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Water Resources
蓝藻水华是一个全球性的水环境问题。近年来，由于全球变暖和水体富营养化，地表蓝藻在一定区域内聚集，形成受风驱动的蓝藻水华。蓝藻水华会改变水的物理和化学性质并造成污染。此外，蓝藻在凋亡过程中向水中释放有机物、N（氮）和P（磷），加速水体富营养化，威胁水生动植物群，影响水中微生物的群落结构和丰度。同时，蓝藻释放的毒素和致癌物可通过食物链/网富集，危害人类健康。
- cyanobacteria blooms
- cyanobacteria toxins
- aquatic environment
一、简介
蓝藻广泛分布于海洋和淡水中，比大多数真核生物具有更强的适应性[ 1 ]。它们具有在极端环境（冰雪、温泉、碱性苏打湖、盐水池、沙漠和极地）中生长和繁殖的能力[ 2 ]。当有机物、N（氮）、P（磷）和其他营养物质在水中富集时，蓝藻就会繁殖并积累成优势群。蓝藻水华在淡水或海洋中形成绿色、红棕色和红色，是养分富集或富营养化最显着的症状之一 [ 3]。蓝藻水华在世界范围内变得越来越普遍，并对水生态系统的可持续性构成严重威胁，例如中国的太湖、美国的伊利湖、加拿大的温尼伯湖和荷兰的新湖 [ 4 ]。1930年代以来，对蓝藻水华进行了大量研究，包括蓝藻水华的成因[ 5 ]、蓝藻产品的危害及藻类与细菌的共生关系[ 6 ]、蓝藻水华的营养作用[ 7 ]等。 ]。
2. 蓝藻对水体的污染
当蓝藻达到10 5 个细胞/mL，或叶绿素a（Chla）浓度达到10 μg/L，水体表面形成可见的覆盖层时，认为蓝藻水华形成[ 12 ]。蓝藻水华的衰变过程对水生环境的影响更为严重。蓝藻降解过程中存在好氧和厌氧反应，释放出毒素和恶臭气体。在蓝藻水华分解过程中，大量有机物和可溶性营养物质会释放到水体中，降低水体透明度，加剧水体富营养化，形成“黑点”[ 13]。蓝藻水华会导致水体呈酸性，电导率呈上升趋势，化学需氧量不断增加，水中有机物浓度增加[ 14 ]。此外，蓝藻堆积形成的有机碎屑在水中分解率高，48 h内可分解41.9%[ 15 ]，对水体生态系统造成危害[ 12、16 ]。蓝藻衰退过程中释放出大量溶解有机物（DOM），随着反应的进行，溶解有机碳（DOC）转化为溶解无机碳（DIC），其中大部分是，最后，转化为腐殖质，难以降解 [17 ]。
2.1。蓝藻水华对水生动物群的影响
蓝藻水华期间，大量死去的蓝藻会沉底分解，消耗氧气，从而降低水中的溶解氧（DO），从而影响水生动物的生存条件，导致部分鱼类、贝类消失。和无脊椎动物，并减少水生生态系统的物种多样性 [ 22 ]。在蓝藻的凋亡过程中，毒素、恶臭物质等物质的次级代谢产物被释放到水中，氨（NH 4 +）和微囊藻毒素（MCs）的浓度会同时升高，对水体造成急性或慢性的不良影响。水生生物 [ 23 ]。
暴露于 MCs 会导致脂质过氧化、DNA 损伤和抗氧化酶的变化，例如不同水生生物中的超氧化物歧化酶 (SOD)、过氧化物酶 (POD) 和过氧化氢酶 (CAT)。MC可对其循环系统、消化系统和免疫系统造成损害。同时，它们会引起解毒酶的变化，例如谷胱甘肽 S-转移酶 (GST) 和谷胱甘肽过氧化物酶 (GPx) [ 24 ]。研究表明，肝脏是 MCs [ 25 ] 的主要目标。通过研究蓝藻花高原湖泊中两只蜗牛的 MCs 的生物积累规律，发现肝胰腺是两只蜗牛的主要目标[ 26]。一些报告表明，鲤鱼的肠道、性腺和肌肉中的 MCs 浓度较低，但在肝胰腺中较高 [ 27 ]。MCs在罗氏沼虾的肝胰腺中富集，破坏肝胰腺的结构和功能，引起剂量依赖性和时间依赖性毒性作用[ 28 ]。安徒生等人。发现高剂量的微囊藻毒素-LR (MC-LR) 可导致大西洋鲑整个肝脏出现弥漫性坏死和肝巨细胞增多症[ 29 ]。以前的研究表明，MCs 可以通过食物链/网络转移到更敏感的生物体中 [ 30 ]。
NH4+ can induce the antioxidant defense of juvenile crucian carp. High concentration NH4+ has toxic effects on CAT, SOD, and glutathione (GSH) in the fish liver [31]. Histopathological changes in the gills, liver, and kidney of Oreochromis niloticus are caused by different concentrations of NH4+, and include gill congestion, telangiectasia, turbid swelling, edema degeneration of liver tissue, kidney congestion, and glomerulonephritis [32]. NH4+ significantly affects the plasma and hematological parameters of juvenile Megalabrama amblycephala, demonstrating histopathological changes in the gills, liver, and kidney of fish. The severity of the lesions is different, with the liver exhibiting the most extensive damage, followed by the gills and kidneys [33].
In addition, it is reported that MCs and NH4+ have synergistic effects on the immunotoxicity of aquatic organisms. After combined poisoning, the peripheral interspace of the lymphocytes of Megalabrama amblycephala is broadened, the nucleus is atrophied, and the mitochondria are swollen. Moreover, the exposure to algae toxin and NH4+ has a significant interaction with macrophage phagocytosis activity, respiratory burst activities, a total number of white blood cells and the transcriptional levels of sIgM, mIgD, and sIgZ genes of Megalabrama amblycephala [23].
2.2. Impacts of Cyanobacteria Blooms on Aquatic Flora
The cyanobacteria blooms have strong inhibitory effect on the photosynthetic activity of aquatic flora, leading to leaf death and irreversible inhibition of photosynthesis [34]. Long-term and high-concentration aggregation of cyanobacteria will shade, consume oxygen, and release allelochemicals and MCs, resulting in the disappearance of submerged vegetation [35]. Cyanobacteria blooms lead to the Chla of Potamongeton malaianus and Stuckenia pectinata decreasing by 50% and 56%, respectively [36].
MCs can induce the reactive oxygen species (ROS) production and an increase in malondialdehyde (MDA), exacerbating the oxidative damage for aquatic flora [37]. MCs can bind irreversibly with phosphatase-1 (PP1) and phosphatase-2A (PP2A) covalently, causing a series of biochemical reactions in cells to be disordered and changing chlorophyll contents and pigment composition in plants [38].
The anatoxin-a produced by cyanobacteria can cause the disorder of oxidative stress reaction in aquatic flora [39]. Treatment with 0.01–0.2 μg/mL MC-LR for 96 h can inhibit the growth of Spirodela oligorrhiza [40]. MC-LR concentration of 1.0 μg/L can significantly impede the development of the roots of Lepidium sativum, and a concentration of 10 μg/L can inhibit the growth of the whole plant [41]. It has also been found that 0.12–3 μg/mL MCs can hinder the growth of Oryza sativa L. [42]. In addition, MCs can cause the gap of aeration tissue in the rhizomes of Phragmites australis to be blocked by callus-like tissue, resulting in the gangrene of outer skin tissue in the reed root. When exposed to 10–40 μg/mL of MC-LR for 120 h, the cytoskeleton of reed root changes (microtubule degradation), and its roots swell and deform [43].
MCs can damage DNA and produce genotoxicity. Nuclear shrinkage and chromatin condensation can be observed in the root tip meristem cells of Phragmites australis treated with MCs, and chromatin condensation is often accompanied by nuclear shrinkage and apoptosis [44]. DNA damage effect of MCs on Oryza sativa root cells by DNA fragmentation and random amplified polymorphic DNA (RAPD) [45]. Furthermore, the affected biochemical processes involved protein folding and stress response, protein biosynthesis, regulation of cell signal and gene expression, and energy and carbohydrate metabolism [46].
The high concentrations of NH4+ and nitrate nitrogen (NO3−-N) released by cyanobacteria decay have toxic effects on aquatic plants, resulting in the yellowing of plant leaves, inhibition of growth, and root morphological changes [47]. A high concentration of NH4+ can also inhibit the absorption of K+, Ca2+ and Mg2+ by plant cells, resulting in a disturbance of ion balance [48]. Studies have also demonstrated that a high concentration of NH4+ leads to the destruction of the antioxidant system balance of aquatic flora, and the accumulation of ROS, which leads to the damage of plasma membrane [49].
2.3. Impact of Cyanobacteria Blooms on Microorganisms in the Aquatic Environment
Studies on the effects of cyanobacteria blooms on microorganisms in water mainly focus on the community structure and activity of microorganisms, especially at the genus level [50]. Cyanobacteria blooms in the summer, and the abundance of Proteobacteria in the water and sediment of Zhushan Bay is the highest at the phylum level, followed by Actinomycetes. At the genus level, the dominant bacteria in the water are GpXI and GpIIa, and the predominant bacteria in the sediment are Gp6 and GpIIa [51]. Meanwhile, the different stages of cyanobacteria blooms will lead to changes in DO, N, and P in surface sediments [52]. Studies have studied and analyzed the bacterial community diversity in Poyang Lake waters and found a specific correlation between DO, Cond, salinity, mineralization, nutrients, and bacterial community diversity index [53]. In addition, debris formed during the degradation of cyanobacteria will precipitate into the surface sediments, stimulating the growth of microorganisms. Studies have demonstrated that the total bacterial diversity of water decreases during cyanobacteria blooms [54]. The decomposition of cyanobacteria will increase the diversity and abundance of ammoniated bacteria in sediments, among which the relative abundance of Nitrosomonas oligotropha is as high as 75% [55].
Studies have demonstrated that the accumulation of cyanobacteria will lead to a change in microbial community structure and a decrease in diversity in the chironomid larvae gut. The relative abundance of β-proteobacteria increased to 40.6%, and the relative abundance of δ-proteobacteria decreased to 4.1%. Moreover, cyanobacteria blooms can promote the expression of the nosZ gene and increase the abundance of nirK denitrifying bacteria [56]. The occurrence of cyanobacteria blooms will lead to the decrease in α-diversity of the bacterial community [57].
3. Impacts of Cyanobacteria Blooms on Human Health
Cyanobacteria blooms directly affect drinking water. In 1996, in Caruaru, Brazil, 50 dialysis clinic patients died because of using water contaminated with MCs [58]. In 1999, the cyanobacteria blooms in Dianchi Lake covered an area of 20 km2. In May 2007, a massive cyanobacteria bloom in Taihu Lake (Wuxi, China) led to a drinking water crisis for 2 million people in the city of Wuxi [59]. In August 2014, cyanobacteria blooms in Lake Erie increased the concentration of MCs in the drinking water, threatening the drinking water safety of nearly half a million people [60].
When cyanobacteria blooms decompose, releasing many odor substances and cyanotoxins, it has been found that 2-methylisoborneol (MIB) and geosmin are the most common substances that cause odor (musty smell) in drinking water, and their odor threshold concentrations are only 9 and 4 ng/L, respectively [61]. Among the eight kinds of odor in the table, except the chemical taste, chloride taste, and medicinal taste, the other five kinds of odor substances are related to odor compounds produced by algae. Excessive odor content in water affects the quality of drinking water and human health [59].
Cyanobacteria can release toxins such as the hepatotoxin class, neurotoxin, and endotoxin. MCs is the most widely distributed in water, which is a cyclic heptapeptide composed of seven amino acids, mainly produced by Microcystis and Anabaena [23]. Microcystis is the dominant species of cyanobacteria blooms in Taihu Lake, and its biomass can account for 40–98% of the total algae biomass [62]. Anabaena is the most common species in cyanobacteria blooms and the only species with hepatotoxic and neurotoxic secondary metabolites [63]. Turner et al. analyzed the MCs of cyanobacteria in freshwater ecosystems in the United Kingdom and found that more than 50% of the water bodies had MCs, and of which about 13% exceeded the World Health Organization (WHO) medium health threshold (20 μg/L) [64].
The toxins can be accumulated by organisms and transferred through the food chain/network. Cyanotoxins are chronically toxic to humans, which lead to acute gastroenteritis, respiratory adverse reaction, eye and ear irritation, skin rash, mouth ulcers, and other diseases [10]. In addition, algal toxins can inhibit the synthesis of protein phosphatase, resulting in hyperphosphorylation of critical regulatory proteins in the signal transduction process that controls cytoskeleton tissues [72].
MCs are hydrophilic and soluble in the blood of organisms. They cannot penetrate the lipid membrane through passive diffusion [73]. Therefore, most ingested toxins cannot pass through the ileal epithelium, stay in the digestive tract, and are most likely excreted through feces [74]. However, some studies have demonstrated that ingested MCs can be transported by bile acid membrane transporters (such as organic anion transporters (OATPs)) through the ileum into the venous blood flow and from the portal vein into hepatocytes [75]. The liver is the main target organ for the accumulation and detoxification of MCs. At the same time, MCs can also be detected in other organs (such as the intestine, kidney, brain, lung, and heart), though to a much lesser extent [74 ]。高剂量的蓝藻毒素可导致急性肝损伤、肝肿大、肝出血、肝细胞结构和功能丧失，甚至生物性呼吸停止[ 76 ]。
This entry is adapted from the peer-reviewed paper 10.3390/toxins14100658
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