International Journal of Marine Science, 2024, Vol.14, No.3, 172-181 http://www.aquapublisher.com/index.php/ijms 176 assemblages. Furthermore, research in the Krka River, Croatia, using environmental DNA (eDNA) metabarcoding, showed significant differences in ciliate community structure across different sampling locations, influenced by hydrological parameters and saprobiological classification (Kulaš et al., 2021). These findings emphasize the need to consider both biogeographical and environmental factors in studies of ciliate diversity. The integration of morphological and molecular approaches is crucial for a comprehensive understanding of ciliate species diversity. Genetic and molecular techniques provide high-resolution tools for species identification, while biogeographical studies reveal the complex interplay between spatial and environmental factors in shaping ciliate communities. This holistic approach enhances our ability to monitor and conserve ciliate biodiversity in various aquatic ecosystems. 4 Ecological Functions of Ciliate Communities 4.1 Role in nutrient cycling Ciliate communities play a crucial role in nutrient cycling within aquatic ecosystems. They contribute significantly to the decomposition of organic matter, thereby facilitating the recycling of essential nutrients such as nitrogen and phosphorus. For instance, studies have shown that ciliate communities in the Northern Beibu Gulf are influenced by the concentrations of phosphorus and nitrogen, which in turn affect their biomass and diversity (Wang et al., 2014). This indicates that ciliates are integral to nutrient dynamics, particularly in eutrophic conditions where nutrient availability is high. Additionally, ciliates are involved in the microbial loop, where they consume bacteria and other microorganisms, thus recycling nutrients back into the ecosystem (Kulaš et al., 2021a; Zhao and Langlois, 2022). 4.2 Contribution to energy flow in ecosystems Ciliates are pivotal in the transfer of energy through aquatic food webs. They occupy intermediate trophic levels, acting as both predators and prey. By preying on bacteria and other protozoa, ciliates help control microbial populations and facilitate the flow of energy to higher trophic levels, such as zooplankton and small fish (Vilas-Boas et al., 2020). In riverine floodplains, for example, ciliates contribute to the overall ecosystem functioning by maintaining high abundances during stable hydrological conditions, which supports the energy flow within these dynamic environments (Vlaičević et al., 2022). Moreover, heterotrichous ciliates, such as those studied in the Pistoia province, play significant roles in material transfer and energy flow in aquatic food webs (Chi et al., 2020). 4.3 Interactions with other microorganisms and higher trophic levels Ciliates interact extensively with other microorganisms and higher trophic levels, influencing community structure and ecosystem health. They serve as bioindicators of environmental conditions due to their sensitivity to pollutants and changes in water quality (Jiang et al., 2013; Kulaš et al., 2021a). For example, ciliate communities in the Krka River have been shown to respond to hydrological parameters and saprobiological classifications, indicating their potential as bioindicators for environmental monitoring (Kulaš et al., 2021a). Additionally, ciliates provide nutrition for higher trophic levels, such as zooplankton and small fish, thereby linking microbial processes to larger food web dynamics (Vilas-Boas et al., 2020). The presence of specific ciliate taxa can also indicate the health of aquatic ecosystems, as seen in the functional groups of marine ciliates that respond to varying levels of pollution and eutrophication (Jiang et al., 2013). 5 Case Studies of Ciliate Communities in Different Environments 5.1 Freshwater ecosystems Ciliate communities in freshwater ecosystems exhibit significant diversity and play crucial roles in the microbial loop, contributing to nutrient cycling and energy flow. Studies have shown that freshwater ciliates are highly diverse, with distinct community structures influenced by environmental parameters such as light exposure and hydrological conditions. For instance, research conducted on the Krka River in Croatia revealed that ciliate communities in biofilm samples from different river sections showed significant differences in community structure, with hydrological parameters being the main structuring factors (Kulaš et al., 2021a; 2021b). Additionally, the diversity of ciliates in freshwater systems of the Brazilian Atlantic Forest was found to be higher
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