Neurotransmitters as Communicative Agents in Aquatic Ecosystems

The present work contains literature data and the authors’ own findings that provide evidence for an important ecosystem-level regulatory role of chemical factors that perform the function of neurotransmitters in animals’ nervous systems. Special attention is given to biogenic amines and related compounds that represent one of the main subgroups of neurotransmitters. The facts considered in this work indicate that such substances are characterized by a wide spectrum of signal functions in diverse components of aquatic ecosystems, including the microbiota, phytoplankton, invertebrates, higher plants, and fishes. Of paramount importance is the involvement of such neurotransmitters in the regulation of the functioning of whole ecosystems. Such ecosystem-level regulators are not only synthesized and released into the environment by various components of aquatic ecosystems but also introduced into them as human-made pollutants.

The present work is aimed at contributing to the resolution of the perennial ecological issue of how a large number of ecosystems can sustainably exist many millions of years without undergoing major changes?
Many natural ecosystems are known to be characterized by complex cooperation and competition among a plethora of coexisting and interacting components, such as populations or associations of living organisms. They produce chemical signals that are recognized by other representatives of the same ecosystem. The signals exert multifarious regulatory effects on them, operating as pheromones, allomones, kairomones, synomones, etc.
In aquatic ecosystems, "chemically mediated interactions strongly affect population structure, community organization, and ecosystem function" [1]. Chemical communication influences "foraging strategies, feeding choices, commensal associations, selection of mates and habitats, competitive interactions, and transfer of energy and nutrients within and among ecosystems" [1].
This work addresses one of the functional groups of chemical agents that regulate the functioning of the ecosystem. The agents in question are neurotransmitters, i.e., substances that transmit impulses among nervous cells (neurons) or between a neuron and a muscular/glandular cell that carries out the neuron's command. Neurotransmitters are subdivided into sev-eral subgroups, and the main attention in this work will focus on biogenic amines, including catecholamines (dopamine, norepinephrine, and epinephrine), serotonin, and histamine.
Many neurotransmitters are multifunctional agents, i.e., they also function as hormones and local tissue factors (histohormones). Some neurotransmitters fulfill communicative and regulatory functions in representatives of various types of animals, plants, fungi, and protozoans [2][3][4], which gives grounds for using the more general term biomediators for them [2].
In the present work, the suggestion is put forward that such neurotransmitters perform important roles within the framework of the ecosystem-level pool of signals that are concomitantly produced and recognized by a large number of ecosystem components. Therefore, they enable communication not just in terms of pairwise interactions in a parasite-host or predator-prey system but also at a more global level related to the whole ecosystem.
The question concerning the ecosystem role of neurotransmitters, their precursors, metabolites, agonists (functional analogs), antagonists (neurotransmitter receptor inhibitors), and other neuroactive compounds is of direct relevance to environmental problems and issues because such compounds increasingly accumulate in wastewater and are present in the REVIEW environment as pollutants. In particular, chlorpromazine and many other preparations used in psychiatry (antipsychotics) accumulate in the food chains of natural ecosystems and affect the behavior of organisms in them [5]. For instance, norepinephrine, a major neurotransmitter and hormone, exerts a lethal effect on the zooplankton daphnia Ceriodaphnia dubia and Daphnia magna; one half of the tested daphnia perished at environmental norepinephrine concentrations of 46 and 38 μM (the LD 50 doses), respectively. Even at much lower (nonlethal) norepinephrine concentrations, the reproduction process and individual development stages (e.g., molting) were disrupted in the tested daphnia species [6].
Environmental pollution caused by neurotransmitters forms a part of the more general issue of uncontrolled spread of neuro-and psychoactive pollutants in water, soil, and food. Pharmaceuticals, disinfectants, and cosmetics are constantly present in water bodies. It is assumed that "a wide array of aquatic organisms, including bacteria, algae, invertebrates, and fishes, have receptors that make them potentially sensitive to these compounds" [7]. They are exemplified by such widely used psychopharmacological drugs as Prozac (an antidepressant) and amphetamine (a cognitive enhancer) that cause significant alterations in the bacterial and algal communities of aquatic ecosystems; the same substances affect the growth rate of insects (Learn, J.R., Some rivers are so drug-polluted, their eels get high on cocaine. National Geographic, 2018. https://www.nationalgeographic.co.uk).

Neurotransmitters in Microorganisms
In a number of recent studies, neurochemical agents formed both by multicellular organisms and microorganisms, e.g., serotonin, glutamic acid, and γ-aminobutyric acid, are envisaged as an "universal language" that enables communication among representatives of different kingdoms of life [8].
In particular, an extensive body of evidence has been presented in the literature concerning the stimulatory effects of catecholamine on the growth of various microorganisms [8][9][10]. Nonetheless, the effects vary depending on the catecholamine concentrations and the taxonomic affiliation of the tested microorganisms. Norepinephrine, epinephrine, and dopamine stimulated the growth of Vibrio parahaemolyticus and V. mimicus but not V. vulnificus and V. cholera; norepinephrine inhibited the growth of Mycoplasma hyopheunomiae, suppressing the expression of genes involved in proliferation. Dopamine stimulated the proliferation of the yeast Saccharomyces cerevisiae; in contrast, norepinephrine caused no significant effect (reviewed, [10][11][12]).
The catecholamines-mediated dialogue in an ecosystem is bidirectional because microorganisms actively produce biogenic amines of this group, apart from responding to them. For instance, norepinephrine at concentrations of 0.2-2 μM was present in the biomass of Bacillus mycoides, B. subtilis, Proteus vulgaris, and Serratia marcescens; dopamine at concentrations of 0.5-2 μM was detected in the biomass of most tested prokaryotes [13]. Based on literature data, a large number of bacterial species form acetylcholine, including Lactobacillus plantarum strains [3].
As for other neurotransmitters, serotonin insignificantly stimulated the growth of Aeromonas hydrophila [15] and statistically verifiably increased the growth rate of the bacteria Enterococcus faecalis [16], Escherichia coli, and Rhodospirillum rubrum [17] and the yeasts Candida guillermondii [16] and Saccharomyces cerevisiae [18]. The serotonin-based dialogue between the microbiota and other ecosystem components is bidirectional: apart from responding to serotonin, the microbiota produces it at physiologically significant concentrations. Serotonin production is sufficiently widely spread in the microbial realm, including representatives of the symbiotic and parasitic microbiota [19,20]. Serotonin was detected in Bacillus subtilis and Staphylococcus aureus cells [13] at concentrations of approximately 1 μM.
Histamine caused an approximately twofold stimulation of biomass accumulation and promoted cell aggregation with colony formation in Escherichia coli K-12 (the maximum effect was attained with ~0.1 μM histamine) [21,22]. At micromolar concentrations, histamine also stimulated cell proliferation in the yeast Saccharomyces cerevisiae, and its effect in this species was similar in magnitude to that of serotonin (both neurotransmitters brought about an approximately 70% stimulation at a concentration of 1 μM) and considerably weaker than the dopamine effect [21,22].
The capacity of synthesizing histamine and releasing it into the medium was established in a wide spectrum of bacteria (reviewed, [10,12]). During seafood processing and storage, they produce histamine and other amines that are toxic at high concentrations (tyramine, phenylethylamine, cadaverine, putrescine, agmatine, spermine, and spermidine) [23].
The serotonin derivative melatonin regulates the response of the dinoflagellate Gonyaulax polyedra to light day shortening and the production of its dormant asexual reproduction forms (cysts) [23].

Neurotransmitters in Aquatic Invertebrates
Neurotransmitters are implicated in regulating physiological processes in various zooplankton representatives. Acetylcholine behaves as a conjugation inhibitor in Paramecium infusorians. These unicellular organisms were revealed to contain nicotine and muscarine receptors and acetylcholine esterase, the acetylcholine-degrading enzyme. Choline acetyltransferase that catalyses acetylcholine synthesis is located on the outer membrane of competent Paramecium primaurelia cells [3]. Dopamine accumulates in the cells of the infusorian Tetrahymena pyriformis [3] There are facts raising the possibility that neurotransmitters are involved in trophic interactions between aquatic invertebrates as predators and algae as their prey, i.e., that neurotransmitters function as kairomones. Chlorella-consuming daphnia, such as Daphnia magna, synthesize dopamine at a concentration of ~0.1 μM (Oleskin and Postnov, unpublished). As noted above, Chlorella growth is stimulated by dopamine [25]. Therefore, it cannot be excluded that the predator, D. magna, stimulates the growth of the algae it uses as food.
In D. magna, an insufficient amount of food (algal cells) in the medium lowers the growth rate and increases the time necessary for an individual's maturation. However, supplementing water with dopamine or the dopamine reuptake-inhibiting antidepressant bupropione (which increases the active dopamine concentration in the daphia's nervous system) results in the growth rate and maturation time approximating those that are characteristic of the medium with a suf-ficient amount of food [32]. Dopamine and bupropione produce no significant effect on growth rate and maturation time if the daphnia have a saturating amount of food [32].
Neuroactive pollutants cause serious changes in the organisms of aquatic invertebrates. For instance, morphine (an agonist of the peptide neuromodulators called endorphins) decreases the serotonin level and increases the dopamine concentration in the organism of the mollusc Elliptio complanata, which manifests itself in a relaxed state of the mollusc [7]. Metamphetamine that disrupts the operation of catecholamines in the nervous system affects long-term memory in the snail Lymnaea stagnalis if it is contained in the aquatic environment [7].
The presence of high serotonin concentrations in the environment exerts a toxic influence on daphnia. A lethal effect is produced if D. magna are fed with the alga Scenedesmus quadricauda that has been preincubated for 7 days with 33 μM serotonin (Oleskin and Postnov, unpublished). This model system demonstrated how bioaccumulation of human-made pollutants makes a negative impact on aquatic ecosystems.

Neurotransmitters in Higher Plants
Many aquatic ecosystems function with the involvement of higher plants that either form a part of these ecosystems (e.g., duckweed and water lilies) or grow on the coast and exchange chemical signals and regulatory substances with aquatic organisms. A large number of plants synthesize neurotransmitters and respond to them [2,3]. In plants, neurotransmitters are implicated in such processes/phenomena as organ formation, flowering, ion transport, photosynthesis, circadian rhythms, fruition, and photomorphogenesis (light-dependent developmental processes) [2,3]. Dopamine, norepinephrine, and serotonin stimulate pollen germination in Equisetum arvense and Hippeastrum hybridum [29].
Acetylcholine is synthesized by at least 65 flowering plant species belonging to 33 families; much acetylcholine is contained in the stinging hairs of the nettle Urtica dioica [3]. In the Thale cress Arabidopsis thaliana, high acetylcholine concentrations are characteristic of the seeds and the roots [29]. Acetylcholine stimulates the growth of a large number of flowering plants exemplified by tomato, wheat, Chinese beans, garden radish, and golden bamboo. It promotes their blooming, accelerates stomatal movement, facilitates the functioning of the phytochrome system, inhibits the formation of the gaseous hormone ethylene, and suppresses leaf rolling [29]. There is evidence that acetylcholine is involved, as a signal molecule, in intraorganismic communication between the roots and the shoots of Vicia faba (kidney beans) [29] and that acetylcholine accumulation in plant stalks, leaves, and roots is stimulated in response to heat shock [23].
Catecholamines were detected in 28 species belonging to the 18 tested plant families [3]. Importantly, an increased amount of dopamine (1-4 mg/g of fresh biomass) is contained in flowers and fruits, including the inflorescences of Araceae and banana pulp [3,32]. Biogenic amines exemplified by catecholamines function as stress response factors. Damaging potato leaves results in a significant increase in dopamine, norepinephrine, and serotonin concentrations; the dopamine level increases in the tissues of an injured Portulacca callus cactus [32].
In a number of plants, norepinephrine-and epinephrine-binding receptors (analogs of animal adrenoreceptors) have been revealed. They are involved in cytoplasm movement regulation, membrane ion permeability, membrane potential generation, and flower development in plants [29]. Plantsynthesized dopamine performs a protective function by preventing plant consumption by mammals or insects [29].
As far as aquatic plants are concerned, norepinephrine and epinephrine, as well as their analog isoproterenol, stimulate the development of flowers [29].
Serotonin and its derivative melatonin represent important plant development regulators at all stages from seed germination to flowering [33]. Serotonin accelerates seedling growth in Hippeastrum hybridum (in which it also promotes seed germination) and Mimosa pudica [29]. Serotonin performs a regulatory role with respect to the formation of plant root apices. In plant tissues, serotonin and melatonin interact with important regulatory compounds, such as nitric oxide, salicylic acid, and jasmonic acid [29].
Melatonin is contained in at least 140 plant species; it is involved in regulating flower and seed development. Melatonin stimulates root elongation, increases the root number, and slows down leaf aging, e.g., in the apple plant and the Thale cress. It increases biomass accumulation in the Thale cress, rice, and, plums. Melatonin facilitates the operation of the plant immune system, stimulating the production of salicylic acid and nitric oxide as antibacterial protective factors. Melatonin's other regulatory functions include stimulating the growth of etiolated (darkgrown) lupin hypocotyls and suppressing root growth in wheat, barley, and oat plants [23,29].
In the tropical part of China, the leaves of coastal plants, such as Plumeria rubra, Syzigium jambos, Buxus megistophylla, and Cinnamomum bodinieri, contain micromolar serotonin amounts and submicromolar or micromolar (in C. bodinieri) catecholamine concentrations. 3-Methyltryptamine, a serotonin metabolite, was also detected in the tested plant leaves, except for B. megistophylla samples [34].
The aquatic plant Lemna minor (the duckweed) synthesizes the neurotransmitter amine tyramine and the ptomains cadaverine and putrescine [35] (which may perform neurochemical functions [11]). Of inter-est in the context of the impact of pollutants on aquatic ecosystems is the fact that the addition of the antibiotic tetracycline, a widespread pharmacological pollutant, to a duckweed culture results in an increase in the biogenic amine level in duckweed cells. This additionally corroborates the idea concerning the role of neurotransmitters in stress responses in both animals and plants [35].
Various neurotransmitters serve as bidirectional signals in terms of interaction between plants and microorganisms that overgrow plants or translocate into their interior [36].

Neurotransmitters in Fishes
Among the comparatively extensive data presented in the literature on neurotransmitter functions in the organisms of aquatic vertebrates, especially fishes, special attention should be paid to ecologically relevant research results. Dopamine that is produced by some algae and invertebrates (see above) exerts an influence on fish behavior. In bony fish species, e.g., in Danio rerio, dopamine suppresses reproduction, behaving as an antagonist of the neurohormone gonadotropin [37]. Therefore, algae-produced dopamine can indirectly limit phyto-and zooplankton consumption by fish to the extent to which it translocates into fish organisms in an aquatic ecosystem and reaches their functional brain areas.
It was emphasized above that, apart from dopamine, catecholamines also include norepinephrine that is released by the microbiota and by other ecosystem components. The literature available to the authors contains no any data on the impact on fish of norepinephrine added to the aquatic environment. However, it was established that experimentally injecting norepinephrine (5-50 ng) into the brain of the goldfish Carassius auratus affected its thermoregulation and induced the fish to prefer relatively cold water [38].
Neurotransmitters contained in the fish organism are partly of microbial origin, since, e.g., histamine is synthesized from the amino acid histidine by symbiotic bacteria, including Escherichia coli, Salmonella spp., Clostridium spp., Proteus spp., Enterobacter spp., and Lactobacillus spp. (reviewed, [12]). In some fish species, especially those with a dark flesh color (the sardinella, scad, and bonito), histamine can accumulate at concentrations that are toxic for their human consumers [12,39,40].
From the ecosystem viewpoint, it is of special interest that histamine and other neurotransmitters contained in the fish organism can be directly released into the water (e.g., if a fish is injured) and influence the other components of the aquatic ecosystem. Importantly, histamine (see the beginning of this work) stimulates the growth of bacteria [10,12] and algae exemplified by Chlorella vulgaris [24] and Vol. 77 No. 1 2022

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Scenedesmus quadricauda [25]. Therefore, neurotransmitters form a part of fish-released kairomones, to the extent to which they are liberated into the aquatic environment, apart from accumulating in the fish organism. Similar to other aquatic ecosystem components, fish are under the strong influence of human-made neuroactive pollutants. The behavior of eels was be disrupted in the presence of cocaine, e.g., in the River Thames near the Houses of Parliament (Palace of Westminster) in London or in the Arno River near Pisa. The behavior disruption manifests itself in eel hyperactivity, so that the eels may fail to reach the spawning area. Cocaine increases the dopamine content in the eel brain, preventing the maturation of young eels; concomitantly, the cortisol concentration is increased, which slows down fat accumulation that is necessary for the migration (Learn, J.R., Some rivers are so drug-polluted, their eels get high on cocaine. National Geographic, 2018 https://www.nationalgeographic.co.uk).

Neurotransmitters as Ecomones (Allelochemicals)
The aforementioned literature data and, in part, the authors' own findings indicate that neurotransmitters are implicated in multilateral interaction among representatives of the aquatic microbiota, phytoplankton, zooplankton, higher plants, and fishes (as well as other vertebrates). Each of the partners involved can both produce neurotransmitters and specifically respond to them. In this work, emphasis was placed on one of the most important neurotransmitter groups. These are biogenic amines (dopamine, norepinephrine, serotonin, histamine, etc.) that probably formed a part of the whole array of means of chemical communication among various life forms that was invented by living nature over 1 billion years ago.
Apart from influencing the functioning of separate components of natural ecosystems, these neurotransmitters are likely to exert regulatory effects on such ecosystems as coherent entities. This is the reason why these chemical factors can be denoted as ecomones, in addition to biomediators [2]. The term "ecomone" was introduced in the works of some ecologists, including Florkin [41] and Pasteels [42,43]. It was stressed that ecomones include intraspecies and interspecies signals. It follows from the facts presented in this work that neurotransmitters operate at both the intra-and interspecies level, combining the functions of pheromones, kairomones, and allomones. Moreover, such ecomones are present in aquatic ecosystems not only because they are synthesized by ecosystem components. While Pasteels in the work cited [43] emphasized the "abiotic environment," it is currently obvious that neuroactive substances and their analogs accumulate in the environment because of environmental pollution. Relevant examples are provided in this review paper.
Ecosystem regulators were also discussed in the classical works of Russian scientists who preferred the term "allelochemicals" to denote active exometabolites in biocenoses, including those of water bodies [44].
Future research should be aimed at measuring the concentrations of diverse neurotransmitters of natural or human origin in water bodies and elucidating their relationship with the various aspects of the functioning of aquatic ecosystems.