Green Hydra is used as a classical example for explaining symbiosis in schools as well as an excellent research model. Indeed the cosmopolitan green Hydra (Hydra viridissima) provides a potent experimental framework to investigate the symbiotic relationships between a complex eumetazoan organism and a unicellular photoautotrophic green algae named Chlorella. Chlorella populates a single somatic cell type, the gastrodermal myoepithelial cells (also named digestive cells) and the oocyte at the time of sexual reproduction. This symbiotic relationship is stable, well-determined and provides biological advantages to the algal symbionts, but also to green Hydra over the related non-symbiotic Hydra i.e. brown hydra. These advantages likely result from the bidirectional flow of metabolites between the host and the symbiont. Moreover genetic flow through horizontal gene transfer might also participate in the establishment of these selective advantages. However, these relationships between the host and the symbionts may be more complex. Thus, Jolley and Smith showed that the reproductive rate of the algae increases dramatically outside of Hydra cells, although this endosymbiont isolation is debated. Recently it became possible to keep different species of endosymbionts isolated from green Hydra in stable and permanent cultures and compare them to free-living Chlorella species. Future studies testing metabolic relationships and genetic flow should help elucidate the mechanisms that support the maintenance of symbiosis in a eumetazoan species.

Hydra is known for its natural occurrence, anatomical simplicity, intricate physiology, regenerative capacity, and ease of maintenance and manipulation in laboratory environments. It has proven to be a valuable model organism in various disciplines. Its applications range from developmental biology, stem cell research, animal physiology to environmental toxicology including ecotoxicology. The sensitivity of Hydra to a variety of environmental stressors and chemical agents such as metals, nanomaterials, and toxic organic compounds provides valuable insights into physiological mechanisms affected by environmental stressors and pollution, and Hydra can be of great use in environmental monitoring. Furthermore, since green Hydra lives in a symbiotic relationship with unicellular photoautotrophic algae, it is a suitable model organism for symbiosis research. Recently, it has become a popular model in holobiont research. The adaptability and importance of Hydra also extends to aquatic science and aquatic ecology, particularly in the context of monitoring and water pollution. Since the 1980s, Hydra has been increasingly used in various fields of research and has established itself as an important versatile model organism in numerous scientific studies. Hydra also represents an outstanding model in the fields of education and STEM. Hydra continues to be an important model in the 21st century, contributing significantly to our understanding of the biology of water and advancing freshwater research, and possibly finding its way to regenerative medicine and tumor pathobiology research.

Algae are gaining attention for their application in aquaculture as a highly sustainable source of useful products. As microalgae have a significant role in primary production in aquatic ecosystems and are the basis of many food chains, it is important to understand the processes that provide them with better survival in a toxicant-polluted environment. In this study the Chlorella bioassay was evaluated: (1) as a potential method for algae farming, (2) as a method for testing advantages or disadvantages of symbiotic association, including two species of aposymbiotic algae, i.e. endosymbiotic algae isolated from green hydra Mychonastes homosphaera (Skuja) Kalina and Punčochářová and Desmodesmus subspicatus (Chodat) Hegewald and Schmidt) and two related free-living algal species ( Chlorella kessleri Fott and Novak. [K&H, 1992] and C. vulgaris Beij. [K&H, 1992]), (3) through algal bioindicator responses related to comparative toxicity and ecotoxicological pollution of iron, and (4) by using algal bioindicators for microscopical and morphometrical application in environmental stress. Increasing concentrations of iron led to cell changes (dry dotted clusters of dying cells, intensive green wet bubbles representing a mucous structure, area, diameter and length), deformations (empty cells, aberrant divisions, irregular coenobia, tetrads and transitional forms) and ultrastructural changes (chloroplasts and nuclei). All modifications were more pronounced in aposymbiotic algae, suggesting a lower degree of adaptation to iron toxicity than their free-living relatives. A free-living species C. kessleri showed the best ability to survive in given unfavorable environmental conditions. High statistical significance was noticed in the cell division parameter, underlining the hormetic effect of increasing the biomass in free-living algal species. This increasing of the cell divisions at the specific concentration of iron demonstrated that the Chlorella bioassay may represent a useful tool for evaluating the growth of different microalgal species, and has a prospective application in a comparative study of algae farming.

The microcosm is a laboratory method frequently used in ecological studies related to population and food web interactions and environmental dynamics. It simultaneously brings into interaction different species in the same controlled laboratory experimental area and provides an opportunity for modeling and reconstruction of relationships in the natural biocenoses and ecosystems. We applied that approach to determine and improve our understanding of predator–prey interactions in different freshwater environments. The inhabitants of the microcosms were isolated endosymbiotic microalga Desmodesmus subspicatus (Chlorophyceae) (Chodat) Hegewald et Schmidt (CZ), green hydras, freshwater turbellarians, and large water fleas. Experiments were performed in five replicates, at 25 °C and 13.5 °C, with fed and hungry predators, respectively. Herein, we proposed a mechanism for microalgal hunting net formation in the freshwater microcosm. Ultrastructural visualization of the endosymbiotic microalgae revealed rod-like structures on the cell wall surface, structures that could possibly fit together and interconnect, suggesting the possibility of microalgal hunting net formation. Interspecific cooperation between isolated microalgae and turbellarians resulted in stronger hunting net formation in preying upon water fleas. This study contributes to the diversity of species interactions and shows the producers as a top link, as opposed to what is generally considered as a basic link in the food web, and presents the microalgae as triggers of the dynamics in the freshwater microcosm.

The freshwater water flea (Daphnia magna Straus, 1820) is prey for numerous predators. Yet it possesses a wide range of strategies to defend itself against predation. The aim of this work is to investigate the defensive mechanisms employed by D. magna to reduce predation by the coelenterate Hydra viridissima, and two planarians, Polycelis felina and Dugesia gonocephala. To do this, we used a freshwater microcosm. An additional aim is to investigate interactions with the presence of the isolated endosymbiotic algae from green hydra, thus combining and observing the interaction of the zooplankton and microalgal component. Each experiment included five replicates (13.5 °C, 25 °C), in crystallizing glass containers (60 mL volume, 60 mm diameter, 35 mm height), including satiated (fed with larvae of Artemia salina) and starved predators, respectively (one or five individuals of a particular predator species in one microcosm). As the isolated microalgae are unique, we tracked the following three mechanisms of Daphnia defense for the first time including precisely this microalgal component: (i) grouping (visual magnification), i.e., two or more Daphnia holding together; (ii) the phenomenon of overproduction, i.e., any number of Daphnia in one container above the 10 initially added individuals; and (iii) accelerated movement (“bullet movement”), i.e., high-speed movements in particular microcosms. The results provide new information for a better understanding of the interspecific relationships in systems that include both zooplankton and microalgal components.

Sodium metasilicate (SM) is a synthetic hazardous water-soluble salt used in industry as an active ingredient in household cleaning products. The impact of SM on the aquatic environment has been discussed worldwide, but its toxicity has not been well documented and researched. Studies have only been performed on a handful of aquatic organisms: algae, plants, blackworms, water fleas, dipteran larvae, and two fish species. Hydra is a simple freshwater cnidarian with diploblastic organisation, where all the cells are in permanent contact with the surrounding aqueous media, and represents a sensitive model organism for environmental toxicity assessments. This research aimed to determine and compare the effect of SM on the morphology, excitability, and behaviour of green and brown hydra and endosymbiotic microalgae as a microbiome of green hydra. The hydras were treated with four sublethal SM concentrations (0.050, 0.365, 0.380, and 0.390 g/L) for 72 h. Standard preparations were made for the cyto-histological analysis of green hydra, and damaged cellular layers and mesoglea and a changed distribution of microalgae were recorded. The SM caused muted responses to mechanical stimuli and damage to the tentacles in both hydra species. The changes were more pronounced in brown hydra, while green hydra showed better adaptability to unfavourable environmental conditions.