently use low light, allows their growth in turbid waters as well as at high biomass concentrations.

Regional inappropriate management of watersheds and high pollution impact induce eutrophication, and are certainly involved in the Nostocales geographic expansion. But these invading species can survive and propagate also under oligotrophic conditions (Figure 3). This is achieved due to their various efficient phosphate acquisition capabilities (Isvánovics et al., 2000; Sprὄber et al., 2003; Posselt et al., 2009; Bar-Yosef et al., 2010) and nitrogen fixation which seems to be more relevant under oligotrophic than under eutrophic conditions. Thus, in oligotrophic systems, nitrogen cannot be considered as a limiting factor. Phosphate availability controls the development of the N₂ fixing population and its growth. Consequently, management efforts to control eutrophication by reducing N loads, apparently effective in reducing blooms of Microcystis and other Chroococcales species, may actually support the growth of Nostocales which easily outcompete native species in such N limited ecosystems (Schindler et al., 2008).

The immediate consequence of Nostocales invasion, besides the ecological impairment, is the expansion of toxic blooms as various strains produce harmful substances. Many Aph. ovalisporum and C. raciborskii strains produce the cytotoxic compound cylindrospermopsin whereas several Brazilian C. raciborskii strains have been reported to produce paralytic shellfish poisoning toxins: neosaxitoxin, saxitoxin, and gonyautoxins (Lagos et al., 1999). Interestingly, many strains isolated from temperate lakes in Europe and Northern America did not produce any known toxin, although they were found toxic in various bioassay experiments (Saker et al., 2004). Recent studies indicated that a given population may be composed of toxic and non-toxic strains (Ballot et al., 2011). The bloom of toxic Nostocales presents an immediate threat to human and animals that use the water for drinking, bathing and recreational activities (Carmichael, 2001). Local and governmental authorities are requested to provide solutions to minimize and control the bloom events by taking scientifically based management measures that should include consideration of the invasiveness of this cyanobacterial group under a scenario of global change (Jones and Sauter, 2005; Schindler et al., 2008).

The domination of recently invaded Nostocales affects the entire ecosystem as native phytoplankton species become rare and displaced and higher trophic levels may be affected via their toxins or due to loss of essential components of their food chain. C. raciborskii is known to affect the growth, reproduction and feeding of zooplankton (Leonard and Paerl, 2005) and cylindrospermopsin was shown to be accumulated in fish and in freshwater mussels (Saker and Eaglesham, 1999; Saker et al., 2004).

The current trend of increasing bloom events of cyanobacteria, including Nostocales, will probably be further enhanced as global warming continues. Any further temperature increase would promote the growth and development of Nostocales species in general, and that of the invasive species in particular, and would enable further expansion in the temperate zone (Mehnert et al., 2010). This trend was further confirmed by a coupled biological–physical model that predicted that high temperatures favor cyanobacteria over other phytoplankton taxa through increased growth rates that boost the development of blooms (Juehnk et al., 2011). A similar conclusion was reached by Wiedner et al. (2007) who evaluated a case of an earlier rise in water temperature associated with climate change. They suggested that such a scenario will prompt further spread of C. raciborskii to the temperate zone as earlier warming permits earlier germination, thereby shifting the pelagic populations to conditions which advance population establishment and growth. However, these models do not consider the ecological variations expected after this earlier germination, as different grazing pressure due to different grazer population (e.g., Winder and Schindler, 2004) or nutrient depletion that may follow an early bloom. These ecological variations are hard to predict and should be further investigated to improve models prediction.

While the possibilities to control and reduce the current trend of global climate change are rather limited, the management of eutrophication processes is feasible. Since a synergistic effect of nutrients and climate was frequently indicated in many sites invaded by Nostocales, it is important that nutrient concentrations in many temperate and subtropical lakes be reduced substantially from present values if cyanobacterial dominance is to be controlled (Kosten et al., 2012). Based on long term experimental manipulation, Schindler et al. (2008) concluded that N-fixing cyanobacteria cannot be limited by a shortage of dissolved N and instead are competitively favored. Thus reducing N inputs could actually intensify the dominance of N-fixing cyanobacteria thus enhancing the expansion of invasive Nostocales. The alternative approach to control and reduce blooms of invasive Nostocales species and their further expansion is to control and reduce external and internal sources of phosphorus, a complex task by itself.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.