Drivers of the Worldwide Distribution of Raphidiopsis raciborskii: Evidence from Experimental to Field Studies

Abstract

Raphidiopsis raciborskii is one of the most widely reported cyanobacteria worldwide, responsible for dense blooms and cyanotoxin production. Classified as invasive, it has been documented across all continents except Antarctica. While its distribution has been extensively studied, abiotic factors have consistently emerged as the main determinants of its success, which are therefore the focus of the present study. The objective of the present review is to synthesize findings from both experimental and field-based studies to identify which are the key drivers of its dominance. In total, 30 abiotic factors were reported, reflecting the broad strategies of the species. Results show the temperature as a consistent universal factor (11–35 °C), while differences were found regarding nutrient dynamics. Particularly, nitrogen forms and N/P ratios predominated in field-based evidence, whereas photosynthetically active radiation was disproportionately emphasized within experimental studies under controlled conditions. Factors such as salinity and micronutrients, and synergistic interactions remain critically understudied, limiting predictive capacity under global change scenarios. Understanding which combinations of these drivers create favorable conditions is essential for anticipating bloom dynamics in order to establish management strategies for avoiding or mitigating the negative impact of them.

1. Introduction

Cyanobacterial blooms are a major cause of water quality deterioration and represent one of the most significant environmental and health threats associated with climate change [1]. Among bloom-forming species, Raphidiopsis raciborskii stands out due to its invasive nature and its wide distribution in continental ecosystems [2]. For decades, it was classified as Cylindrospermopsis raciborskii; however, recent molecular analyses have revealed significant genetic similarities between the genera Cylindrospermopsis and Raphidiopsis [3,4]. This finding supports the synonymy of these names and has resulted in its reclassification as Raphidiopsis raciborskii. This species is morphologically characterized by isopolar trichomes that feature terminal heterocytes and akinetes distributed along their length [5]. Due to its invasive nature, its capacity to outcompete other cyanobacteria, like Microcystis aeruginosa [6], and its propensity to form dense blooms and produce toxins [7], R. raciborskii has become significant concern in recent decades. Toxic strains of this species are known to produce cylindrospermopsin and saxitoxin, potent cyanotoxins that pose a serious risk to human health; however, important biogeographic differences exist, as saxitoxin producing strains of R. raciborskii have to date been reported exclusively from South America, whereas cylindrospermopsin producing strains show a broader global distribution [8]. Cylindrospermopsin is hepatotoxic, nephrotoxic, and cardiotoxic, while saxitoxin is a neurotoxin that can induce paralytic shellfish poisoning and neuromuscular paralysis [9,10,11]. Although these studies do not involve Raphidiopsis raciborskii, recent research on New Zealand Scytonema crispum strains has revealed the largest saxitoxin biosynthetic gene clusters described to date, with genetic differences in modifying enzymes correlating with distinctive toxin profiles among strains [12,13]. Furthermore, blooms of R. raciborskii critically degrade water quality, potentially causing severe ecological imbalances and increasing the economic costs associated with water treatment [14].

R. raciborskii exemplifies the paradox of a “generalist specialist”, a cyanobacterium with remarkable phenotypic plasticity that enables it to thrive in diverse habitats, from oligotrophic reservoirs to hypereutrophic lakes [15]. Since its first report in Indonesia by Woloszynska (1912), this species has achieved a global distribution [16], occurring in environments ranging from rivers to reservoirs of varying trophic status [14]. It has formed blooms in tropical and subtropical regions, such as South America, as well as in temperate zones like Europe and North America; to date, blooms have not been reported in Antarctica [15]. Proposed mechanisms for its dispersal include the transport of akinetes by migratory birds (on their legs or via digestive tracts) or by fish dispersing vegetative cells, while accidental human transport via ship ballast water may also play a significant role [17]. The success of R. raciborskii is largely attributed to its physiological plasticity and tolerance. Although the specific factors driving its expansion remain unclear, most cyanobacteria are influenced by eutrophication and rising temperatures associated with climate change [18,19]. The distribution and ecology of R. raciborskii have been extensively studied, with research focusing on its dispersal mechanisms [20,21], morphological traits such as akinete number and size [22], different morphotypes and environmental factors regulating toxin production [23]. Overall, increased temperatures and high nitrogen and phosphorus concentrations appear to be the main conditions favoring its dispersal and colonization of new environments [24].

Identifying the key drivers behind the expansion and proliferation of this species is crucial for predicting its behavior and designing effective prevention and mitigation strategies. This requires a synthesis of the extensive literature now available. Combining insights from field observations and controlled experiments offers a powerful approach to achieve this goal. Thus, this study aims to integrate evidence from both sources to elucidate the principal factors determining the success of Raphidiopsis raciborskii.

2. Materials and Methods

A database was compiled from scientific publications retrieved from Scopus covering the period 1990–2024. Searches were performed on English, Spanish, and Portuguese titles and abstracts using the keywords raciborskii AND factor OR condition* OR driver*. The results were restricted to articles within the subject areas of Agricultural and Biological Sciences, Environmental Science, and Earth and Planetary Sciences.

The bibliometric search yielded 561 potential papers. In the first stage, 251 papers were excluded based on titles and abstracts that did not meet the criteria. Of the remaining studies, 209 were initially classified as “field-based” and 101 as “experimental”. Field-based studies were defined as field investigations conducted in natural systems (e.g., reservoirs, lakes, shallow lakes), whereas experimental studies referred to research carried out under controlled conditions, either in laboratories or mesocosms. After an in depth review, 88 field-based and 60 experimental studies were retained. In total, 148 papers were included in this study (Figure 1). The literature search was finalized in August 2025. For the creation of the graphs, RStudio version 4.5.1 was used, and QGIS software version 3.40.7 was used for the creation of the maps.

3. Results

A total of 16 variables were identified as drivers for field-based studies, and 10 for experimental ones (Figure 2). Some studies reported more than one driver simultaneously, while others, although recognizing the variable as a driver, did not specify value ranges for it.

Regarding the geographic distribution of field-based research, Brazil had the highest number of studies (n = 33), followed by China (n = 8), the United States (n = 5), and Australia (n = 4). In contrast, African and most Asian countries were represented by comparatively fewer studies. Although Europe contributes many studies overall, most European countries did not exceed three studies each (Figure 3). The most studied natural systems were lakes and reservoirs, the former being well represented in Europe and North America, while the latter are mainly found in Brazil, China, and parts of Asia. Studies on wetlands, ponds, and rivers are more scattered and are mainly found in Africa and Europe (Figure 3).

Meanwhile, 87.9% of laboratory experiments were conducted in Erlenmeyer flasks, 12.1% in open-air microcosms, mesocosms, or limnocorrals.

3.1. Physical Drivers

3.1.1. Temperature

Our analysis identified 65 field-based studies that reported temperature as the most frequent abiotic driver for R. raciborskii dominance (Figure 2 and Figure 4). Among the field studies, optimal temperatures were detected across temperate, subtropical, and tropical climates, ranging from a minimum of 11.2 °C [25] to a maximum of 35 °C in lakes in Saudi Arabia [26]. The remaining 13 studies identified temperature as a driver without specifying the exact ranges. However, the most common temperatures in reservoirs with blooms were generally above 23 °C. Records of lower temperatures were rare, for example, 12.6 °C and 12.7 °C in Brazil and Taiwan [27,28], 13 °C in China [29], 16 °C in New Zealand [30], 17 °C in Taiwan [31], and 19.5 °C in the United States [32].

When comparing the mean optimal temperature ranges reported by experimental and field studies (Figure 5), a notable consistency emerges. Experimental studies, conducted under controlled conditions, tend to report slightly narrower and higher mean optimal ranges (e.g., 25–32 °C), reflecting the ability to isolate temperature effects without confounding environmental variables. In contrast, field studies show broader mean optimal ranges (e.g., 20–30 °C), capturing the natural variability and the interactive effects of other factors such as nutrients, light, and hydrodynamics. This comparison highlights that while temperature is a key driver across both approaches, field conditions may modulate the realized thermal niche, often extending the lower and upper boundaries of tolerance due to local adaptations and synergistic interactions with other environmental variables.

In experimental studies using cultivated R. raciborskii (n = 29), temperature was also reported as the most important driver among other variables. Occurrence and survival of the species in its vegetative phase under unusually low water temperatures, below 12 °C, the lowest value recorded [33], have been documented, with lower growth rates and more fragile trichomes observed at 12 °C [34], and even dominance reported at temperatures below 15 °C [35]. The highest optimal temperature range for growth reported under laboratory conditions was between 33 and 35 °C [36,37]. In addition, Soares et al. [34] observed denser blooms, accompanied by reduced trichome length, at temperatures above 32 °C. Depending on competition with Microcystis aeruginosa, R. raciborskii exhibited variation in optimal growth temperature, maintaining a competitive advantage between 16 °C and 32 °C [38].

3.1.2. Light

Only nine field studies reported the photosynthetically active radiation (PAR) as relevant drivers. From these, eight concluded that the optimal range of PAR was between 50 and 150 µmol photons·m⁻²·s⁻¹, while a slightly lower range of 30–80 µmol photons·m⁻²·s⁻¹ was reported by Briand et al. [39]. In experimental studies, 35% recognized PAR as influential in growth, with the most common radiation between 50 and 150 µmol photons·m⁻²·s⁻¹, although lower intensities were also used. Evaluations at 25 and 50 µmol photons·m⁻²·s⁻¹ showed no significant differences in akinete differentiation [40]. The highest growth rates were recorded at light intensities around 80–86 μmol photons·m⁻²·s⁻¹, based on testing of 10 strains across a range of 30–400 μmol photons·m⁻²·s⁻¹ [36,39], suggesting that the species’ ability to colonize mid-latitude regions may be linked to its broad light tolerance.

Ultraviolet light was reported in only one experimental study, finding that exposure to UV-B light increased saxitoxin concentration and altered growth, morphology, and heterocyst numbers [41].

3.1.3. Secchi Depth and Turbidity

Eleven field studies evaluated Secchi disk depth, and six evaluated turbidities. In experimental studies, these parameters were not assessed. Both Secchi depth and turbidity showed high variability across ecosystems and were closely linked to R. raciborskii abundance. Low transparency (Secchi depth < 1 m) and high turbidity were consistently associated with the dominance of this species, though there are also consequences of the bloom itself, particularly when considering inorganic turbidity [42,43,44]. For instance, in New Zealand lakes, Secchi depths ranged from 0.01 to 1.3 m, while turbidity reached extremes of 932 NTU [45]. Similar patterns were observed in Serbia’s Ponjavica River, where summer cyanobacterial blooms coincided with low Secchi depth (0.15 m) and turbidity up to 145 NTU, both positively correlated with Raphidiopsis sp. abundance [46]. High turbidity was also associated with blooms in Brazilian reservoirs [28,47] and Lithuanian lakes [48].

However, the relationship between turbidity and R. raciborskii dominance is not unidirectional and depends critically on the origin of the turbidity: organic (phytoplankton biomass) versus inorganic (mineral suspended matter, often following a heavy rainfall). In Muskegon Lake, for example, relatively low turbidity values of 5.62 NTU were sufficient to favor the species’ growth, and turbidity showed strong positive correlations with cyanobacterial biovolume and microcystin concentrations [42]. In semi-arid Brazilian reservoirs, Secchi depth and water volume were key predictors of R. raciborskii blooms, confirming its competitive advantage in turbid, light-limited environments [43]. In contrast, negative correlations have also been reported: in Argentina, turbidity values between 5 and 10 NTU recorded during intense rainfall events, reflecting inorganic suspended matter, were associated with lower abundances of R. raciborskii [14].

It is important to note that in the field studies compiled here, Secchi depth and turbidity were recorded both as pre-existing environmental conditions and as concurrent measurements during bloom events. Consequently, it is not always possible to determine whether reduced water transparency preceded and facilitated the bloom, or whether it resulted from the high phytoplankton density associated with the bloom itself. The negative correlation observed in Argentina (5–10 NTU during heavy precipitation) likely reflects inorganic light limitation driven by mineral suspended matter, rather than a biological response to bloom-derived turbidity. This highlights that similar turbidity ranges can have opposite effects depending on the source of the suspended particles.

3.1.4. Hydrodynamics and Water Column Stability

Water body morphology and hydrological parameters identified as predictors in several field studies included total volume (n = 7), water column depth (n = 4), thermal stratification (n = 5), mixing (n = 1), and water column stability (n = 9). Stable conditions were noted as favorable but not essential for species success [24]. Dominance of Raphidiopsis sp. in Brazilian reservoirs affected by prolonged El Niño droughts was attributed to stability and water volume diminishing [49]. Thermal stratification in tropical reservoirs was associated with species thriving under both stratified and mixed conditions, with high abundances at both the surface and bottom despite a 2.8 °C temperature difference [47]. Decreases in cyanobacterial abundance during cold-dry seasons due to mixing were observed [50]. Proliferation of Raphidiopsis sp. during stratified periods in Lake das Garças was reported, with disappearance during mixing periods. Up to 50% of phytoplankton biomass during stratified periods was attributed to Raphidiopsis sp. [51]. Dominance of Raphidiopsis sp. during mixing periods in Funil Reservoir was also reported, indicating tolerance to mixing and an ability to prosper under such conditions due to redistribution of nutrients and oxygen [52].

Only one experimental study identified aeration as relevant, showing that ambient aeration favored R. raciborskii [53].

3.2. Chemical Drivers

3.2.1. Nutrients

A total of 45 field-based studies considered nitrogen as a driver, considering all its forms: total nitrogen, nitrite, nitrate, and ammonium. Particularly, in reservoirs and shallow lakes, total nitrogen was reported as the abundance driver in 12 studies, with concentrations ranging from 0.18 mg/L [54] to 3.67 mg/L [55] and up to 5.23 mg/L in a pond in Thailand [56]. Relative abundance of Raphidiopsis raciborskii of 92% was found with total nitrogen concentrations of 1.694 mg/L in Laguna Blanca, Uruguay [57]. Ammonium showed highly variable ranges, from 0.033 mg/L [43] to 1.009 mg/L in Gazan Dam Lake [26]. Nitrate and nitrite concentrations were, in comparison, less often reported as predictors than ammonium. On the other hand, in experimental studies, nitrogen was less often reported as a driver, with the N/P ratio being more relevant than nitrogen forms, showing a broad range of variation [58,59]. Experiments in limnocorrals tested low (7:1) and high (122:1) N/P ratios, with the community being dominated by R. raciborskii at the end of the experiments, showing the species’ capacity to dominate under extreme ratios [58].

Related to phosphorus, 40 field-based studies identified it as a driver in one of its forms: total phosphorus, soluble reactive phosphorus, etc. The most relevant form in natural water bodies was total phosphorus, while soluble reactive phosphorus was reported only two times as a driver. Opportunistic use of dissolved inorganic phosphorus by the species was suggested [32,39], while a lack of direct dependence on total phosphorus concentrations was reported [49]. In contrast, the number of experimental studies was lower (n = 16). Laboratory experiments demonstrated that different ecotypes of R. raciborskii showed lower growth rates at higher phosphorus concentrations, suggesting a phosphorus storage strategy [60]. Continuous and discontinuous experiments further indicated strong uptake affinity and adaptation of ecotypes to low phosphorus conditions, prioritizing nutrient storage over the growth rate [7]. Lower cell numbers at phosphorus concentrations of 0.00 and 0.01 mg/L compared to ≥0.5 mg/L were observed, supporting the hypothesis of metabolic reorganization as an adaptive response [61].

Regarding micronutrients, zinc, copper, and iron were identified as factors in field, while none of them were reported as relevant in experimental studies.

3.2.2. Carbon

Carbon dynamics were reported as a driver in only six experimental studies. A relationship between R. raciborskii, inorganic carbon, and pH was observed. The species efficiently utilized dissolved inorganic carbon (DIC), particularly bicarbonate (HCO₃⁻), under conditions of high DIC availability and elevated pH, an ability enhanced under high light intensity [62]. Light-dependent CO₂ concentrating mechanisms enabling active uptake of both CO₂ and HCO₃⁻ were reported [62,63,64]. An increase in partial pressure of CO₂ (pCO₂) in water by injection was shown to raise cellular concentrations and specific growth rates, although this effect varied between strains [64,65]. Elevated pCO₂ levels doubled maximum photosynthetic rates (Vmax) despite a decrease in affinity for CO₂ [63,64]. However, very high pCO₂ (e.g., 1000 ppm) resulted in lower cell concentrations than at intermediate levels (750 ppm), potentially due to light limitation or imbalances in carbon assimilation [65].

Phosphorus availability critically modulated the response to carbon: high phosphorus availability amplified the positive effects of elevated CO₂ on photosynthetic parameters and nitrate uptake, while low P availability suppressed them [64]. Furthermore, carbon availability mediated competitive outcomes, with R. raciborskii dominating under CO₂-favorable conditions (low alkalinity), whereas Microcystis aeruginosa competed better under high bicarbonate conditions [53].

3.2.3. pH

pH was among the most frequently reported factors influencing R. raciborskii abundance, with 27 field studies identifying it as a driver. Higher values were predominant, with tolerance reported between 6.9 and 10 [24,66,67,68]. Tolerance starting from 5.8 was also reported [23,27,51]. In experimental studies, growth between 6.9 and 10 was reported [69], as well as ranges between 6.8 and 8.2 [53,70].

3.2.4. Salinity and Conductivity

Salinity was rarely evaluated as a factor in field studies (n = 7). Considering electrical conductivity as a driver adds 11 field studies, with values ranging from 80.9 to 864 µS/cm. Preference for low salinity conditions between 0.5 and 5 ppm was reported [16,46,71,72]. Broader ranges of 0.5–8 ppm were also noted, allowing survival in mesohaline waters [15,39]. Only three experimental studies identified salinity as relevant, reporting tolerance within 0.5–5 ppm [73]. Tolerance limits up to 4 g L⁻¹ NaCl was demonstrated, with high estuarine salinity suggested as a controlling factor of species dominance [74].

3.2.5. Dissolved Oxygen

Dissolved oxygen (DO) was recorded as a driver in 8 field studies. DO concentrations varied significantly across ecosystems and seasons, ranging from 2.0 to 12.5 mg L⁻¹. Seasonal trends showed higher DO values typically recorded during the dry season in Vietnam and Brazil [75,76]. A positive correlation between DO concentrations and both the biovolume of R. raciborskii and cylindrospermopsin toxin concentrations was reported [75]. In Nanpeng Reservoir (China), the highest DO levels coincided with the bloom stage of R. raciborskii, while lower DO occurred during the low-bloom stage [77].

3.3. Other Factors

Some factors were scarcely analyzed in field studies. Precipitation was addressed in only three papers. Intense cyanobacterial blooms dominated by R. raciborskii during years with the lowest rainfall were reported, with rainfall promoting destratification and turbidity, thereby limiting bloom intensity [14]. Positive relationships between rainfall and R. raciborskii abundance were also reported, combined with temperature and nutrient levels, although extreme rainfall events decreased abundance by more than 60% [61]. In Brazilian reservoirs, rainfall was positively correlated with abundance, as nutrient input via runoff reorganized the community and favored species establishment [78].

Climate change (global warming) was addressed in only two studies. Expansion of R. raciborskii was proposed to result from earlier increases in water temperature, facilitating earlier germination and greater light availability [79]. A review reached similar conclusions, highlighting global warming as a driver of expansion into temperate regions and emphasizing physiological adaptability as a key factor [80].

Alkaline phosphatase activity was analyzed in only one study. Activity was reported as positively associated with R. raciborskii, accounting for 89% of total phosphorus uptake by phytoplankton. Elevated activity coincided with higher phytoplankton biomass and predominance of cyanobacteria, including R. raciborskii [81].

4. Discussion

4.1. Temperature as a Universal Driver

Our results are consistent with the extensive literature that positions temperature as a primary factor promoting cyanobacterial blooms in general [82,83,84]. The broad thermal niche (11–35 °C) helps explain its global expansion from tropical to temperate regions. However, its preference for temperatures above 23 °C suggests that warming due to climate change could favor its competitive dominance over other phytoplankton. Although historically unreported in polar regions or at temperatures below 10 °C [16,85,86,87], its documented survival at temperatures as low as 5 °C indicates a greater thermal tolerance than previously assumed [88]. Rare reports of growth at lower temperatures (e.g., 12–13 °C) [29], often from colder climates, suggest that local ecotypes may be adapting, a phenomenon requiring further investigation.

Some recent studies have begun to question whether the influence of temperature has been overestimated, emphasizing that nutrients may be more decisive [1]. Indeed, in warmer regions, water bodies often exhibit higher trophic states, which strongly promote cyanobacterial blooms. Aquatic productivity and trophic status generally decrease from the equator toward the poles, highlighting that temperature and nutrients act in concert [89,90]. Thus, while temperature is a key driver, it rarely acts in isolation; its effects are frequently modulated by nutrient availability, as discussed below. This interplay also extends to colder environments: cyanobacteria can be abundant in the benthos of oligotrophic subarctic lakes, underscoring that their success is not solely dependent on warm, nutrient-rich conditions [83]. Such considerations align with studies by Håkanson et al. [91] and further support the need to examine temperature–nutrient interactions more explicitly.

4.2. Discrepancies in Nutrient Relevance

In natural systems, especially lentic environments, nutrients play a key role for cyanobacteria and phytoplankton in general, with nitrogen and phosphorus concentrations being critical factors. However, under experimental conditions, nutrient availability has been shown to significantly affect the growth and physiological performance of Raphidiopsis (Cylindrospermopsis) raciborskii; nevertheless, most culture-based studies rely on standard media with fixed nutrient concentrations, with comparatively few exploring modified media or nutrient interactions in depth. In general, authors have recognized that nitrogen concentration is related to the dominance of the species, although ammonium appears to be the most favorable form of nitrogen [8,15,92]. Importantly, this species is capable of fixing atmospheric nitrogen under nitrogen-depleted conditions through the differentiation of specialized cells (heterocytes), which provides a competitive advantage in environments with low dissolved inorganic nitrogen availability [93]. In addition, it grows well on ammonium and nitrate [94].

Phosphorus is another determining element, although it has long been considered the limiting nutrient for phytoplankton growth. While the species may be associated with eutrophic environments, it has been shown to grow easily in environments with phosphorus limitation [24], as long as ammonium or nitrates are present [95]. The species’ success under phosphorus limitation can be explained by its ability to efficiently uptake and store dissolved inorganic phosphorus (DIP) when available, prioritizing storage over immediate growth. This strategy, coupled with its capacity to utilize ammonium as a preferred nitrogen source and to fix atmospheric nitrogen when necessary, allows R. raciborskii to survive and dominate in fluctuating nutrient conditions where other phytoplankton might be limited.

4.3. Light, Context-Dependent Influence

In experimental setups, radiation comes second as an important parameter (based on the number of reports), particularly light and photoperiod. In contrast, PAR in natural environments was less reported as a driver for the species, even though numerous field studies have recognized that light availability influences the species across a wide range of intensities [16,41,87,91]. Several studies suggest that the species is actually tolerant to low light conditions due to chromatic adaptation and the ability to increase the concentration of phycobiliproteins, expanding the light spectrum it can absorb [25,69,91,92]. These adaptations enable the species to compete successfully against other cyanobacteria in turbid waters.

Field observations reinforce this view: the inverse relationship between Secchi depth and R. raciborskii dominance underscores this species’ adaptation to light-limited, turbid waters. Its tolerance and efficient light-harvesting strategies allow it to outperform competitors in eutrophic systems where high phytoplankton density reduces light penetration. This creates a feedback loop: blooms increase turbidity and shading, consolidating the species dominance. Collectively, these findings position Secchi depth and turbidity as practical, integrative indicators for bloom risk assessment. Managing water clarity through sediment and nutrient control could disrupt the cycle of bloom persistence, particularly in shallow, eutrophic systems vulnerable to R. raciborskii invasion.

The dual role of Secchi depth and turbidity—as both drivers and consequences of cyanobacterial blooms—must be interpreted with caution. High phytoplankton density itself reduces water transparency, creating a feedback loop that consolidates bloom dominance. However, turbidity driven by inorganic suspended matter, such as that mobilized by intense precipitation events, may instead impose light limitation that suppresses bloom development. Future field studies should explicitly distinguish between organic and inorganic turbidity sources to better resolve the causal direction of these relationships.

4.4. pH and Water Chemistry

pH and carbon availability are tightly interconnected drivers shaping the success of R. raciborskii. While pH tends to be less relevant in controlled experimental systems, where it is maintained stable, it plays a major role in natural environments, where intense blooms tend to naturally elevate pH through high photosynthetic activity. The depletion of CO₂ concentration and corresponding increases in bicarbonate (HCO₃⁻) under these conditions can disadvantage species restricted to CO₂ uptake but favor those capable of bicarbonate assimilation. This mechanism partly explains why cyanobacteria often dominate high-pH systems, especially in hard waters where alkalinity further enhances bicarbonate availability.

In this context, carbon emerges as a critical yet understudied factor in field research. The species’ efficiency in utilizing bicarbonate via CO₂ concentrating mechanisms (CCMs) provides a decisive advantage in the high-pH conditions typical of eutrophic, cyanobacteria-dominated blooms where CO₂ is depleted [62,63]. These adaptations not only support growth in elevated-pH environments but also interact with broader environmental changes. Rising atmospheric pCO₂, for instance, may further fuel its growth by reducing the energetic cost of operating CCMs, potentially freeing resources for other processes like toxin production or nutrient acquisition [63,64]. Additionally, the strong interaction between CO₂ and phosphorus [64] suggests that eutrophication and climate change act synergistically to reinforce the species’ dominance. The high strain-specific variability reported [21,65] further emphasizes its plasticity and invasive potential. Addressing this knowledge gap requires incorporating carbon speciation and pH dynamics into long-term monitoring to better anticipate bloom trajectories under changing environmental conditions.

4.5. Geographical Patterns

The comparison between field-based and experimental studies highlights that while temperature consistently drives R. raciborskii success, other factors received different amounts of attention depending on the type of study. Nutrients, pH, and water chemistry dominate in the field, while light is more relevant in laboratory settings. Many variables, including salinity, micronutrients, and their interactions, remain poorly understood.

Caution is warranted when interpreting country-level patterns, particularly for nations with vast and climatically heterogeneous territories. Russia, for instance, spans climatic zones ranging from high Arctic conditions in northern Siberia to subtropical climates along the Black Sea coast, with summer air temperatures that can reach 30–35 °C in boreal and steppe regions and up to 45 °C in the Lower Volga basin. Similarly, China, Brazil, and the United States encompass multiple climatic zones within their borders. In such countries, attributing bloom drivers to a single climatic context oversimplifies the ecological reality. The prevalence of temperature as a reported driver in Russia may therefore reflect not the paradox of a cold country responding to heat, but rather the role of seasonal temperature increases—which coincide with thermal stratification and nutrient upwelling in lakes and reservoirs—as key triggers for R. raciborskii proliferation during the productive summer season, even in subarctic and boreal regions.

Considering the effects of climate change in recent years, it is reasonable to assume that rising temperatures in traditionally cold areas are becoming an ecologically determining factor. Eastern Europe, China, South America, and Oceania also show high percentages for temperature, suggesting that in temperate or subtropical regions—where R. raciborskii is expanding toward higher latitudes—temperature acts as a key factor controlling its proliferation (Figure 6). However, the nutrient map in Figure 6 reveals that many of these same regions also exhibit elevated nutrient levels, particularly in Eastern Europe, parts of China, and agricultural areas in South America. This pattern aligns with the broader understanding that temperature and nutrients rarely act independently; rather, their combined effect often determines the success of cyanobacterial expansion. Thus, while temperature may be the limiting factor at the advancing front of the distribution range, nutrient availability likely modulates the magnitude and intensity of blooms once thermal conditions become suitable.

Areas with a stronger emphasis on nutrients are concentrated in China, India, South America, and Africa. In China, for instance, agricultural expansion may increase nutrient availability through fertilizer application, which could explain why nutrients are more frequently reported as bloom-promoting factors.

Overall, temperature exhibits a more consistent global pattern as the dominant driver, especially in temperate and cold regions where climate warming facilitates the species’ expansion. In contrast, nutrients show a more localized distribution, playing a key role in tropical regions or in countries with eutrophic water bodies, where insufficient sanitation infrastructure may exacerbate nutrient enrichment and eutrophication. In temperate regions, thermal limitation appears to be the main constraint for the proliferation of R. raciborskii, making temperature a key factor in explaining its recent success. Conversely, in warm regions where temperature is already optimal, nutrients gain greater relevance in determining the magnitude of the bloom. Thus, temperature acts as an enabling factor for the geographic expansion of the species, whereas nutrients function as modulators of bloom intensity once populations are established.

Short-term fluctuations in environmental conditions, even on the scale of days or months, can explain discrepancies between experimental studies and those conducted in the field. Climate change, in fact, serves as evidence of this. The interaction between rising temperatures in certain areas and changes in precipitation or runoff could explain the emergence of blooms in locations where they were previously absent; however, this interaction has been virtually unassessed in studies.

4.6. Limitations

It is important to consider that experimental studies often focus on one or a few factors, while ecological reality is far more complex. Interactions among variables and the ranges in which they occur can produce unexpected effects on the growth of a species. This methodological schism not only reinforces the paramount importance of abiotic conditions but also highlights a critical limitation in our current understanding, suggesting that a holistic, multi-factorial approach is essential for accurately predicting bloom dynamics in a changing global climate. In this sense, outdoor mesocosm studies should be an important tool for reducing the gap.

Studies often assume variables act independently, but there may be synergistic effects. Designing multifactorial experiments can therefore help identify the combinations that truly enhance blooms. Conversely, understanding which combinations promote abundance can also provide a starting point for identifying those that limit it, contributing to potential solutions for the challenges posed by cyanobacteria.

Moreover, the studies compiled in this work did not consider biotic interactions such as allelopathy or grazing [82,83]. And finally, the geographically skewed distribution of the studies, with a clear underrepresentation of Africa and Asia, makes it difficult to extrapolate conclusions on a global scale. While phytoplankton dynamics have traditionally been linked to environmental drivers, cyanobacteria are increasingly recognized for thriving under higher temperatures and elevated nutrient levels, conditions intensified by climate change and global agricultural practices. More importantly, our synthesis indicates that, although field and experimental studies generally focus on similar drivers, there are still notable gaps, and several drivers identified in the field are rarely tested under controlled conditions.

5. Conclusions

Based on the analyzed studies, Raphidiopsis raciborskii dominance is primarily influenced by temperature, showing optimal growth between 23 and 35 °C and survival at lower temperatures (11–12 °C), which reflects high physiological plasticity. Temperature, however, interacts with nutrient availability rather than acting independently.

A methodological gap persists between field and laboratory studies: the former emphasizes nutrient concentrations, especially ammonium nitrogen and pH, while the latter focuses on light and N/P ratios. Integrative approaches, such as mesocosm experiments, are needed to bridge this divide.

Nutrients, particularly nitrogen and phosphorus, emerge as key factors in field environments, whereas experimental research has more often examined carbon dynamics. The species’ capacity to use bicarbonate efficiently, store phosphorus, and adapt to low light provides strong competitive advantages in fluctuating eutrophic systems, explaining its invasive potential.

R. raciborskii tends to prefer eutrophic, low-salinity, and stable conditions; high salinity appears to be a limiting factor, though this aspect remains understudied. Finally, the geographic bias in current research, with limited representation of Africa and Asia, restricts the global applicability of these findings, highlighting the need for broader spatial coverage to better understand its ecological behavior across climates.