Some strains of cyanobacteria are known to produce potent hepato-, cyto-, neuro- and/or dermatotoxins and other bioactive compounds [1]. Blooms of cyanobacteria and cyanobacteria-associated health problems have been documented throughout the world ^[2][3][2,3]. Many cyanobacterial poisonings can be traced back to microcystins, cyanobacterial peptide hepatotoxins, tumor promoters and possible carcinogens, which may pose a serious threat to health through contaminated drinking water [4]. Other cyanobacterial toxins (cyanotoxins) include compounds from the groups of cylindrospermopsins (cytotoxins), anatoxins and saxitoxins (neurotoxins) and nodularins (hepatotoxins, tumor promoters and possible carcinogens). All of these toxins have been implicated in cyanobacterial poisonings of either humans or animals.

One interesting compound in the spectrum of cyanobacterial metabolites is the non-proteinogenic amino acid β-N-methylamino-L-alanine, abbreviated BMAA. This compound, according to systematic chemical nomenclature (S)-2-amino-3-methylaminopropanoic acid (L-BMAA), has the natural isomers (S)-2,4-diaminobutyric acid (L-DAB), N-(2-aminoethyl)glycine (AEG) and β-amino-N-methylalanine (BAMA) [5].

BMAA was first observed in cycad seeds some 55 years ago [6], and its occurrence and properties continue to inspire scientists of various disciplines [7]. BMAA has been suggested to play a causal role in amyotrophic lateral sclerosis/Parkinsonism-dementia complex (ALS/PDC), found at an elevated incidence in the Chamorro people of the Pacific island of Guam [8]. The Chamorro people are thought to become exposed to the neurotoxin through the use of cycad seed flour contaminated by BMAA originating from symbiotic cyanobacteria. As BMAA can be found or even biomagnified in certain food web compartments ^[8][9][10][8,9,10] it is also possible that a major share of the BMAA exposure on Guam occurs through the traditional consumption of cycad-feeding bats.

Early research pointed to a wide occurrence of BMAA in free-living and symbiotic cyanobacteria in many parts of the world, at up to mg/g DW (dry weight) levels [11]. Another paper from South Africa confirmed the taxonomic ubiquity of BMAA in freshwater cyanobacteria, but the reported toxin concentrations were lower [12]. BMAA was found in combination with additional cyanotoxins in British waterbodies [13]. In contrast, a study involving 62 cyanobacterial samples of worldwide origin found no BMAA in any of the cyanobacterial samples [14]. BMAA has also been reported in other groups of microalgae. The extent of BMAA production by cyanobacteria and other organisms may not thus be fully understood, but it seems a widely observed phenomenon. Out of 74 publications dealing with BMAA detection and quantification in samples of cyanobacteria or human tissues, only 12 failed to detect a BMAA signal in environmental samples [15]. Less is known about the transfer and possible biomagnification of BMAA in food webs. Further, it is not fully understood how much of the toxin occurs in free, protein-bound or soluble-bound forms [16] and whether such a bound toxin is bioavailable.

Some of the BMAA analytical protocols have been shown to suffer from methodological problems or at least from poor reporting [17], and therefore, especially older literature must be treated with caution. Analytical methods relying on liquid chromatography and fluorescence detection of derivatized BMAA seem to be more prone to report either false positives or an overestimation of BMAA concentration in the studied samples. More selective and sufficiently validated analytical protocols involving appropriate sample preparation and relying on liquid chromatography and tandem mass spectrometry (MS/MS) for the detection and quantification of (derivatized) BMAA have overcome most, but not all, of the methodological challenges ^[15][18][15,18]. The AOAC (Association of Official Analytical Chemists)-accepted method for BMAA in a cyanobacterial matrix is based on ultra-performance liquid chromatography of AQC (6-aminoquinolyl-N-hydroxysuccinimidyl carbamate )-de-derivatized BMAA and tandem mass spectrometry [19].

2. Occurrence of BMAA

2.1. Occurrence of BMAA in Cyanobacteria and Microalgae

Based on the published papers (Occurrence of BMAA and its isomers in environmental samples and organisms), BMAA and its isomers (DAB and AEG) were found to be produced by cyanobacteria belonging to 29 genera: Anabaena ^{[11][13][14][20][21][22][23]}[11,13,14,23,24,25,26], Anabaenopsis ^[23][26], Aphanizomenon ^{[11][13][20][22][24][25]}[11,13,23,25,27,28], Calothrix ^[11][20][26][11,23,29], Chlorogloeopsis [11], Chroococcidiopsis ^[11][23][11,26], Cyanobium ^[27][28][30,31], Cylindrospermopsis ^[11][14][11,14], Fischerella ^[11][23][11,26], Gomphosphaeria [13], Leptolyngbya ^{[24][26][27][28][29][30]}[27,29,30,31,32,33], Lyngbya ^[11][20][23][11,23,26], Microcoleus ^[27][28][30,31], Microcystis [11,13,14,23–25, 34], Myxosarcina ^[11][31][11,35], Nodularia ^{[11][13][14][23][24][27][28]}[11,13,14,26,27,30,31], Nostoc ^{[8][11][20][23][26][27][28][31][32]}[8,11,23,26,29,30,31,35,36], Oscillatoria ^[13][23][13,26], Phormidium ^{[11][23][27][28]}[11,26,30,31], Planktothrix ^{[11][13][20][22]}[11,13,23,25], Plectonema [11], Prochlorococcus [11], Pseudanabaena [13], Scytonema ^[11][20][11,23], Symploca ^[11][26][11,29], Synechococcus ^{[10][11][20][26][27][28]}[10,11,23,29,30,31], Synechocystis ^[23][27][28][26,30,31], Trichodesmium [11] and Woronichinia ^[22][25]. These cyanobacteria represented: (a) a biomass of cyanobacteria collected from: freshwater (40 reports from different localities), marine (5 reports) and brackish (1 report) environments and (b) cultures of cyanobacterial strains originating from freshwaters (28 strains), marine habitats (14 strains), brackish waters (33 strains), symbiotic plants (10 strains) and terrestrial environments (4 strains). For 32 strains, the origin could not be found in the corresponding publications. There are also some reports about the presence of BMAA and/or its isomers in the biomass of collected cyanobacteria, but the producers were not identified ^{[9][10][33][34][35][36][37][38][39][40]}[9,10,37,38,39,40,41,42,43,44]. BMAA and its isomers have been found in 22 supplements made of biomass of the cyanobacteria Spirulina ^[19][32][41][19,36,45] and Aphanizomenon ^[14][42][14,46]. The compounds have been also detected in cyanobacterial biocrust samples from Qatar ^{[43][44][45][46]}[47,48,49,50] and aerosols (air-filters from the lakeshore) ^[47][51]. In total, the presence of BMAA and/or its isomers was linked to more than 200 findings related to cyanobacteria from nature (freshwaters, marine and brackish environment, terrestrial habitats and plant symbionts), market samples and specimens from culture collections. Although BMAA and its isomers are found in many ecosystems, the occurrence of these compounds is not ubiquitous. A total of 387 environmental and biological samples (water, fish, aquatic plants) taken from Nebraska (USA) were analysed for BMAA, DABA and anatoxin-a (a compound unrelated to BMAA and DABA). Measurable levels of BMAA, DABA and anatoxin-a were found in 18%, 17% and 12% of the samples, respectively ^[34][38]. In a study involving bloom-impacted lakes and reservoirs of Brazil, Canada, France, Mexico and the United Kingdom, 390 samples were taken from 45 lake sampling sites. AEG and DAB isomers were detected in 30% and 43% of the samples, respectively, while BAMA was found in less than 8% of the samples and BMAA was not observed in any sample ^[33][37]. No BMAA was found in any of the analyzed biological loess crusts (BLCs, terrestrial samples) taken from various locations in Serbia, China and Iran ^[48][52]. These results indicate that while BMAA occurs in many ecosystems, it is not present in every ecosystem. The results also underline the need to look for the isomers of BMAA (DAB, AEG, BAMA) in order to get a more complete picture of the occurrence of the BMAA family of compounds. It can also be noted that the BMAA and isomer concentrations are not stable in one ecosystem but vary temporally and spatially. They depend on when (daily, monthly and annual variations), where and how the samples have been taken from a waterbody, and how the samples have been analyzed. Further, BMAA can be found in either free or bound forms which necessitate due attention during the analysis and reporting [16]. Comparisons are therefore more meaningful within one publication with a consistent methodology. The physico-chemical environment also seems to have an influence on the BMAA concentration. BMAA in environmental phytoplankton samples ranged from 1 µg/g to 276 μg/g DW, while cultures had higher values ranging from 20 µg/g to 6.4 mg/g DW ^[49][53]. One of the most extensive analytical studies on the occurrence of BMAA isomers in bloom-impacted lakes and reservoirs analyzed environmental water samples from five countries ^[33][37]. The study did not detect BMAA in any of the samples but observed isomers at the following min-max concentrations: 10–1100 ng/L (DAB), 5–19,000 ng/L (AEG) and 15–56 ng/L (BAMA). The same paper ^[33][37] further presented widely varying BMAA isomer concentrations in environmental water samples reported in previously published studies. For instance, the reported BMAA concentrations in environmental waters in the paper ^[33][37] and four earlier papers cited therein varied dramatically: not detected, 6.5–7 ng/L (mean values from two years), 10–300 ng/L, 110 ng/L, 1800–25,300 ng/L. There are reports that planktonic diatoms (Bacillariophyta), dinoflagellates (Pyrrhophyta), green algae (Chlorophyta), euglenas (Euglenophyta), red algae (Rhodophyta), Haptophyta and Cryptophyta also produce BMAA and/or its isomers. Diatoms are represented by 15 genera: Achnanthes ^[30][33], Asterionellopsis ^[50][54], Aulacoseira ^[51][55], Chaetoceros ^[10][52][10,56], Cyclotella ^[51][55], Fragilaria ^[51][55], Halamphora ^[50][54], Navicula ^[30][51][33,55], Odontella ^[50][54], Phaeodactylum ^[10][52][10,56], Proboscia ^[30][33], Pseudo-nitzschia ^[50][54], Skeletonema ^[10][30][10,33], Tabellaria ^[51][55] and Thalassiosira ^[10][30][52][10,33,56]; dinoflagellates by seven genera: Alexandrium ^[10][50][10,54], Gymnodinium ^[53][57], Heterocapsa ^[50][54], Prorocentrum ^[50][54], Pyrocystis ^[50][54], Scrippsiella ^[50][54] and Symbiodinium ^[50][54]; green algae by four genera: Ostreococcus [10], Chlamydomonas ^[50][54], Chlorella ^[50][54] and Dunaliella ^[50][54]; Euglenophyta and Rhodophyta by one genus each: Eutreptiella ^[50][54] and Porphyridium ^[50][54], respectively; Haptophyta by two genera: Tisochrysis ^[50][54] and Emiliana ^[50][54]; and Cryptophyta by four genera: Hemiselmis ^[50][54], Proteomonas ^[50][54], Rhinomonas ^[50][54] and Rhodomonas ^[50][54] isolated from marine and freshwater environments.

2.2. Occurrence of BMAA in Animals

BMAA has also been detected in the zooplankton community in the Baltic Sea ^[9][40][9,44]. The presence of BMAA and its isomers in zooplankton organisms, molluscs, crustaceans, fish, birds, mammals and other animals is a consequence of the bioaccumulation of BMAA in food webs ^[8][37][54][8,41,58]. In the group of molluscs, BMAA was found in bivalvia Anodonta woodiana ^[55][34], Antigona lamellaris ^[56][59], Arca inflate ^[56][59], Atrina pectinate ^[56][59], Cerastoderma edule ^[53][57], Chlamys farreri ^[56][59], Corbicula fluminea ^[55][34], Crassosstrea sp. ^[56][59], Crassosstrea gigas ^[26][50][57][29,54,60], Crassostrea virginica ^[58][61], Gafrarium tumidum ^[56][59], Mactra chinensis ^[56][59], Mercenaria mercenaria ^[56][59], Moerella iridescens ^[56][59], Mytilus coruscus ^[56][59], Mytilus edulis ^{[9][50][57][59]}[9,54,60,62], Mytilus edulis platensis ^[59][62], Mytilus galloprovincialis ^{[10][26][50][56]}[10,29,54,59], Ostrea edulis ^[9][24][57][9,27,60], Periglypta petechialis ^[56][59], Perna canaliculus ^[59][62], Perna viridis ^[56][59], Placopecten magellanicus ^[59][62], Ruditapes philippinarum ^[56][59], Scapharca subcrenata ^[56][59], Sinonovacula constricta ^[56][59], Solen strictus ^[56][59], Tegillarca granosa ^[56][59] and an unidentified mussel ^[60][63]. BMAA was also found in gastropods Bellamya aeruginosa ^[55][34], Neverita didyma ^[56][59], Neptunea cumingii ^[56][59], Natica maculosa ^[56][59], Haliotis discus hannai ^[56][59], Volutharpa ampullaceal ^[56][59] and Rapana venosa ^[56][59]. The arthropods (Crustacea) in which BMAA was found are represented by ten species: Callinectes sapidus ^[32][58][61][36,61,64], Cancer pagurus ^[59][62], Heterocarpus ensifer ^[57][60], Eriocheir sinensis ^[55][34], Macrobrachium nipponense ^[55][34], Mysis mixta ^[40][44], Neomysis integer ^[40][44], Palaemon modestus ^[55][34], Panulirus sp. ^[62][65] and Procambarus clarkia ^[55][34]. In the tissues of the fish, there was evidence of accumulation of BMAA after the consumption of BMAA producers, mostly cyanobacteria. A total of 39 species of fish showed the presence of BMAA in their tissues: Abramis brama ^[53][57], Anguilla anguilla ^[53][57], Aristichthys nobilis ^[55][34], Carassius auratus ^[55][34], Carcharhinus acronotus ^[63][64][66,67], Carcharhinus leucas ^[63][64][66,67], Carcharhinus limbatus ^[63][64][66,67], Clupea harengus ^[9][57][9,60], Coilia ectenes taihuensis ^[55][34], Coregonus lavaretus ^[9][24][9,27], Cyprinus carpio ^[47][55][34,51], Erythroculter ilishaeformis ^[55][34], Esox lucius ^[53][57], Galeocerdo cuvier ^[64][67], Ginglymostoma cirratum ^[63][64][66,67], Gymnocephalus cernua ^[53][57], Hemiramphus kurumeus ^[55][34], Hypophthalmichthys molitrix ^[55][34], Neosalanx taihuensis ^[55][34], Negaprion brevirostris ^[63][64][66,67], Osmerus eperlanus [9], Parabramis pekinensis ^[55][34], Parasilurus asotus ^[55][34], Pelteobagrus fulvidraco ^[55][34], Perca fluviatilis ^[53][57], Pleuronectes platessa ^[57][60], Protosalanx hyalocranius ^[55][34], Pseudorasbora parva ^[55][34], Rhizoprionodon terraenovae ^[64][67], Rhodeus sinensis ^[55][34], Rutilus rutilus ^[53][57], Salvelinus alpinus ^[57][60], Sander lucioperca ^[53][57], Scophthalmus maximus [9], Sphyrna mokarran ^[63][64][66,67], Sphyrna tiburo ^[63][64][66,67], Sphyrna zygaena ^[64][67], Tinca tinca ^[53][57] and Triglopsis quadricornis [9]. Fish samples from Nebraska reservoirs (carp, white crappie, bass, shad, walleye, catfish, wiper and bluegill) showed the presence of BMAA, and in many samples, also the presence of DAB ^[34][38]. BMAA, DAB and/or AEG were found in 16 fish-based dietary supplements (shark cartilage powders) from seven manufacturers ^[65][68]. The reports about mammals showed that BMAA was detected in flying foxes ^[37][66][41,69], in dolphins ^[67][70] and in human brain tissue from some patients who died from ALS ^[8][37][68][8,41,71]. It was also found in human hair ^[69][72].

2.3. Occurrence of BMAA in Plants

BMAA and/or its isomers were found in parts of the following symbiotic and other plants: Azolla filiculoides [8], Brassica oleracea ^[24][27], Cycas micronesica ^{[8][25][37][70]}[8,28,41,73], Cycas revoluta ^[14][20][24][14,23,27], Cycas debaoensis ^[41][45], Gunnera kauaiensi [8], Lathyrus latifolius [14] and aquatic plants from Nebraska ^[34][38]. BMAA was found in flour prepared from the gametophyte of cycad seeds ^[37][41].

2.4. Exposure to BMAA

EThe reports presented in Table S1 suggest that exposure to BMAA can occur through the same routes of exposure that are known for other cyanotoxins: through drinking water (e.g., in a case when a cyanobacterial mass development occurs in a drinking water reservoir), recreational activities (e.g., swimming, canoeing or bathing), the aquatic food web, terrestrial plants and animals and food supplements ^{[49][71][72][73]}[53,74,75,76]. Synthesized by cyanobacteria and microalgae, BMAA is transported through some food webs in aquatic ecosystems from zooplankton and benthos invertebrates, planktivorous fish, shellfish, snails and crustaceans to carnivorous fish and mammals. Human contact with BMAA is possible through all these food web compartments in aquatic ecosystems. In terrestrial ecosystems, BMAA can be found in some symbioses of cyanobacteria with higher plants, but it can also be transferred from aquatic ecosystems into terrestrial ecosystems through irrigation. As a consequence of irrigation with BMAA-contaminated water, BMAA can be accumulated in plant tissues and thus reach animals and humans.  As presented in Figure 1, BMAA and/or its isomers have been found throughout the world and exposure scenarios either through contaminated drinking water or consumption of contaminated foodstuffs are present in most parts of the world. There are clearly some hotspots of BMAA occurrence: parts of Europe, the United States and China as well as some islands including Guam. The largest number of reports deal with European countries which could be an indication of active research on the topic there. The absence of reports from, e.g., most African, Asian and South American countries probably does not mean the absence of BMAA and its isomers in these parts of the world, but that those territories were not as thoroughly investigated as for instance Western European countries. The situation with BMAA is similar to that of cylindrospermopsin which was first thought to be a tropical toxin. Generally speaking, cylindrospermopsin has been found in most countries where it has been looked for carefully enough and this is the likely scenario with BMAA, too. According to the map shown, BMAA and also its isomers are frequently recorded in regions where analyses have been performed. Assuming methodological robustness, BMAA and its isomers are thus found in most parts of the world, and therefore there are potential exposure scenarios in many countries. This exposure would be reflected in the incidence of neurodegenerative diseases if BMAA is regarded as a causative factor for such diseases and the exposure levels are high enough. Likewise, the incidence of neurodegenerative diseases would increase upon BMAA exposure even if BMAA is only regarded as a contributing factor in the presence of other risk factors present in the investigated areas. Whether exposure to BMAA in natural conditions really has an effect on the incidence of neurodegenerative diseases has to be proven and confirmed by epidemiological research ^[2][4][2,4].