Sodium and nitrate favor the steady state of cyanobacteria in a semiarid ecosystem

This work is licensed under Creative Commons Attribution 4.0 International.

Depending on the abiotic conditions, some dominant species in the environment may not show a significant variation in biomass within a time interval, constituting a steady state event, regulated by the production and loss of biomass in a balanced manner (Rojo & Álvarez-Cobelas, 2003; Teubner et al., 2003; Hui, Xie, Guo, Chu, & Liu, 2014). In accordance with Sommer, Padisák, Reynolds, and Juhász-Nagy (1993), criteria for identification of steady state phases in natural systems are: up to three species contributing with, at least, 80% total biomass for over two weeks without significant variation in total biomass.

In tropical environments, temperatures are high, but with small annual range compared to temperate environments (Komárková & Tavera, 2003). Because of its relative stability, it is assumed that the steady state is more likely to occur at lower latitudes environments (Becker, Huszar, Naselli-Flores, & Padisák, 2008; Baptista & Nixdorf, 2014). Steady state has been reported in tropical ecosystems in drier seasons, when higher thermal stability is observed. During rainy periods, there may be disturbances in the water column and avoid the steady state maintenance (Sutherland, Turnbull, & Craggs, 2017; Havens, Beaver, Fulton III, & Teacher, 2019).

Long steady state events have been reported in studies conducted in tropical ecosystems, which report that the steady state phases are typically dominated by high biomass of a single species, commonly Microcystis aeruginosa (Kützing) Kützing and species of the order Nostocales, especially Cylindrospermopsis raciborskii (Woloszynska) Seenaya and Suba Raju (Komárková & Tavera, 2003; Rojo & Álvarez-Cobelas, 2003).

In tropical zones, semiarid regions cover a large area and possess a peculiar climate dynamics. The Northeast Brazil, for example, is characterized by a semiarid climate, where it is observed water shortage, high temperatures, high water residence time, and artificial eutrophication, with rainfall concentrated in about three months and the remaining months comprise the dry season (Moura, Tavares, & Amorim, 2018). In this way, in semiarid environments, it would be expected longer steady state events.

Despite these traits, studies in semiarid regions addressing the occurrence of steady state, considering all the pre-requisites established by Sommer et al. (1993), are scarce. Instead, authors have attempted to explain the factors leading to the dominance and persistence of phytoplankton species in these ecosystems, carrying out studies with monthly samplings (Moura, Oliveira, Dantas, & Arruda-Neto, 2007; Dantas, Moura, & Oliveira, 2011; Tavares, Moura, & Dantas, 2017), so it is not possible to apply the concept of steady state in these studies.

Studies focusing on the dominance and persistence of cyanobacteria are common in the Northeastern Brazil. Authors have pointed out the trophic level to explain the persistence of cyanobacteria in these semiarid ecosystems. Bouvy, Falcão, Marinho, Pagano, and Moura (2000) analyzed 39 reservoirs in Brazilian semiarid ecosystems and noted that C. raciborskii corresponded to deeper, warmer and hypertrophic reservoirs characterized by the combination of nutrient concentrations, high temperature, thermal stability and pH values. Volume reduction, water column stability and long water retention time in the reservoirs combined with drought would have created excellent conditions of temperature and irradiation for the dominance of Cylindrospermopsis sp. Studies carried out by Tavares et al. (2017) also showed that the high nutrient concentrations (nitrate, nitrite and orthophosphate) influenced persistence of C. raciborskii throughout the study in semiarid reservoirs in Brazil.

Conversely, C. raciborskii persistence has also been reported under P-limited conditions, shallowness and low water column transparency (Henemmann & Petrucio, 2016) as well under N-limited conditions, since this is a species able to fix atmospheric nitrogen (Chellappa, Borba, & Rocha, 2008). Studies have reported that reservoirs located in the Brazilian semiarid region, characterized by high trophic levels, phosphorus and nitrogen concentrations, temperatures and water retention time; low turbulence and pH alkaline did not present dominance or persistence of cyanobacterial species (Barbosa et al., 2012). Persistence of cyanobacterial species in eutrophic, oligotrophic, and mesotrophic reservoirs is often related (Chellappa et al., 2008, Okello, Ostermaier, Portmann, Gademann, & Kurmayer, 2010; Fonseca, Vieira, Kujibida, & Costa, 2015), suggesting that eutrophication level is not the solely responsible for steady state events.

Climatic variables are also used to assess the persistence of cyanobacterial species. In semiarid ecosystems, dry climate favors evaporation and decrease the reservoir volume and should lead to dominance of cyanobacteria species (Bowling et al., 2013; Braga et al., 2015), and could contribute to the steady state in dry seasons (Naselli-Flores, Barone, Chorus, & Kurmayer, 2007). Water level rise can lead to a reduction in the biomass of filamentous cyanobacteria and a dilution of M. aeruginosa colonies (Li et al., 2018), probably due to disturbance of the water column or changes in the concentration of nutrients dissolved in water, compromising the persistence of cyanobacteria (Bakker & Hilt, 2016). However, several studies have shown that in semiarid ecosystems, it is common to observe persistence of cyanobacteria throughout the year (Moura et al., 2007; Dantas et al., 2011), suggesting that seasonality and stability have less influence than in other ecosystems.

It is noteworthy that the studies that aim to relate climatic variables to cyanobacteria, often consider air temperature and rainfall. Nevertheless, other climatic variables should also exert influence, such as wind, humidity and air speed. According to Bouvy, Molica, Oliveira, Marinho, and Becker (1999), the fluctuating nutrient supply, probably associated with sediment resuspension by wind in shallow waterbodies, is an advantageous factor for cyanobacteria.

The growth and dominance of phytoplankton species in reservoirs are controlled by a complex combination of physical and chemical factors, acting synergistically (Huszar, Kruk, & Caraco, 2003; Çelik & Ongun, 2008). Therefore, it is expected that other abiotic variables, besides rainfall, phosphorus and nitrogen, are involved in establishing the steady state of cyanobacterial species. Considering this scenario, the influence of other variables not included in other studies, such as micronutrients and other macronutrients, besides nitrogen and phosphorus, should be considered. According to Du, Creed, Sorichetti, and Trick (2019), lake managers concerned with the persistence of cyanobacteria should incorporate sampling for trace metals, such as Fe (iron), and other micronutrients into routine sampling campaigns. Studies have shown that micronutrients are required for essential metabolic functions in photosynthetic electron transport, respiratory electron transport, nitrate and nitrite reduction, sulfate reduction, nitrogen fixation, and detoxification of reactive oxygen species (Sorichetti, Creed, & Trick, 2016; Zhang et al., 2019), which could help to understand the growth and persistence of some species of cyanobacteria.

In this way, the present study hypothesized that in semiarid reservoirs steady state events are independent from seasonality and influenced by other nutrients besides nitrogen and phosphorus. Thus, aiming to better understand how cyanobacteria establish steady state in semiarid regions, this study evaluated the influence of rainfall, nitrogen, phosphorous and other thirteen abiotic variables in a semiarid, eutrophic ecosystem from Northeast of Brazil dominated by cyanobacteria.

Throughout the study period, dominance of cyanobacteria was observed, especially P. agardhii and C. racibrorskii; however, in only 36% of the weeks, the steady state assumptions, according to Sommer et al. (1993), were completely met. The results also showed short SS periods, lasting, on average, four weeks and occurred in both drier and rainy periods with no difference over time.

Nitrate, color, turbidity and sodium were related to SS periods, while sulfate was related to nSS periods. Our results showed that P. agardhii (Oscillatoriales) was related to the SS periods and its biomass was directly related to darker water color and more turbid samples.

Nitrogen is a macronutrient that participates in the formation of various organic molecules, such as proteins and nucleic acids. This nutrient can be absorbed in various chemical forms, including nitrate. In non-nitrogen fixing cyanobacteria, such as P. agardhii, nitrate is required for activation of nitrate reductase that catalyzes the conversion of nitrate to ammonium and glutamine, which participates in the construction of amino acids and proteins (Jha, Ali, & Raghuram, 2007). As reported by Von Rückert and Giani (2004), nitrate serves as a long-term nitrogen storage reservoir and this may explain its relationship with the persistence of the high biomass of P. agardhii during SS periods.

Studies carried out by Kim, Jo, and Kim (2017) on M. aeruginosa show that its growth is affected by nitrate, mainly if a certain minimum PO₄^3- concentration is present. Besides, as this species grows in less turbid waters, it moves towards the high-intensity light at the surface, using its gas vacuole, and thereby generates blooms. M. aeruginosa recovers its buoyancy more quickly when the nitrate concentration is higher. However, our results detected a negative relationship between this species and nitrate concentrations, corroborating Von Rückert and Giani (2004). According to these authors, under low nitrate concentrations, there is dominance of non nitrogen-fixing species, like Microcystis species. They observed Microcystis blooms when ammonium concentrations are very high and nitrate not detectable.

Color and turbidity may influence the growth of phytoplankton species, regulating, mainly, the light input and the extension of the euphotic zone, necessary for photosynthetic reactions. Studies suggest that increases in water color favor low-light-adapted phytoplankton species, such as Nostocales and Oscillatoriales, because of efficient light harvesting systems and buoyancy regulation (Lebret, Langenheder, Colinas, Östman, & Lindström, 2018). According to Reynolds, Huszar, Kruk, Naselli-Flores, and Melo (2002), C. raciborskii belongs to the codon Sn and tolerates light deficiency, while P. agardhii is a species belonging to the codon S1, whose growth is favored in turbid mixed layers, with highly light deficient conditions. Thus, the SS of P. agardhii may have occurred due to the self-shading effect caused by its biomass, inhibiting the development of other phytoplankton species. On the other hand, Reynolds et al. (2002) framed M. aeruginosa within the codon M, adapted to high insolation. Thus, it is expected that this species will grow in an environment with lower values of color and turbidity, corroborating the results observed herein, which show turbidity and color on the negative side of axis 1, while M. aeruginosa is plotted on the positive side of this axis.

Nitrate and color were variables related to SS periods, however oscillations in their values were not significant to distinguish these periods as the SS dominant species. For this, turbidity, as well as the concentrations of sulfate and sodium, were suitable.

Our study showed high biomasses of C. raciborskii throughout the study period, with significant correlation with both the nSS and SS sampling units. Steady states with dominance of C. raciborskii and/or P. agardhii occurred in the periods X and XI and were concomitant with turbid waters and higher sodium concentrations.

Ruiz, Sampaio, Oliveira, and Venegas (2004) report that the soils from semiarid regions, due to the intense evaporation and low rainfall, generally contain high soluble salt concentrations and elevated sodium concentrations are often. This ion can migrate from soil to the water column and influence the dynamics of phytoplankton communities. However, studies on the influence of sodium on the growth and dominance of these organisms in freshwater ecosystems are scarce. Hence, our knowledge comes from several laboratory studies that highlight the importance of these elements in the physiology of phytoplankton species.

Increased sodium concentrations appear to be correlated with filamentous cyanobacteria (Seale, Boraas, & Warren, 1987). For nitrogen-fixing cyanobacteria species, such as C. raciborskii, this nutrient is required for the nitrogenase activity, responsible for the fixation of atmospheric nitrogen, which occurs in differentiated cells, called heterocytes (Apte & Thomas, 1980). Studies conducted by Valiente and Avendaño (1993) show that when sodium is present, Anabaena has higher nutrient uptake efficiencies and lower rates of organic matter loss. For non-nitrogen fixing cyanobacteria species, such as P. agardhii, sodium also could enhance phosphorus uptake (Seale et al., 1987). Thus, phosphorus input together with sodium could lead to proliferation of both nitrogen-fixing and non-nitrogen fixing cyanobacteria, particularly in warmer conditions. Semiarid ecosystems are characterized by high temperatures throughout the year. The mean values of phosphate ranged from 0.11 to 0.25 mm L^-1. These variables, combined with higher sodium concentrations, could have favored the steady state of both C. raciborskii and P. agardhii.

Different water bodies are characterized by dominance of cyanobacteria, which can be explained by the resistance to zooplankton predation (Dai et al., 2016), through cyanotoxin production. Synthesis of saxitoxin was observed in M. aeruginosa (Martínez-Ruiz & Martínez-Jerónimo, 2016), P. agardhii and C. raciborskii (Sato-Liebe et al., 2012) exposed to high sodium concentrations. Studies have shown that saxitoxin release to the extracellular medium is dependent on the concentration of monovalent cations due to the increase in intracellular osmotic pressure generated by higher sodium concentrations (Sato-Liebe et al., 2012).

In addition to sodium, sulfate anions may also influence the physiology of cyanobacteria, aiding in the production of cyanotoxins. Sulfate is a common ion in eutrophic freshwater systems, mainly from the discharge of domestic and industrial effluents. It is an essential nutrient and plays a pivotal role in biosynthesis of sulfate-containing metabolites in organisms (Chen, Gin, & He, 2016), such as the amino acids cysteine and methionine and biosynthesis of toxins and proteins that act as electron donors in reductive steps in photosynthesis reactions (Dai et al., 2016; Hughes, Sulesky, Andersson, & Peers, 2018; Zalutskaya, Minaeva, Filina, Ostroukhova, & Ermilova, 2018).

Sulfate starvation has induced a characteristic reduction of cylindrospermopsin, a sulfur-containing cyanotoxin known to be produced by C. raciborskii (Bácsi et al., 2006). For another group of cyanotoxins, microcystins, studies developed by Long (2010) have suggested an important role for sulfur-containing metabolites in microcystin biosynthesis by M. aeruginosa. According to this author, sulfur is required to support N-methylation and thiolation reactions and to provide sulfur-containing cofactors for microcystin synthesis.

Since SS periods have been related to sodium and sulfate and, according to the literature, they stimulate the synthesis of toxins, it can be assumed that the problem associated with water pollution by these elements may be more damaging than has been discussed in the literature, favoring the stabilization of blooms of toxic species of cyanobacteria in freshwater reservoirs.

In the present study, higher sulfate concentrations were related to nSS periods, although participating in sporadic episodes of SS with dominance of M. aeruginosa. Despite its importance for the establishment of SS of M. aeruginosa, it is reported that the increase in sulfate in freshwater systems may inhibit photosynthesis, cause oxidative stress and inhibited the growth of M. aeruginosa (Chen et al., 2016). High sulfate concentrations can also reduce nitrogen fixation by cyanobacteria, due to the inhibition of nitrogenase activity (Marino, Howarth, Chan, Cole, & Likens, 2003). Nevertheless, low sulfate concentrations can limit the growth of C. raciborskii, G. amphibium and P. agardhii in freshwater ecosystems (Bácsi et al., 2006; Oliveira et al., 2015; Hughes et al., 2018). High sulfate concentrations have been found to cause higher phosphate and ammonium concentrations, which often results in eutrophication and subsequently can causes algal blooms (Chen et al., 2016).

Studies have shown that the disruption of SS can occur after heavy rainfall events (Naselli-Flores & Barone, 2003; Stoyneva, 2003), due to the water column instability (Havens et al., 2019). According to Mowe, Mitrovic, Lim, Furey, and Yeo (2015), the rainy period should break the dominance of M. aeruginosa, favoring the development of species more adapted to high turbidity, as P. agardhii. However, in our study, no climatic variable presented significant influence on SS events, either in the occurrence, or in the duration or even in the species composition.

Many studies on phytoplankton communities highlight a correlation between SS of cyanobacteria species and nitrogen and/or phosphorous concentrations. Baptista and Nixdorf (2014) reported that low availability of nitrate and phosphorus favored the exclusive SS of C. raciborskii. On the other hand, Nasselli-Flores & Barone (2003) showed SS of cyanobacteria species related to high concentrations of total phosphorus and low N: P ratio. Nevertheless, this is the first study that shows the influence of sodium, sulfur and other nutrients on SS of phytoplankton communities.

Semiarid ecosystems have peculiar climatic conditions characterized by high evaporation, scarce and unevenly distributed rainfall. These conditions favor the growth and dominance of cyanobacteria, which is reported in several studies carried out in the region (Bowling et al., 2013; Fonseca et al., 2015; Silva & Costa, 2015). Therefore, it is expected that these climatic conditions, together with other abiotic variables, should contribute to the establishment of SS in these regions. However, there are no studies on the occurrence of SS in semiarid ecosystems. Further, studies explaining the occurrence of SS in ecosystems located in regions with other types of climate test the influence of nitrogen, phosphorus or reservoir stability, not evaluating other abiotic variables that direct phytoplankton growth.

This study shows the robustness of a 130 week-monitoring and presents an important contribution to the understanding of steady state events, highlighting the participation of other abiotic variables besides nitrogen, phosphorus and reservoir stability. In addition, it explains the dynamics of steady state in semiarid ecosystems, whose abiotic characteristics are peculiar to other tropical and temperate ecosystems.

Our findings confirm the hypothesis that SS events occur in semiarid reservoirs, independent of seasonality, but influenced by the concentrations of nutrients.

The authors are grateful to Compesa (Companhia Pernambucana de Saneamento) for the availability of the data used in this study.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

The authors declare that they have no conflicts of interest.