Control measures against cyanobacterial growth and restoration of eutrophic environments.

Introduction

Eutrophication is considered one of the most important polluting process in aquatic ecosystems. The result of this process is a decrease in the water’s quality and potability, as well as a shift in the natural dynamic of the ecosystem. (Fig. 1). One of the processes linked to eutrophication are cyanobacterial blooms, which is the case of study of this entry, including the toxicological implications of said blooms and the restoration of the ecosystem.

Figure 1: eutrophication process diagram.

Cyanobacteria blooms

Cyanobacterial growth is mainly determined by the ratio of nutrients found in the water. A higher trophic level index (TSI) corresponded to a lower Nitrogen : Phosphorus ratio (N:P), which can be attributed to a loss of N and an increase in P. Specifically in more eutrophic lakes, the enrichment of total organic carbon and all forms of P in sediments could fuel the potential denitrification rate (PDR) by shaping community composition, resulting in a loss of N.¹  The capacity to fix nitrogen of the cyanobacteria are probably an important factor to increase their efficiency when blooming in low levels of N, in comparison to other cyanobacteria.²

The cyanobacteria benefited by this situation in freshwater eutrophic environments are species like Microcystis aeruginosa., Anabaena flos-aquae. and Cylindrospermopsis raciborskii.³ All three of these  fix nitrogen, carry out photosynthesis and create bioaccumulative toxins, which are all under the denomination of microcystins. As reported by D. Turner et al. in an analysis of microcystins in cyanobacterial blooms from freshwater bodies in England⁴, the estimated total concentration of toxins per cyanobacterial cell results in approximately 40 pg/cell to 85 ng/cell, assuming scum-containing samples have cell-counts of 500,000 cells/mL.

Microcystis aeruginosa when forming blooms can cause anaerobic conditions below the water surface, being even problematic for itself. Despite this, it has been found Microcystis is tolerant to living in dark anaerobic conditions. It’s been reported that M. aeruginosa had a slight increase in cell metabolic activity, no visible death of cells and absence of decay of chlorophyll-a fluorescence in individual and competition cases under dark anaerobic conditions. This cyanobacteria creates toxins named microcystins, cyclic peptides which are potent hepatotoxins to humans and rodents.⁵  The International Agency of Research on Cancer has classified MC-LR as a possible human carcinogen and the WHO has set a provisional guideline of MC-LR in drinking water of 1 μg/L.⁶ Microcystin-LR (MC-LR) has the highest toxicity of this group of toxins. 

Figure 2: Microcystis aeruginosa bloom

The Anabaena genus in eutrophic environments is mainly represented by Anabaena flos-aquae, among others. Anabaena genus produces, besides microcystins, three other toxins that aren’t as widely produced by cyanobacteria: Anatoxin-A, Homoanatoxin-A and Anatoxin-A(S), the former only known to be produced by A. flos-aquae.⁷ Anabaena is a genus of filamentous cyanobacteria, known for its nitrogen fixing abilities.⁸

Cylindrospermopsis species like C. raciborskii are dominant cyanobacterial organisms, which create specific toxins (Cylindrospermopsin) as well as microcystins when in blooms. This species has gained attention due to its toxicity, bloom formation capacity and invasiveness.⁹

Cyanobacterial toxins

MC-LR (Fig. 3) attacks hepatic activity in both humans and animals. The most troublesome characteristic found in the MC-LR is that it attacks the liver but also testis, as it crosses the blood-testis barrier and affects the DNA damage repair pathway, as well as increases expression of the protooncogenes (genes involved in the response to DNA damage). MC-LR also reduces the motility and morphology of sperm and affects the hormone levels of the male reproduction system.

Figure 3: Microcystin L-R molecular structure

Anatoxin-A (Fig. 4) is a neurotoxic cyanobacterial toxin, just like Homoanatoxin-A (Fig. 4) and Anatoxin-A(S) (Fig. 4a6). Anatoxin-A attacks neuromuscular receptors, as it’s a structural analogue to the neutrotransmitter acetylcholine, and cannot be degraded by its enzyme (acetylcholinesterase). This produces muscle overstimulation, which leads to fatigue, paralysis, gasping, convulsions and death by respiratory arrest in vertebrates¹⁰, although no human deaths have been reported. Homoanatoxin-A is analogue to Anatoxin-A and are both highly toxic, with a LD₅₀ = 200 - 250 µg/kg (mouse, intraperitoneal).¹⁰ Anatoxin-A(S) has a similar effect to Anatoxin-A and Homoanatoxin-A but is also an anticholinesterase¹¹: it inhibits the destruction of the neurotransmitter acetylcholine by the enzyme acetylcholinesterase within the nervous system. If this happens, the toxin allows high levels of the neurotransmitter to build up, slowing the heart activity, lowering blood pressure and inducing contraction of the smooth muscles¹².

                                                          Figure 4: Anatoxin-A, Homoanatoxin-A and Anatoxin-A(S)'s molecular structure

Cylindrospermopsin (CYN) (Fig. 7) is an hepatotoxin alkaloid, as well as being associated with being cytotoxic and genotoxic, due to its effects in other organs and in DNA. As reported by Ohtani et al. (1992), administration in mice of the toxin had  a LD₅₀ of 2-1 mg/kg over 24h.¹³

Control measures against cyanobacterial growth

There are 4 main ways shown to manage cyanobacterial growth: the control of the nutrients in the water, mechanical extraction of the cyanobacteria, the use of chemicals to decrease their expansion and finally biological control.

Nutrient control measures

As mentioned above, the low N:P ratio is crucial for the microorganisms’ growth, and recent studies have shown that the reduction of P concentration is the most effective way of decreasing the expansion of cyanobacteria, contrary to the initial hypothesis of N being the limiting nutrient.¹⁴ The mechanisms are: the precipitation of P, the dredging of the sediments or by limiting the nutrient resources by breaking the stratification. These methods are meant to prevent optimal situations for cyanobacterial growth and survival.

Mechanical control measures

The mechanical control methods include the direct removal of cyanobacteria and decreasing their growth by reducing the light incidence or cellular deaths. Filtering the water’s surface with specific nets has been useful for the removal of filamentous species. Depending on the water’s characteristics mass agitation can be used, causing cyanobacterial destabilization on the surface and therefore preventing its growth. The addition of flocculant agents for the later skimming of the resulting sludge flocs in suspension is a common practice too.

 Chemical control measures

These control measures regulate the cyanobacterial growth by adding chemical products. Most used pesticides for cyanobacterial control are hexazinone (though it may degrade water quality for wildlife)¹⁵, diquat (competitive inhibitor of photosynthetic electron transport)¹⁶ and terbutryn (a photosystem II inhibitor)¹⁷. These methods may persist on the ecosystem and can cause more negative effects than positive in an overall view due to bioaccumulation and biomagnification on other organisms.

 Biological control measures

These methods involve the use of both organisms and natural substances derived from them. A great number of aquatic organisms have been considered and studied based on depredation phenomena, parasitism or liberation of metabolites which suppress cyanobacterial growth.¹⁸

• Virus: LPP-1 virus is the first cyanophage found. A drastic decrease of cyanobacteria biomass was reported when they were introduced. The specificity of the method is what makes it non-viable: cyanobacteria attacked by the cyanophage can be easily replaced by another species resistant to it.

• Bacteria: there have been found bacteria that lyse cyanobacteria acting differently depending on the size of the population, the nutrient levels and the water’s state.

• Fungi and actinobacteria: actinomycetes can act on a direct manner producing antibiotic substances as well as lytic agents.  Farmers also noted that when straw bales (Hordeum vulgare) accidentally fell into eutrophicated basins, eutrophication was reduced or even eradicated. This was due to the salicylic acid produced when H. vulgare is decomposed by fungi, which is toxic for the cyanobacteria causing the eutrophication.

Treating eutrophication

The key points and goals in order to restore eutrophic environments are to reduce the organic content in the water (to decrease the BOD), to reduce the nutrient load (mainly N and P) and to reduce the bacterial load to inactivate pathogens.¹⁹

Some mechanical measures have been found to incorporate oxygen to the water and to stir the sediments in order to break the stratification of the sediments, homogenizing the water.²⁰ This would also affect the turbidity of the water and aid the light to penetrate again and affect positively the benthic photosynthetic organisms.

Regulation

The Health Department of the Generalitat de Catalunya’s regulation states that when a lake or pond receives nutrient-dense waters, a plan is to be designed to control the eutrophication, with samples being taken monthly, controlling this way fecal coliforms and heavy metals and the N and P content.²¹

Impacts of cyanobacterial blooms

On top of the toxicity that certain cyanobacterial blooms may imply, like the death of indigenous flora and fauna due to the toxins secreted, economical impacts are to be taken in consideration too. Nearby restaurants or premises are also affected by the eutrophication of the freshwater bodies, as they are to remain closed due to the toxicity of the waters.

Conclusions

The relation between cyanobacteria and eutrophication is a recurring issue. When cyanobacteria bloom, they cause disturbances to the ecosystem that can become lethal for other organisms inhabiting the water mass, as well as the economical impacts that come hand in hand with the toxicity of the water. This is why regulation and treatments are to be applied to prevent, reduce and solve the issue, although prevention is the most effective way of acting. Although there are ancient methods like the straw bales in England they are not to be discarded as they are known to be efficient, as opposed to other biological methods studied only in lab.

Bibliography

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