As global temperatures rise and ecosystems change, attention should be paid to microbial communities and environmental stressors [1]. While bacteria, viruses, and fungi have received focus, the role of (pathogenic) free-living amoebae is overlooked [2]. There are major knowledge gaps in understanding how climate change affects distribution, behaviour, and pathogenic potential of free-living amoebae [3]. Bacterial indicators: Escherichia coli, Legionella, and enteric viruses are routinely monitored in water/clinical settings, pathogenic free-living amoebae are rarely included in surveillance, (Pathogenic/non-pathogenic refer to recognised clinical associations and not distinct ecological/climate patterns, as current evidence remains limited) despite presence in environments that are monitored [4]. Conventional indicators primarily reflect faecal contamination or acute water-quality failures, whereas amoebae integrate microbial interactions through grazing and encystment dynamics. Free-living amoebae occur across soil, freshwater, biofilm, and built environments, capturing broader microbial community shifts not detectable through bacterial or viral indicators alone.

Climate-driven changes in temperature/precipitation/salinity/soil moisture and ultraviolet radiation likely affect amoebic survival/encystment/virulence, altering ecosystems and increasing human exposure through soil disturbance/aerosols/recreational waters and urban water systems [5] particularly in coastal zones and regions affected by extreme weather events. By sheltering microorganisms from harsh conditions, amoebae facilitate horizontal gene transfer, promote microbial persistence, and contribute to evolution of traits enhancing survival/virulence [6].

Study designs may include experimentation linking amoebal abundance, prevalence of cysts and intracellular microbiota to temperature, moisture and land use, alongside climate-simulation systems assessing warming- and drying-induced shifts in encystment and grazing. Free-living amoebae are ubiquitous, inhabiting diverse environments: soil/dust/air/water forming a critical part of the microbial ecosystem [7]. Acanthamoeba spp., Naegleria fowleri, Balamuthia mandrillaris, Sappinia pedata, Vahlkampfia spp., Paravahlkampfia spp. and Vermamoeba spp., can cause infections such as keratitis and fatal encephalitis [8]. To our knowledge, no global regulations addressing amoebae presence and standardized prevention/disinfection protocols exist. Acanthamoeba act as dominant bacterial grazers responsible for bacterial reduction in soil ecosystems [7] consuming primary decomposers and releasing mineral nutrients sequestered within bacterial biomass, as secondary decomposers. Amoebal grazing reshapes bacterial communities and accelerates nutrient mineralisation, linking microbial turnover to soil fertility and carbon cycling. As these processes are climate-sensitive, shifts in amoebal activity under warming conditions may influence soil function and microbial succession [9].

Bacterial species have evolved mechanisms to survive digestion and persist within both trophozoite/cyst stages [6]. Highly resilient cysts enable free-living amoebae and internalized bacteria to withstand desiccation/temperature fluctuations/chemicals/standard disinfection procedures, potentially facilitating entry of pathogenic bacteria into human-related environments. Cysts can survive exposure to extreme desiccation/ultraviolet radiation and common disinfectants for prolonged periods, demonstrating a level of environmental persistence that exceeds that of many free-living bacteria [7]. The life cycle of amoebae includes an active trophozoite/dormant cyst stage (and a temporary flagellate stage in Naegleria). Recent work in soil microbial ecology reveals protist driven grazing enhances litter decomposition, increasing microbial carbon flux, varying according to temperature, indicating that protist activity responds to climate shifts and carbon cycling processes within soil microbiomes [10].

Human and planetary health are interconnected, with microbial ecosystems, particularly those found in soil [11]. One Health recognises the health of humans, animals, plants, and environment as connected [12], and has expanded to include microbial communities [13]. The planetary microbiome refers to interconnected microbial systems spanning soil, water, air, and host organisms that shape and respond to planetary-scale ecological processes, linking environmental, animal, and human health within a One Health framework [11], with amoebae representing an important yet overlooked component.

Rising temperatures, altered precipitation patterns, and anthropogenic land use may shift distribution/behaviour of free-living amoebae and associated microbiomes (Fig 1). Salinity fluctuations impose osmotic stress on amoebae, with tolerance varying across taxa; experimental evidence demonstrates differential growth responses among pathogenic genera under elevated salt conditions [14].

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Fig 1. Climate stress-amoeba-pathogen-health interface.

The proposed mechanistic pathway linking climate-driven environmental stressors to cross-interface health risks through free-living amoebae. Climate-related changes, including rising temperatures, altered hydrology, land-use disturbance, and extreme weather events, influence amoebal distribution, persistence, and behaviour. In response, free-living amoebae exhibit enhanced encystment, ecological resilience, and increased capacity to harbour intracellular microorganisms. These interactions promote pathogen persistence, stress adaptation, and potential amplification within environmental reservoirs. Subsequent release through soil disturbance, aerosols, water systems, and animal–environment interfaces facilitates transmission across environmental, animal, and human domains. This highlights free-living amoebae as biological connectors within One Health and planetary microbiome, linking environmental change to emerging health risks.

https://doi.org/10.1371/journal.pesy.0000016.g001

Amoebae respond rapidly to changes in moisture/temperature, and nutrient availability, making them sensitive indicators of environmental instability. Their ability to transition between trophozoite/cyst and flagellate stages, allows them to persist during drought/heat waves and chemical stress. Amoebae can shift microbial composition by selectively grazing on some groups while protecting others that survive intracellularly. Many microorganisms that resist amoebal digestion possess traits enhancing survival under stress, including antimicrobial resistance, increased biofilm formation, and improved tolerance to desiccation/temperature fluctuations. Climate change may favour microorganisms that are pre-adapted to amoebal predation and human infection.

Amoebae are sensitive bioindicators, with shifts in abundance, cyst ratios and intracellular microbiota detectable at species or genus level and typically over seasonal timescales. Rising temperatures may expand the geographic range of species such as Naegleria fowleri, while moisture conditions may alter behaviour of Acanthamoeba spp. and Balamuthia mandrillaris in soil/dust. Factors influencing free-living amoebae, together with predicted ecological responses/implications for planetary microbiome and One Health, are summarised in Table 1. These are plausible ecological links rather than direct causal relationships, likely influenced by environmental mediators.

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Table 1. Climate-sensitive drivers of free-living amoebae and their implications for the planetary microbiome and One Health.

https://doi.org/10.1371/journal.pesy.0000016.t001

Free-living amoebae should be incorporated into environmental/ecological, and public-health frameworks to understand/mitigate impact of climate change. Their ubiquity/ ecological importance and capacity to harbour diverse intracellular microorganisms make them indicators of ecosystem stability. Incorporating Acanthamoeba spp., Naegleria fowleri, and Balamuthia mandrillaris into routine soil/water surveillance will strengthen early detection of microbial shifts, particularly in regions experiencing sustained warming (e.g., > 0.2–0.3 °C per decade) or marked precipitation variability (e.g., > 20% interannual variation) via development of biosensor-based early warning tools for climate-sensitive hazards [15].

Systems that simulate future climate scenarios will allow exploration of how amoebae influence microbial succession, biogeochemical cycling, and persistence of opportunistic pathogens. Application of multi-omic approaches to characterise intracellular microorganisms within amoebae in different climates, and comprehension of gene transfer/emergence of antimicrobial resistance should be conducted, prioritising representative taxa such as Acanthamoeba, Naegleria and Balamuthia, together with intracellular microbiota. Useful metrics include relative abundance of amoebae relative to total microbial or eukaryotic community reads, prevalence of cysts, and detectable intracellular bacteria. Feasible detection approaches include microscopy with molecular confirmation, targeted qPCR for key taxa/metagenomic screening. Conducting seasonal sampling in coastal, urban water and agricultural soils will capture climate linked fluctuations. These approaches face limitations, including non-climatic drivers/technical constraints, highlighting the need for simplified, comparable monitoring tools. Incorporating free-living amoebae into planetary microbiome and One Health models will offer a practical indicator system, tracking microbial responses to climate change.