Lacustrine Cyanobacteria, Algal Blooms and Cyanotoxins in East Africa: Implications for Human and Ecological Health Protection

: Advected cyanobacteria, algal blooms and cyanotoxins have been increasingly detected in freshwater ecosystems. This review gives an insight into the present state of knowledge on the taxonomy, dynamics, toxic effects, human and ecological health implications of cyanobacteria, algal blooms and cyanotoxins in the East African Community lakes. The major toxigenic microalgae in East African lakes include Microcystis , Arthrospira , Dolichospermum , Planktolyngbya and Anabaenopsis species. Anatoxin-a, homoanatoxin-a, microcystins (MCs), cylindrospermopsin and nodularin have been quantiﬁed in water from below method detection limits to 81 µ g L − 1 , with peak concentrations characteristically reported for the wet season. In whole ﬁsh, gut, liver and muscles, MCs have been found at concentrations of 2.4 to 1479.24 µ g kg − 1 , which can pose human health risks to a daily consumer. While there have been no reported cases of cyanotoxin-related poisoning in humans, MCs and anatoxin-a (up to 0.0514 µ g kg − 1 ) have been identiﬁed as the proximal cause of indiscriminate ﬁsh kills and epornitic mortality of algivorous Phoeniconaias minor (lesser ﬂamingos). With the unequivocal increase in climate change and variability, algal blooms and cyanotoxins will increase in frequency and severity, and this will necessitate swift action towards the mitigation of nutrient-rich pollutants loading into lakes in the region.


Introduction
Industrialization has been the key driver of economic growth and inclusive prosperity because it does not only foster economic and infrastructural development, but also enhances the realization of some vital targets enshrined in the 2015-2030 Sustainable Development Goals [1,2]. This is evident from the employment opportunities it creates, improved working conditions, optimal resource use [3] and the innovations that have led to nascent and environmentally benign (greener) production technologies [4,5]. Despite this, there are various environmental challenges that have been associated with industrialization. One of

Occurrence of Cyanobacteria, Algal Blooms and Phycotoxins in EAC Lakes
Toxic and non-toxic CYB are photosynthetic prokaryotes that occur naturally in terrestrial as well as aquatic ecosystems [10]. They are typically larger than normal bacterial cells, and their inherent mass production of phycobilin pigment confers upon them a bluish tint at high concentrations, hence their naming as blue-green algae [7]. They are Gram-negative bacteria that may be filamentous, unicellular or multicellular (occuring as colonies), contingent on the prevailing conditions. Under suitable environmental conditions that afford competitive advantages (e.g., alkaline pH, buoyancy, high sunlight-for conversion of ferric ion to ferrous ion, moderate temperature, i.e., 20 • C to 30 • C (10 • C in winter for Planktothrix rubescens), nutrients phosphorous and nitrogen, and water column stability), CYB are capable of proliferating and forming CYBHAB or scums in the upper sunlit layers. Such unsightly scums and blooms contain malodorous compounds such as geosmin and methylisoborneol, which are responsible for the aesthetically unpleasant taste of CYB-contaminated water. While the biology and ecology of CYB has been a subject of intensive research globally, there is a paucity of clearly articulated information regarding factors and processes that regulate toxin production in most cyanobacterial species [12]. In lentic freshwater resources (such as L. Victoria, Lake Tanganyika and Lake Kivu in the EAC), the occurrence of CYB is favored by climate variability, anthropogenic activities, hydrological shifts and high nutrient loads [19]. In part (for Ugandan lakes such as Mburo and Kachera), loading is from influx of nutrient-rich hippopotamus and cattle dung wastes [20].
Cyanobacteria are usually associated with the production of nocive cyanotoxins. The great diversity and high metabolic potential of CYB implies that there are other unknown or at least little studied cyanotoxins. According to CyanoMetDB (a comprehensive database of cyano-metabolites), at least 2000 molecules, including more than 300 microcystin congeners, are already known [21]. Cyanotoxins are contaminants of emerging concern that are potentially (eco)toxic. They can adversely impact ecosystem services provided by water resources by depleting oxygen, altering food webs, species assemblages and poisoning animals and humans [22]. Examples of cyanotoxins include cyclic hepatotoxic peptides (microcystins, nodularins), dermatoxic, cytotoxic, genotoxic or neurotoxic alkaloids, polyketides and amino acids (lyngbyatoxin-a, cylindrospermopsins, anatoxins, saxitoxins, aetokthonotoxin, lipopolysaccharides (endotoxins), guanitoxin, beta-N-methylamino-L-alanine and aplysiatoxins) [22].
The frequently encountered cyanotoxins are anatoxins, cylindrospermopsin (CYN), nodularins (NODs), saxitoxin, and microcystins (MCs), but the most-studied members are MCs (the -LR variant). Thus, in addition to anatoxin-a (ATX), they are the main cyanotoxins that garnered early scientific interest in Eastern Africa ( Figure 2). In the EAC, toxigenic freshwater CYB and CYBHAB have been implicated in the reoccurrence of eutrophic and hypoxic conditions in L. Victoria [14,[23][24][25][26][27]. Table S1 shows a summary of reports on the occurrence and abundance of CYB and other phytoplankton, their dominant species and MCs in EAC lakes. These are discussed per country in the following. Phycology 2023, 3, FOR PEER REVIEW 5

DRC
In the Congolese part of the oligotrophic Lake Tanganyika, the occurrence of CYB (Dolichospermum flosaquae, Anabaenopsis species and Limnococcus limneticus), along with Nitzschia asterionelloides (Bacillariophyta), has been reported [28,29]. These diazotrophic CYB were implicated in the CYBHAB witnessed in 1955 and 2018, but cyanotoxin concentrations of water sampled from the lake have not been established.
Another unique lake in DRC is Lake Kivu, a deep oligotrophic and meromictic water resource. Though it contains copious volumes of exploitable methane, CYB (Synechococcus species and Planktolyngbya limnetica) dominate the phytoplankton biomass in this lake, followed by pennate diatoms (Nitzschia bacata and Fragilaria danica) [30,31]. These reports resonate well with that of Hecky and Kling [32], who showed that CYB and chlorophytes (with biomass contents that are higher than in the neighboring Lake Tanganyika) dominated in Lake Kivu. Their report, however, pointed to the presence of additional CYB Lyngbya circumcreta West, Anabaenopsis, Cylindrospermopsis and Raphidiopsis species. Sarmento et al. [33] found a strikingly contradicting result, with pennate diatoms being more abundant than CYB in Lake Kivu. Nevertheless, picocyanobacterial Synechococcus species was found to still form a significant proportion of the annual autotrophic plankton of the lake [34]. Further, Urosolenia and Microcystis genera are dominant holopelagic species under certain stratification scenarios [33]. The dominance of diatoms in Lake Kivu thus seems to occur solely during dryer periods, when deep mixing occurs [35].

DRC
In the Congolese part of the oligotrophic Lake Tanganyika, the occurrence of CYB (Dolichospermum flosaquae, Anabaenopsis species and Limnococcus limneticus), along with Nitzschia asterionelloides (Bacillariophyta), has been reported [28,29]. These diazotrophic CYB were implicated in the CYBHAB witnessed in 1955 and 2018, but cyanotoxin concentrations of water sampled from the lake have not been established.
Another unique lake in DRC is Lake Kivu, a deep oligotrophic and meromictic water resource. Though it contains copious volumes of exploitable methane, CYB (Synechococcus species and Planktolyngbya limnetica) dominate the phytoplankton biomass in this lake, followed by pennate diatoms (Nitzschia bacata and Fragilaria danica) [30,31]. These reports resonate well with that of Hecky and Kling [32], who showed that CYB and chlorophytes (with biomass contents that are higher than in the neighboring Lake Tanganyika) dominated in Lake Kivu. Their report, however, pointed to the presence of additional CYB Lyngbya circumcreta West, Anabaenopsis, Cylindrospermopsis and Raphidiopsis species. Sarmento et al. [33] found a strikingly contradicting result, with pennate diatoms being more abundant than CYB in Lake Kivu. Nevertheless, picocyanobacterial Synechococcus species was found to still form a significant proportion of the annual autotrophic plankton of the lake [34]. Further, Urosolenia and Microcystis genera are dominant holopelagic species under certain stratification scenarios [33]. The dominance of diatoms in Lake Kivu thus seems to occur solely during dryer periods, when deep mixing occurs [35].
Other reports on CYB in Kenya are from its L. Victoria part (bays, gulfs and satellite lakes), where CYB (>35%) and diatoms (>30%) of Microcystis, Merismopedia and Dolichospermum species are the primary phytoplankton (Table S1). As far as cyanotoxins are concerned, Sitoki and others [46] detailed the incidence of MCs in L. Victoria water. They concluded that the levels varied greatly between seasons. This comes in concordance with later inferences of other researchers [47,48] who investigated MCs contamination of water and fish consumed by fisher communities of Winam Gulf, Homa Bay, Kisumu, Siaya and Busia counties of L. Victoria (Kenya). Regrettably, up to 30% of water from these points exceeded the regulatory set value (1.0 µg L −1 ) of the WHO [48]. The study suggested that CYBHAB pose potential year-round health risks to riparian communities [48].
Recently, a team of researchers collated the insights and awareness of L. Victoria shore community on MCs toxicity [47]. The authors appreciated that more than 70% of the fisherfolk are conversant with the toxic effects of MCs, and showed the urgency required to mitigate them. An earlier investigation [49] echoed that higher average values of MCs (5 to 109 µg kg −1 ) occurred in fish from Nyanza Gulf (Kisumu Bay) compared with those from Rusinga channel water (14 µg kg −1 ). These reports reaffirmed that CYBHAB are recurrent in L. Victoria.
For L. Victoria, the occurrence of CYB (upto 82%) was quantified in several parts of the southern part. Miles et al. [53] found putative MCs analogues in extracts of a cyanobacterial bloom from Mwanza Gulf but did not quantify them. On 27 islands of Ukerewe district, MCs (0.0028 to 0.0102 µg L −1 ) were reported [54]. Other studies in L. Victoria (several bays, open water and Gulfs) have found MCs (up to 13 µg MC-LR eq L −1 ; Table S1). An incidence of multiple cyanotoxins: CYN (0.004 to 0.01 µg L −1 ), NODs (0.010 µg L −1 ) and MCs (0.0028 to 0.0118 µg L −1 ) in water from L. Victoria has been communicated [54]. The report emphasized that multiple and repeated exposure to phycotoxins could amplify their toxicity and/or adverse effects.
Phycology 2023, 3, FOR PEER REVIEW 7 MCs (0.0028 to 0.0118 µg L −1 ) in water from L. Victoria has been communicated [54]. The report emphasized that multiple and repeated exposure to phycotoxins could amplify their toxicity and/or adverse effects.
In the Ugandan part of L. Victoria, Microcystis, Dolichospermum and Cylindrospermopsis species are the prevalent CYB (>80%) ( Table S1). Cyanotoxin analyses have reported concentrations of UDT to 93 µg L −1 of MCs in water from Murchison Bay, Napoleon gulf and open lake water. Worth citing are pioneering studies in Murchison Bay where MCs were quantified in Oreochromis niloticus (Nile tilapia fish), unveiling that the concentrations in biota and aqueous phase were correlated. The study highlighted that there has been an increase in MCs-producing CYB in the lake which are plausibly ingested by fish, agreeing with previous research findings [61,65,66]. The maximum concentration of total MCs reported for guts, liver and muscles of phytoplanktivorous Oreochromis niloticus (Nile tilapia) and Lates niloticus (Nile perch) from Murchison Bay of L. Victoria (1.86 to 1479.24 µg kg −1 ) is slightly higher than those from other Ugandan lakes such as Lake Mburo (73.10 to 1312 µg kg −1 ) [65]. A study published in 2022 unveiled for the first time the occurrence of homoanatoxin-a (HTX; <0.04 HTX L −1 in water from an inshore station of Murchison Bay [67], along with MCs (0.15-11.7 µg MC-LR eq L −1 ). At recreation sites, MCs (0.180 to 14.800 µg MC-LR eq L −1 ) equally occurred. The study demonstrated that whereas CYB were eliminated by water treatment, MCs remained detectable in water during and posttreatment (0.14 µg L −1 ) [67]. This shows that remediation of cyanotoxins in water from L. Victoria will require more efficient technologies to avoid exposing the local population to potential effects of MCs.
At this point, it can be suggested that shallow lakes in Uganda exhibit less seasonality in their CYB composition when compared with satellite lakes and others in the main L. Victoria basin. Unlike in oligotrophic lakes in the region, the CYB dynamics (spatial and temporal variations in prevalent cyanobacterial genera) in L. Victoria are, however, inconsistent in its different parts. This may be related to external anthropogenic influences, especially nutrient loading, because the lake receives a cocktail of pollutants from different countries.

Rwanda
The only report on CYB in a Rwandese Lake (Lake Muhazi) showed that it contains mainly Microcystis aeruginosa, followed by the dinoflagellate Cerutium hirundinellu [68].
These are ingested by Nile tilapia present in the lake [69], suggesting the need to establish the concentrations of cyanotoxins in water and fish from this lake.
Overall, volcanic and tectonic lakes in the East African Great Rift Valley possess distinguished extents of hydrological connections. Volcanicity in the region resulted in endorheic basins whose bedrock, groundwater connection and climate have favored schizohaline water formation [70]. These, in turn, have contributed to the dominance of CYB, and occurrence of CYBHAB and cyanotoxins. The literature reveals that toxigenic microalgae recorded from EAC lakes are Dolichospermum, Microcystis, Arthrospira, Planktolyngbya and Anabaenopsis species. The prevalence of CYBHAB and cyanotoxins in EAC lakes is of concern due to potential bioaccumulation and trophic transfer in zooplanktivorous and carnivorous fish species [61,66]. Moreover, the observed levels of MCs in whole fish, gut, liver and muscles (2.4 to 1479.24 µg kg −1 ) could pose human health risks to a daily consumer, as the WHO daily intake limit of MCs in fish is 0.04 µg kg −1 [65].
In L. Victoria, cyanobacterial biomasses and MCs levels in water from gulfs and bays comparatively surpasses their levels in the open lake water, with Microcystis and Dolichospermum species being the most prevalent CYB genera. Further, CYBHAB in the lake has increased costs associated with water treatment, e.g., National Water and Sewerage Cooperation Uganda reported increased chlorine demand for water treatment, unpleasant odors and tastes in untreated water supplies, and clogging of pumps and filters. Fishermen have reported that CYB has hampered fishing operations on the lake [71]. Anecdotal reports point that portions of the lake covered by CYB were observed to have small dead fish, whereas larger fish from such brackish waters are often weak and stressed. Earlier (in 1984), indiscriminate fish die-offs were witnessed in L. Victoria (Kenya), and this was plausibly connected with CYBHAB [72]. Similar mass mortalities were observed in 1991 for fish in Lake Magadi, Kenya [73], and this event was anticipated to have been caused by reduction in the algae Spirulina platensis. These effects, according to a recent report [67], may increase in severity in the coming decades. For example, MCs detected from Murchison Bay of L. Victoria now range from 0.20 to 15.00 µg MC-LR eq L −1 , which is higher than those reported previously (0.20 to 0.70 and UDT to 1.6 µg MC-LR eq L −1 between 2004 and 2005, and then 2007 to 2008) [66,74], possibly due to the doubling of the mean Microcystis biovolume [67]. The EAC recreational waters are not often screened for pelagic cyanobacterial species, implying that the sanitary activities in contaminated lakes may expose both humans and animals to the potential negative effects of CYBHAB and cyanotoxins.

MCs
MCs are hepatotoxins, majorly produced as secondary metabolites of planktonic cyanobacterial species from genera such as Microcystis, Cylindrospermopsis, Anabaena, Oscillatoria (Planktothrix), Anabaenopsis, Nostoc, Arthrospira, Hapalosiphon, Limnothrix, Lyngbya, Phormidium, Rivularia, Synechocystis and Synechococcus [75]. Acute effects such as nausea, diarrhea, dermal, eye and throat irritations have been associated with their ingestion. Chronic exposure to MCs culminates in hepatic necrosis, retarded growth, reduced reproduction potential and, ultimately, death in fish and humans. The neurotoxicity of MCs is also known, but this applies specifically to invertebrates without livers [76]. In addition, exposure to MCs is associated with colorectal and primary liver cancer, with MC-LR receiving classification as a possible human carcinogen (group 2B) [77]. For humans, exposure to MCs occurs principally through the ingestion of contaminated aquatic organisms (e.g., fish) or water, as well as through the recreational use of water. Upon ingestion and absorption into the liver by organic anion transport proteins, MCs inhibit protein phosphatases, thereby selectively distorting cytoskeleton formation, degrading hepatic ultrastructure in eukaryotic cells, resulting in hepatic failure, intrahepatic hemorrhage and shock [78,79].
In EAC, MCs and ATX were implicated in the death of Phoeniconaias minor Geoffroy Saint-Hilaire 1798 (lesser flamingos) [80]. The pink birds (Figure 4a) feed on A. fusiformis [81], which confers upon them the pink plumage following the accumula-tion of ingested cyanobacterial pigments [82]. While this phenomenon is not new (e.g., in the Greater flamingos and Western Tanager [83,84]), it should be anticipated that other nutrition-based compounds may become bioaccumulated in lesser flamingos, e.g., potentially toxic metals. Event-driven reports of lesser flamingo die-offs are available for soda lakes such as Bogoria and Nakuru of Kenya [85,86], Momela, Natron, Rishateni, Manyara and Empakai Crater of Tanzania [13,51,87,88] (Table 2). Some of these reports substantiated the anecdotal claims by quantifying MCs and ATX levels in carcasses of the birds. Krienitz et al. extended the hypothesis further and examined the concentration of MCs and ATX in Lake Bogoria, in the surrounding hot springs and in flamingo birds [89]. They concluded that the cyanotoxins from the hot-spring mats could be responsible for the mass mortalities of the birds because: (i), there were evident cyanobacterial cells, fragments in hot spring mats, and elevated levels of ATX and MCs (0.00434 and 0.000196 µg kg −1 ); and (ii) there were clinically indisputable signs of flamingo intoxications. Such intoxication with the biotoxins could plausibly have been caused by direct or indirect intake of CYB or their cells [89]. Wings, breasts and head feathers of the flamingos reportedly had ATX and MCs concentrations ranging from UDT to 0.03 MC-LR eq µg kg −1 [90]. Moreover, amino acid neurotoxins β-N-methylamino-L-alanine (0.0035 µg kg −1 DW) and 2,4-diaminobutyric acid (0.0085 µg kg −1 DW) were recently quantified in lesser flamingo feathers from Lake Nakuru [91].      10th June, 1st and 4th July 1993 Birds (unreported number), and 1 coot, 1 duck. ATX was recorded at 3.30 µg kg −1 while MCs occurred at 0.0001 to 0.0009 µg kg −1 28th June and 6th July 1994 1 duck 9th July 1995 1,2,3 In part, Pseudomonas aeruginosa, Mycobacterium avium, Escherichia coli and heavy metals were claimed to be contributors to these flamingo die-offs [96,103,104]. * In 2004, over 43,800 flamingo die-off was experienced in this lake [105]. ** Several other animals (6 dogs, cows, crows and some fish) succumbed to cyanotoxins after swimming in this lake or drinking its water, which was rich in Anabaena species.
While it is still debated that MCs may be a potential initiator of avian botulism, other probable causes of the unnatural mass death of wild birds include avian tuberculosis [96], cholera, botulism, heavy metals [95,106], pesticide residues, or combinations of these [85,98,103,107,108]. Indeed, mycobacteriosis was reported in lesser flamingos from Lake Nakuru, Kenya [109]. Nevertheless, anatoxins and MCs (at concentrations higher than reported in EAC flamingos) have been associated with avian mortalities [98,101,102,110,111]. From an ecological perspective, lesser flamingos are "Near Threatened" supported by the limited and ungazetted nesting areas, as well as the reduced bird populations [112][113][114]. At present, they reportedly breed in only five sites: one in EAC (Lake Natron, with at least 75% of the breeding birds), Etosha and Makgadikgadi Pans in Namibia and Botswana, and Purabcheria Salt and Zinzuwadia Pans of India [115]. An attempt has been made to hand-rear lesser flamingos so as to restore and conserve this rare species of birds [116], but there is no available literature with any registered success.

Anatoxin-a
ATX is toxicologically known as Very Fast Death Factor for its fast lethal effect in animals, which could be related to its high rate of absorption into the gastrointestinal tract [117]. ATX is a secondary bicyclic amine alkaloid with peracute neurotoxic effects. Its discovery and identification in the 1960s and 1972 from CYB (Anabaena flos-aquae) followed the mortality of cattle herds that ingested contaminated water from Saskatchewan Lake in Ontario [117]. It is known to be biosynthesized by CYB from Arthrospira, Anabaena, Microcystis, Planktothrix, Oscillatoria, Aphanizomenon and Cylindrospermum genus [117].
Exposure to ATX (through ingestion of contaminated water or dried algal crusts, accidental swallowing/inhalation) has been associated with burning, tingling, respiratory paralysis and dysrhythmias, which are fatal. ATX antagonizes the activity of neuronal α4β2 and α4 nicotinic acetylcholine receptors (nAchRs) of the central nervous system and (α1)2βγδ muscle-type nAchRs of the neuromuscular junction [118]. With an affinity >20 times that of acetylcholine, ATX has the same effect as the former when it binds with nAchRs, i.e., it induces a conformational effect on the receptor, opening the channel pore to permit the passage of ions (Ca 2+ and Na + ) into the neuron. This culminates into cell depolarization, the generation of action potentials and thus muscle contraction. During ATX-mediated toxicity, the acetylcholine neurotransmitter does not dissociate from the nAchRs, resulting into irreversible inhibition and blockage of neuromuscular transmission [119]. This inhibitory effect generally accumulates the neurotransmitter within the synaptic cleft, eventually causing paralysis, asphyxiation and death, specifically if respiratory muscles are affected. In the epornitic mortalities of algivorous EAC lesser flamingos (such as in Lake Bogoria; Figure 4b), clinical symptoms have included opisthotonus, supporting that such die-offs are (at least in part) due to ATX intoxication. Other than the foregoing, ATX possess modulatory effects on nAChRs, which can result in the release of dopamine and noradrenaline [120].

Homoanatoxin-a
HATX being structurally a higher homologue of ATX has the same toxic effects as ATX. In addition to its nicotinic agonistic effects, HATX also upregulates acetylcholine release from cholinergic nerves [121]. This may explain why the potency of HATX is greater than that of ATX. Mortalities from CYBHAB with HATX are rare, but a report of dog neurotoxicosis from New Zealand (where the animals ingested CYB from Hutt River, lower North Island with 4400 µg kg −1 wet weight of HATX) has been published [122].

Cylindrospermopsin
CYN is a hydrophilic potentially hepatotoxic and immunotoxic cyclic guanidinium alkaloid, with characteristic tricyclic hydroxymethyl uracil [76]. It has some analogues such as deoxy-CYN (lacking an oxygen atom), demethoxy-CYN and 7-epiCYN (difference in the orientation of hydroxyl group) isolated in CYB Cylindrospermopsis raciborskii. The discovery of CYN toxicity happened when more than 100 children from Palm Island in Queensland, Australia suffered from unprecedented gastroenteritis and hepatomegaly. The ordeal was finally found to be due to the ingestion of CYN in contaminated water with CYBHAB of C. raciborskii [123]. However, CYN is also produced by other CYB, including Aphanizomenon flos-aquae, Anabaena species (bergii, and lapponica), Aphanizomenon ovalisporum, Lyngbya wollei, Raphidiopsis curvata Oscillatoria (Planktothrix) species and Umezakia natans [75]. With guideline values of 0.5 to 3 µg L −1 in drinking water across continents, CYN is the second-most-studied cyanotoxin known to target the liver, kidneys, heart, spleen, ovary, eye, lung, T lymphocytes, neutrophils and vascular endothelium [124]. CYN elicit toxicity through inhibition of protein synthesis, which can also occur at subtoxic concentrations [125]. Other toxicologists stated that CYN (with its inherent reactive guanidine) could be largely toxic through the induction of DNA wreckage and disruption of the kinetochore spindle. This could possibly result in chromosome loss, aneugenic and clastogenic effects [126]. Chichova et al. [124] found that CYN elicited moderate toxicity in human intestinal epithelial cells with suppression of cellular regeneration of the epithelial layer. CYN shows hepatotoxic, nephrotoxic, and cytotoxic effects, suggesting potential carcinogenicity. The neurotoxic potential of CYN has also been cited, though this could be a direct consequence of its cytotoxicity. To this end, the full underlying mechanisms of CYN toxicity needs to be elucidated [76].
In the EAC, there are no toxicity reports on CYN, which may be due to the absence of robust data on this cyanotoxin. There are, however, episodes of human and animal CYN-related poisoning from other countries. The most notable human poisoning is the 1979 Solomon dam gastroenteritis and hepatomegaly incidence in children from Palm Island [123]. The mortality of a cow and three calves after drinking water from McKinley Shire dam, Northern Queensland (Australia) was also reported. The animals had severe abdominal and thoracic haemorrhagic effusion, hyperaemic mesentery, pale and swollen liver, extremely distended gall bladder with dark yellow bile and epicardial haemorrhages [127]. In the subsequent 21 days, another eight animals (two cows and six calves) died, and analyses implicated CYN in C. raciborskii as the cause [127]. In Lake Aleksandrovac (Serbia), indiscriminate fish deaths due to the ingestion of CYN (range: 1.91 and 24.28 µg L −1 ) were reported [128]. This report may point to the need to establish CYN levels in EAC lakes where indiscriminate fish deaths have been reported, as CYN may be a contributing factor in addition to MCs.

Nodularins
Nodularins, a class of hepatotoxic non-ribosomal cyclic pentapeptides, possess toxicity mechanisms similar to those of MCs [129]. They are structurally analogous to MCs, but differentiable from MCs in their amino acid components (Figure 2). To date, ten naturally occurring variants (isoforms) of NODs have been discovered, but nodularin-R (with Z amino acid = arginine) is the most common, most commercially available and most studied variant. The toxicity of NODs mainly targets the liver, but they also accumulate in the intestines, blood and kidneys [130]. Upon ingestion, NODs diffuse from the proximal and distal ileum into the liver [131], where they inhibit active sites of serine/threonine protein phosphatases (PP) namely: 1 (PP-1), 2A (PP-2A) and 3 (PP-3). A non-covalent interaction occurs at first with the side chain (ADDA part) and a free D-glutamyl carboxyl group in the cyclic structure of the PP, followed by the inhibition of the phosphatase activities. NODsphosphatase complexes (NODs-PP-1 and NODs-PP-2A) are formed with exceptionally stable bonds. Thus, the key difference between NODs and MCs in their toxicity via protein phosphatases inhibition is that the former binds non-covalently to phosphatases, while the latter forms a covalent bond [130].
Furthermore, NODs also elicit toxicity through formation of superoxide and hydroxyl radicals (reactive oxygen species) according to a yet incompletely elucidated pathway [130]. Their tumor-promoting activity is, on the other hand, mediated through the induced gene expression of TNF-alpha and proto-oncogenes, the exact mechanism of which is yet to be unraveled. In addition, the deactivation of the resultant tumor suppressor gene products (retinoblastoma and p53) progresses via phosphorylation, and this inevitably promotes tumorigenesis [132]. Overall, the cascade of reactions following NOD ingestion causes cellular disorganizations and damages, apoptosis, necrosis, loss of cell integrity, DNA fragmentation and strand breaks, intrahepatic bleeding and rapid blistering of hepatocytes which results in blood pooling and doubling of the liver weight [133]. Thus, mortalities associated with NOD poisoning is mediated through hemorrhagic shocks, which occurs in a few hours when ingested at high concentrations [134].
There are no toxicity events involving NODs in the EAC. Nevertheless, animal (cattle, dog, sheep, horse, pig and guinea pig) NOD-poisoning-related mortalities have been reported in other parts of the world. For example, hepatotoxicosis of a South African dog following the ingestion of NODs (0.00000347 µg kg −1 DW) was reported [135]. Main et al. [136] recorded 52 sheep deaths in South Western Australia from drinking water contaminated with NODs from Nodularia spurnigena. These reports emphasize that more studies on this cyanotoxin are warranted in EAC lakes.

Conclusions and Recommendations
CYB, CYBHAB and cyanotoxins have increased in EAC lacustrine ecosystems. Dolichospermum, Microcystis, Arthrospira, Planktolyngbya and Anabaenopsis species are the major groups of toxigenic CYB prevalent in EAC lakes producing ATX, HATX, MCs, CYN and NODs. Shallow EAC lakes exhibit less seasonality in their CYB composition, with Microcystis being the CYB producing MCs under shallow and eutrophic lacustrine conditions. The only direct ecological effects of cyanotoxins in EAC lakes is indiscriminate fish deaths and mass die-offs of lesser flamingos. With the unequivocal increase in climate change and variability, it is inferred that CYBHAB and cyanotoxins will increase in frequency and severity. This calls for urgent action to mitigate nutrient-rich pollutants loading into water resources and the expansion of CYBHAB from eutrophic lakes to the surrounding marine environments. The (eco)toxicological relevance of co-production of phycotoxins should be assessed in the EAC because such exposure may amplify the toxicological outcomes in aquatic biota and humans. As some CYB encountered in EAC lakes produce other cyanotoxins (such as β-N-methylamino-L-alanine and saxitoxins), studies targeting these cyanobacterial metabolites should be initiated. While there are no reports of cyanotoxin poisoning of humans in the EAC, future studies should examine the risk of hepatocellular cancer, the ingestion of CYB and mycotoxin-contaminated water and foods, and hepatitis virus, which were earlier linked to increased primary liver cancer cases in Asia. Another potential relationship with microplastics should be assessed because they are known to accumulate toxins and amplify their toxicity.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/phycology3010010/s1, Table S1. Phytoplankton composition and cyanotoxins in East African Community lakes. Data Availability Statement: This is a review article, and no raw data were collected.
Acknowledgments: Ivan Kahwa (Mbarara University of Science and Technology, Uganda) is acknowledged for his insights on crater lakes in Western Uganda, and for the picture of Lake Kamweru presented in this review. The graphical abstract was created with BioRender.com.

Conflicts of Interest:
The authors declare no conflict of interest.