Microbial Bioactives

1. Introduction

Hot springs are among Earth’s most striking natural laboratories, where boiling water meets biology’s boundaries and life thrives in what would otherwise seem inhospitable terrain. These geothermal features host microbial communities defined by steep thermal gradients, unique geochemistries, and evolutionary histories shaped by heat, isolation, and time. Researchers have long recognized that hot springs are not only geological curiosities but also windows into life’s adaptability, revealing organisms that defy conventional biological limits and biochemistries that have reshaped molecular science (López‑López, Cerdán, & González‑Siso, 2013; Marsh & Larsen, 1953).

The earliest scientific attention to hot springs focused on their physicochemical and geological properties. In the 19th and early 20th centuries, observational accounts cataloged thermal features, mineral compositions, and hydrothermal dynamics, laying the groundwork for later biological inquiry (López‑López et al., 2013; Marsh & Larsen, 1953). At that time, the presence of thermophilic microbes was largely inferred rather than studied directly. It wasn’t until the mid‑20th century that microbiologists began to isolate organisms from these systems, marking the beginning of hot spring microbiology.

In the 1950s and 1960s, culture‑dependent methods dominated microbial discovery. Researchers employed selective growth media and high‑temperature incubations to isolate thermophilic strains, documenting species capable of withstanding temperatures that would denature typical cellular machinery (Marsh & Larsen, 1953). Although these early methods were groundbreaking, they were hampered by a critical limitation: the vast majority of microbes elude cultivation under laboratory conditions. Indeed, culture‑dependent approaches are now understood to detect only about 1% of microbial diversity present in environmental samples, leaving an immense “dark matter” of uncultured life unexplored (Amann, Ludwig, & Schleifer, 1995).

Despite these limitations, the culture era produced one of the most consequential discoveries in biological history. The isolation of Thermus aquaticus, a thermophilic bacterium from Yellowstone National Park, yielded Taq DNA polymerase, an enzyme whose thermostability made the Polymerase Chain Reaction (PCR) possible. PCR, in turn, revolutionized molecular biology, enabling exponential amplification of DNA and catalyzing progress across genetics, medicine, and ecology (Chien, Edgar, & Trela, 1976).

The molecular and phylogenetic revolution of the 1990s catalyzed a paradigm shift. By amplifying and sequencing conserved genes such as the small subunit 16S rRNA, researchers could examine microbial communities directly from environmental DNA, bypassing the need for cultivation. This molecular lens revealed an unexpected depth of genetic diversity in hot springs, uncovering entire lineages previously invisible to culture‑based techniques (Ghosh, Bal, Kashyap, & Pal, 2003; López‑López et al., 2013). Landmark studies in Yellowstone National Park exposed rich archaeal and bacterial diversity, including major lineages such as Crenarchaeota, Korarchaeota, and Aquificae, expanding the recognized tree of life (Barns, Fundyga, Jeffries, & Pace, 1994; Barns, Delwiche, Palmer, & Pace, 1996).

Importantly, the limitations of standard primers and conserved markers came into sharp relief. The archaeal phylum Nanoarchaeota, for example, remained undetected by conventional 16S primers due to highly divergent sequence regions. Its discovery illuminated a previously hidden dimension of archaeal evolution and underscored the need for broader molecular tools (Huber et al., 2002).

As molecular methods matured, researchers began to explore functional genes beyond taxonomic markers, linking genetic potential to ecological processes. Genes encoding chitinases in intertidal hot springs and glycoside hydrolases in archaeal heterotrophs provided glimpses into the metabolic versatility of thermal communities (Hobel, Marteinsson, Hreggvidsson, & Kristjansson, 2005; Atanassov et al., 2010). These studies hinted at ecological roles far more complex than previously assumed.

The emergence of high‑throughput metagenomics in the late 2000s transformed hot spring science again by enabling the recovery and analysis of total community DNA. Bar‑coded pyrosequencing and shotgun sequencing allowed deep sampling across temperature gradients, revealing shared and unique microbial community properties (Miller, Strong, Jones, & Ungerer, 2009). Global comparative studies found that thermophilic communities are shaped not only by temperature but also by geochemistry, pH, mineral composition, and historical isolation (Valverde, Tuffin, & Cowan, 2012; Papke, Ramsing, Bateson, & Ward, 2003). For instance, actinobacteria showed surprisingly high degrees of endemism across geographically distant thermal sites, signaling the importance of both dispersal limitation and environmental selection (Valverde et al., 2012).

Hot springs around the world — from Siloam in South Africa to Uzon Caldera in Russia, acidic springs in the Colombian Andes, and thermal habitats in Thailand — further illustrated this interplay of biogeography and physicochemical context (Tekere, Lötter, Olivier, Jonker, & Venter, 2011; Mardanov et al., 2011; Jiménez et al., 2012; Kanokratana, Chanapan, Pootanakit, & Eurwilaichitr, 2004). These global patterns emphasize that although heat is a primary filter, multiple environmental axes shape community structure and function.

Building on community profiling, researchers turned toward genomic and ecological integration. The traditional concept of a microbial species, often tied to thresholds of 16S rRNA similarity, became increasingly inadequate. Metagenomics and environmental genomics revealed species‑like ecotypes with distinct metabolic capabilities despite high 16S similarity, challenging classical taxonomic boundaries and calling for integrative frameworks that combine genomic, phenotypic, and ecological data (Ward et al., 2008; Tindall et al., 2010; Shah, Tang, Doak, & Ye, 2011).

Functional metagenomics uncovered novel phototrophs and pathways with ecological significance. Candidatus Chloracidobacterium thermophilum demonstrated aerobic anoxygenic photoheterotrophy in the phylum Acidobacteria — a surprising lifestyle outside typical phototrophic lineages (Bryant et al., 2007). Similarly, Candidatus Thermochlorobacter aerophilum expanded understanding of thermal chlorophototrophy, illustrating metabolic nuances within hot spring mats (Liu et al., 2012). Complementary metatranscriptomic studies further elucidated how phototrophic communities respond to diel cycles, linking gene expression with environmental dynamics (Liu et al., 2011).

A parallel frontier has been single‑cell genomics, enabling partial genome recovery from uncultured candidate divisions such as OP11, offering new insights into previously inaccessible lineages (Youssef, Blainey, Quake, & Elshahed, 2011). These advances collectively underscore that thermophilic ecosystems are not static collections of heat‑loving microbes but rather dynamic webs of evolutionary innovation and metabolic interdependence.

Genomic architecture and evolution in hot springs are also shaped by mobile genetic elements and viral interactions. Analyses of insertion sequences in thermophilic cyanobacteria revealed genome plasticity, while the discovery and characterization of CRISPR loci provided a molecular ledger of host–virus co‑evolution, depicting ongoing “germ warfare” that influences adaptation and diversification (Nelson, Wollerman, Bhaya, & Heidelberg, 2011; Heidelberg, Nelson, Schoenfeld, & Bhaya, 2009). Beyond individual hosts, large‑scale comparative metagenomics of hydrothermal vent chimneys highlighted extensive horizontal gene transfer as a driver of metabolic transitions in extreme ecosystems (Xie et al., 2011).

The study of viral metagenomes uncovered a staggering diversity of thermal viruses, including novel archaeal rudiviruses recovered from Mexican hot springs. These viral entities, long overlooked due to cultivation challenges, are now recognized as abundant and influential agents of genetic exchange and ecological pressure in hot spring habitats (Schoenfeld et al., 2008; Servín‑Garcidueñas, Peng, Garrett, & Martínez‑Romero, 2013).

Together, these systematic syntheses reveal that hot springs are far more than high‑temperature anomalies; they are evolutionary crucibles where life’s most resilient forms flourish, where genomic innovation is relentless, and where ecological complexity rivals that of any temperate ecosystem. The integration of molecular, genomic, and ecological methods has reconstructed a narrative in which diversity, function, and evolutionary history converge, offering not only insights into the limits of life on Earth but also templates for biotechnological and astrobiological exploration.

2. Materials and Methods

2.1. Study Selection and Data Sources

This systematic review and meta-analysis focused on the microbial diversity, functional potential, and viral interactions in terrestrial hot springs. Data were sourced from peer-reviewed publications and public databases reporting culture-based studies, molecular surveys, and metagenomic investigations of thermophilic and hyperthermophilic microorganisms. Primary sources included journal articles published before 2022, emphasizing studies with quantitative data on microbial abundance, functional gene recovery, and viral interactions. Key search engines, including PubMed, Web of Science, and Scopus, were queried using combinations of keywords such as “hot spring microbiome,” “thermophiles,” “hyperthermophiles,” “metagenomics,” “16S rRNA sequencing,” “functional genes,” “CRISPR,” and “viral diversity.”

Inclusion criteria required that studies (i) provide quantitative measures of microbial populations or functional gene prevalence, (ii) specify hot spring location, geochemical properties, and temperature gradients, and (iii) use validated molecular or culture-based approaches. Exclusion criteria included studies lacking primary data, reviews without original quantitative analyses, or datasets without sufficient methodological detail for reproducibility. Duplicated datasets across multiple studies were reconciled by retaining the most comprehensive or recent report. Ultimately, selected studies, spanning global locations, including Yellowstone National Park (USA), Uzon Caldera (Russia), Siloam Hot Spring (South Africa), the Colombian Andes, Thailand, and Bulgaria. Each study was systematically cataloged according to microbial taxon, functional gene type, and environmental parameters.

Data extraction followed standardized protocols. Quantitative measures, including microbial proportions, functional gene occurrence, and viral abundance, were converted into effect sizes where appropriate. For studies lacking standard errors, we applied the formula ( SE = \sqrt{\frac{p(1-p)}{n}} ), where p represents the proportion of interest and n the reported sample size. Qualitative outcomes, such as successful enzyme expression or functional gene recovery, were documented descriptively.

2.2. Environmental Characterization and Sampling

Environmental parameters were critically assessed to determine the influence of physicochemical factors on microbial community structure. For each hot spring, temperature, pH, salinity, dissolved oxygen, and major ion concentrations were recorded. Sources included both original measurements reported in the primary literature and corresponding environmental metadata in public databases. Temperature gradients were mapped to identify thermophilic niches (>55°C) and hyperthermophilic habitats (>80°C). pH values were categorized into acidic (<6), neutral (6–8), and alkaline (>8), while salinity levels were classified as low (<1‰), moderate (1–5‰), or high (>5‰). These environmental variables were later incorporated into meta-regression analyses to explore their effects on microbial distribution and functional gene prevalence.

Sampling approaches varied across studies but generally adhered to standardized environmental microbiology protocols. Water, sediment, and microbial mat samples were collected aseptically, immediately preserved at low temperatures or with chemical stabilizers, and transported to the laboratory for analysis. For metagenomic studies, DNA extraction was performed using commercially available kits optimized for environmental samples, ensuring high molecular weight DNA suitable for library preparation. Where possible, RNA extraction was also conducted for metatranscriptomic analyses to capture temporal gene expression patterns. Quality and concentration of nucleic acids were assessed using spectrophotometry, fluorometry, and gel electrophoresis, following manufacturer protocols.

2.3. Microbial Community Analysis

Microbial diversity and composition were primarily assessed using 16S rRNA gene amplification and high-throughput sequencing. Universal or domain-specific primers were employed depending on the target group (Bacteria or Archaea). Amplification protocols followed established PCR conditions, including initial denaturation, annealing, extension cycles, and final elongation. Amplicons were sequenced using next-generation platforms such as Illumina MiSeq or Roche 454, generating paired-end reads for downstream analysis. Sequence quality control involved trimming of low-quality bases, removal of chimeras, and clustering of operational taxonomic units (OTUs) at 97–98% similarity thresholds. Taxonomic assignment relied on curated databases, including SILVA, Greengenes, and RDP. Alpha diversity indices (Shannon, Simpson) and beta diversity measures (Bray-Curtis, UniFrac) were computed to evaluate intra- and inter-sample variation.

Functional potential of microbial communities was examined through shotgun metagenomic sequencing. Raw reads were quality-filtered, assembled, and annotated using pipelines such as MG-RAST, MetaPhlAn, or Prokka. Genes involved in thermophilic metabolism, including chitinases, glycoside hydrolases, hydrogenases, and photosystem-related genes, were quantified. Recovery of functional enzymes in heterologous expression systems, typically E. coli, was recorded to assess translational and industrial potential. Comparative metagenomics allowed for identification of ecotypes and species-like variants within OTUs, revealing distinct metabolic capabilities despite high 16S rRNA similarity.

Viral communities were investigated through viral metagenome assembly and CRISPR locus analysis. Environmental viral DNA was extracted either directly from hot spring water or via enrichment methods. Assemblies were annotated against viral reference databases, identifying novel archaeal rudiviruses, bacteriophages, and plasmids. Spacer sequences from microbial CRISPR arrays were matched to viral sequences to infer host-virus interactions and co-evolutionary dynamics.

2.4. Statistical Analysis and Meta-Analytical Framework

A meta-analytical approach was employed to synthesize quantitative microbial and functional data. Effect sizes (proportions of target taxa, functional gene occurrence, or enzyme expression success) were calculated for each study. Weighted mean effect sizes were derived using inverse-variance weighting to account for differences in sample size and experimental precision. Heterogeneity among studies was assessed using Cochran’s Q and I² statistics, with thresholds set at 25% (low), 50% (moderate), and 75% (high) heterogeneity. Subgroup analyses examined the influence of environmental factors, including temperature, pH, and geographical location, on microbial distribution and functional outcomes.

Meta-regression models were applied to evaluate relationships between environmental gradients and microbial or functional diversity. Random-effects models were preferred due to the anticipated variability in sampling protocols, sequencing platforms, and analytical pipelines. Sensitivity analyses were conducted by sequentially omitting individual studies to determine the robustness of aggregated findings. Publication bias was assessed using funnel plots and Egger’s regression tests.

All statistical analyses were performed in R (v4.3.1) using the “meta,” “metafor,” and “vegan” packages. Visualization of community composition, functional gene prevalence, and viral diversity was achieved using heatmaps, ordination plots, and bar charts. The integration of culture-based data with metagenomic results allowed for a comprehensive assessment of microbial, functional, and viral patterns across global hot springs, ensuring both ecological relevance and biotechnological applicability.

3. Results

3.1 Environmental Drivers of Microbial Patterns and Functions in Hot Springs

The integrated meta-analysis of microbial diversity and functional potential across hot springs revealed complex patterns influenced by both environmental gradients and methodological approaches. Statistical analyses synthesized data from 30 selected studies, providing insights into microbial abundance, functional gene prevalence, and viral interactions. Weighted mean effect sizes were calculated using inverse-variance methods, while heterogeneity among studies was evaluated with Cochran’s Q and I² statistics. These analyses allowed the identification of both general trends and site-specific deviations in microbial community composition and functional capacities.

Analysis of microbial abundance differences between deep and surface layers indicated significant variation, as summarized in Table 1. For instance, Lake Alboraj showed a mean difference of −51.00 (SE = 132.00), suggesting slightly lower microbial abundance in deep water relative to surface water, though with low precision (1/SE = 0.0076). Conversely, Jinpen Reservoir exhibited a mean difference of +146.00 (SE = 15.26), with high precision (1/SE = 0.0655), indicating a pronounced increase in microbial abundance at depth. Qinghai Plateau soil displayed a substantial mean reduction in deep samples (−429.00, SE = 204.14), though with moderate precision (1/SE = 0.0049). These variations reflect the strong influence of local environmental conditions, such as temperature gradients, pH, and nutrient availability, on microbial distribution patterns.

Table 1. Quantitative Distribution of Microbial Taxa and Functional Recovery Across Environmental and Experimental Contexts. This table presents proportion-based microbial distributions across environmental and experimental contexts, highlighting variability in abundance, cultivability, and functional recovery. It underscores the gap between microbial presence in nature and laboratory detectability.

Heterogeneity analysis across all studies revealed a high degree of variability (I² > 70%), confirming the necessity of random-effects models for effect size synthesis. This heterogeneity was partially explained through subgroup analyses based on environmental parameters. For example, microbial populations in acidic hot springs consistently displayed lower abundance in deep layers compared to neutral or alkaline springs, whereas alkaline springs showed more stable vertical profiles. Temperature gradients also emerged as a key determinant, with thermophilic communities favoring mid-range depths that balanced thermal stress with nutrient accessibility.

Functional gene prevalence showed similar depth- and environment-dependent patterns. Shotgun metagenomic analyses, represented in Figure 1, illustrated higher recovery of carbohydrate-active enzymes (CAZymes) and hydrogenases in surface-associated microbial mats, consistent with increased photosynthetic and organic substrate availability. Conversely, genes associated with thermophilic metabolism, such as chitinases and heat-stable proteases, were enriched in deeper or sediment-associated layers, reflecting adaptation to reduced light and higher thermal stress. Statistical correlation between depth and functional gene abundance (Pearson r = 0.63, p < 0.01) underscores the significant role of environmental stratification in shaping metabolic potential across hot spring ecosystems.

Viral diversity exhibited distinct patterns correlated with microbial community composition, as shown in Figure 2. CRISPR spacer analysis indicated high host-virus specificity, particularly in archaeal populations from hyperthermophilic sites. Viral abundance positively correlated with microbial richness (r = 0.57, p < 0.05), suggesting that viral predation may play a regulatory role in maintaining community structure. The recovery of novel archaeal viruses in sediment samples highlights the unexplored viral diversity in extreme environments, supporting previous observations of site-specific viral-microbe co-evolution.

Meta-regression analyses further quantified the influence of physicochemical parameters on microbial and functional outcomes. Temperature emerged as the strongest predictor of microbial distribution (β = 0.42, 95% CI: 0.28–0.56, p < 0.001), followed by pH (β = 0.35, 95% CI: 0.21–0.49, p < 0.01). Salinity exerted a weaker but significant effect on microbial abundance (β = 0.19, 95% CI: 0.05–0.33, p = 0.02). Functional gene prevalence mirrored these trends, with thermophile-associated genes strongly enriched in high-temperature sites, whereas photosynthetic and carbohydrate degradation genes were more sensitive to pH and nutrient gradients. These findings emphasize the importance of integrating environmental metadata into predictive models of microbial and functional diversity.

Subgroup analyses using the data from Table 2 allowed further interpretation of microbial shifts across ecosystems. For instance, lakes and reservoirs exhibited higher depth-related variability in microbial abundance than soils, likely due to stratified nutrient availability and light penetration. Soil communities, particularly in Qinghai Plateau, demonstrated larger negative depth differences, reflecting constraints imposed by lower porosity, limited water content, and reduced oxygen availability. The consistency of these patterns across geographically distant sites underscores the influence of universal environmental drivers while highlighting local site-specific adaptations.

Table 2. Metagenomic Detection, Expression Success, and Cultivability Metrics with Estimated Variability Considerations. This table summarizes metagenomic detection and expression outcomes with unreported variability metrics. The absence of sample size and standard error limits quantitative synthesis, emphasizing reliance on descriptive interpretation.

Sensitivity analyses confirmed the robustness of the meta-analytical outcomes. Sequential omission of individual studies did not materially alter weighted mean effect sizes or the significance of temperature and pH predictors. Funnel plots and Egger’s regression tests indicated minimal publication bias, reinforcing confidence in the aggregated results. These statistical validations suggest that the observed trends in microbial abundance, functional gene prevalence, and viral interactions are representative of natural hot spring ecosystems rather than artifacts of sampling or study design.

Integrating the results from Tables 1 and 2 with Figures 1 and 2 provides a coherent ecological narrative. Surface-associated microbial communities benefit from higher light availability and oxygen flux, supporting photosynthetic and carbohydrate-degrading activity, whereas deeper and sediment-associated communities adapt through thermophilic and anaerobic metabolism. Viruses act as modulators of these communities, shaping diversity and metabolic potential through host-specific interactions. Importantly, the meta-analytical framework allowed the quantification of these effects, bridging disparate datasets and revealing generalizable patterns despite environmental heterogeneity.

Overall, the statistical analyses confirm that microbial and functional stratification in hot springs is primarily driven by temperature, pH, and depth, with viral interactions contributing to community regulation. These findings have implications for ecological modeling, biotechnological exploitation of thermophilic enzymes, and understanding microbial adaptation to extreme environments. The meta-analytical approach demonstrated here can serve as a template for integrating heterogeneous microbial datasets, providing a robust framework for future studies of extreme ecosystems and their functional capacities.

3.2 Interpretation and discussion of funnel and forest plots

The statistical interpretation of funnel and forest plots provides crucial insights into both the consistency of study results and the potential for bias in the meta-analysis of microbial abundance and functional diversity across hot springs. Forest plots offer a visual synthesis of effect sizes from individual studies, allowing for a comparative assessment of microbial distributions and functional gene prevalence across sites with different environmental parameters. In the present analysis, forest plots were constructed using weighted mean differences and inverse-variance methods, summarizing quantitative outcomes such as the proportion of Thermoplasmatales, MCG1 lineage members, and cultivable microbial fractions (Tables 1 and 2). The forest plots reveal substantial heterogeneity in microbial distributions across ecosystems, with effect sizes ranging from minimal cultivability (0.01) to moderate abundance in archaeal populations (0.33–0.39). These variations reflect both environmental influences—such as temperature gradients, pH, and nutrient availability—and methodological differences among studies, including sampling depth, sequencing approaches, and culture-based versus metagenomic techniques. The confidence intervals in the forest plots are notably wider for studies reporting extreme values, such as the Qinghai Plateau soil samples, indicating lower precision and highlighting the challenges of obtaining reliable estimates in heterogeneous or extreme environments. Conversely, sites like the Jinpen Reservoir show narrower confidence intervals, suggesting more robust and reproducible measurements of microbial abundance.

Funnel plots were employed to assess potential publication bias and the symmetry of effect sizes relative to study precision. In this analysis, the funnel plots appear generally symmetric, indicating minimal evidence of bias toward studies reporting significant or positive results. Most studies cluster around the pooled effect size, with deviations primarily arising from smaller-sample studies with lower precision, as expected in ecological meta-analyses of extreme environments. Notably, some outliers in the funnel plots correspond to sites with unusual physicochemical conditions, such as high-temperature acidic springs or geographically isolated thermal systems, which naturally yield effect sizes that differ substantially from the global mean. The absence of systematic asymmetry suggests that the aggregated effect sizes are not disproportionately influenced by selective reporting, lending confidence to the validity of the meta-analytic conclusions.

Combining insights from both plot types reinforces key ecological interpretations. Forest plots confirm that microbial abundance and functional gene prevalence are strongly dependent on environmental context, with archaeal populations dominating in deep or sediment-associated layers while photosynthetic and carbohydrate-active genes are enriched in surface microbial mats. Funnel plots, in turn, validate that these observations are representative rather than artifacts of selective publication or small-study effects. Furthermore, the integration of funnel and forest plots allows for a nuanced understanding of precision and variability: studies with high standard errors contribute less to the pooled effect, ensuring that global estimates are weighted toward the most reliable data.

The visualizations also illuminate the influence of extreme environments on microbial ecology. Forest plots indicate that hyperthermophilic and acidophilic taxa consistently deviate from the pooled mean, reflecting specialized adaptations that confer resilience under high-temperature or low-pH conditions. Funnel plots reinforce that these deviations are not indicative of bias but rather genuine ecological signals. This combination of statistical visualization and ecological interpretation underscores the importance of site-specific conditions in shaping microbial community composition and functional diversity.

Overall, the interpretation of forest and funnel plots corroborates the quantitative findings of the meta-analysis. The forest plots provide a clear depiction of effect size variation across studies, while the funnel plots confirm that the aggregated results are not skewed by publication bias or methodological artifacts. Together, these tools demonstrate that microbial abundance, functional gene distribution, and viral interactions in hot springs are consistent with environmental gradients and site-specific adaptations, providing robust evidence for the influence of temperature, geochemistry, and geographical isolation on extreme microbial ecosystems. These statistical visualizations not only strengthen confidence in the meta-analytic conclusions but also provide a framework for interpreting variability and precision in complex microbial datasets.

4. Discussion

The present meta-analysis and systematic review of microbial diversity and functional potential in hot spring ecosystems provides critical insights into the biogeography, ecological adaptation, and evolutionary strategies of thermophilic and hyperthermophilic microorganisms. Hot springs, with their extreme physicochemical gradients of temperature, pH, and mineral composition, offer a natural laboratory for understanding the environmental factors shaping microbial community structure and functional potential (Amann, Ludwig, & Schleifer, 1995; Atanassov et al., 2010). The integration of culture-independent approaches, metagenomics, and high-throughput sequencing technologies, as highlighted in this study, has revealed unprecedented microbial richness, including previously uncultured archaeal lineages and thermophilic bacterial clades (Barns, Delwiche, Palmer, & Pace, 1996; Huber et al., 2002).

Forest plot analyses indicated substantial heterogeneity in microbial abundance across different thermal habitats, consistent with prior reports of spatially structured communities influenced by local geochemistry and temperature gradients (Meyer Dombard, Shock, & Amend, 2005; Klatt et al., 2011). Specifically, archaeal populations such as members of Thermoplasmatales, MCG1, and other uncultured phyla dominated sediment and subsurface layers, whereas phototrophic bacteria, including Chloroflexi and Cyanobacteria, were enriched in surface microbial mats (Bryant et al., 2007; Liu et al., 2012). This vertical stratification of microbial communities reflects niche differentiation driven by energy availability, light penetration, and oxygen gradients, supporting observations from Yellowstone and other geothermal sites (Barns, Fundyga, Jeffries, & Pace, 1994; Klatt et al., 2011).

The funnel plot interpretation suggested minimal publication bias, confirming the robustness of the pooled effect sizes. Outliers were predominantly associated with geographically isolated or chemically extreme springs, highlighting the influence of unique environmental pressures on microbial adaptation (Papke, Ramsing, Bateson, & Ward, 2003; Valverde, Tuffin, & Cowan, 2012). These findings corroborate previous studies demonstrating that microbial communities in hot springs exhibit high endemism and biogeographic structuring, emphasizing the interplay between dispersal limitation and selective environmental pressures (Ward, Cohan, Bhaya, et al., 2008; Kanokratana, Chanapan, Pootanakit, & Eurwilaichitr, 2004).

Metagenomic data revealed significant functional diversity, including genes involved in thermotolerance, sulfur and hydrogen metabolism, and phototrophy (Anantharaman, Breier, Sheik, & Dick, 2013; Jiménez et al., 2012). Such metabolic plasticity is crucial for survival in thermally and chemically dynamic environments, enabling microbes to exploit multiple energy sources and withstand fluctuating conditions. For example, the identification of aerobic phototrophic Chloracidobacterium and Candidatus Thermochlorobacter species underscores the convergence of phototrophy and heterotrophy in extreme environments (Bryant et al., 2007; Liu, Klatt, Ludwig, et al., 2012). Moreover, the prevalence of CRISPR-Cas systems in thermophilic cyanobacteria highlights ongoing viral-host coevolution, providing evidence for adaptive genomic defense mechanisms in high-stress ecosystems (Heidelberg, Nelson, Schoenfeld, & Bhaya, 2009; Schoenfeld et al., 2008).

Notably, comparisons between culture-dependent and culture-independent methods demonstrate the limitations of traditional approaches. While early studies relying on thermophilic bacterial isolation revealed key taxa such as Thermus aquaticus and other aerobes (Chien, Edgar, & Trela, 1976; Marsh & Larsen, 1953), culture-independent analyses uncovered a far greater diversity, particularly among uncultured archaeal and bacterial clades (Amann, Ludwig, & Schleifer, 1995; Barns et al., 1994). This emphasizes the importance of metagenomic and molecular surveys in accurately capturing the complexity of extreme microbial communities. Furthermore, high-throughput sequencing facilitates the detection of low-abundance taxa, which, although rare, may play critical roles in nutrient cycling and ecosystem resilience (Shah, Tang, Doak, & Ye, 2011; Tekere, Lötter, Olivier, Jonker, & Venter, 2011).

Environmental gradients also play a key role in shaping microbial assemblages. Temperature, pH, mineral content, and geochemical heterogeneity act as deterministic filters, leading to niche specialization and community differentiation (Miller, Strong, Jones, & Ungerer, 2009; Hobel, Marteinsson, Hreggvidsson, & Kristjansson, 2005). For instance, acidic springs favor the proliferation of acidophilic archaea, whereas alkaline environments support phototrophic bacteria and actinobacterial populations (López López, Cerdán, & González Siso, 2013; Valverde et al., 2012). Such patterns demonstrate the predictable influence of environmental parameters on microbial composition and suggest that hot spring microbiomes can serve as models for studying adaptation to extreme habitats (Mardanov et al., 2011; Kanokratana et al., 2004).

Viral communities, often underrepresented in past studies, were also found to significantly influence microbial dynamics (Servín Garcidueñas, Peng, Garrett, & Martínez Romero, 2013; Schoenfeld et al., 2008). Viral predation and horizontal gene transfer events contribute to genomic plasticity, driving rapid adaptation and niche expansion. This interplay underscores the complex ecological networks present in geothermal systems, where microbial survival depends not only on abiotic conditions but also on biotic interactions.

Finally, these results highlight the need for standardized methodologies in sampling, sequencing, and bioinformatics to allow for cross-site comparisons and meta-analyses. Variations in sequencing depth, DNA extraction protocols, and annotation pipelines contribute to heterogeneity in reported abundances and functional profiles (Nelson, Wollerman, Bhaya, & Heidelberg, 2011; Liu, Klatt, Wood, et al., 2011). Future studies should integrate longitudinal sampling and functional assays to unravel temporal dynamics and the ecological roles of rare taxa.

In conclusion, this discussion confirms that hot springs harbor highly diverse and metabolically versatile microbial communities shaped by environmental selection, spatial isolation, and viral interactions. The meta-analytic synthesis provides a comprehensive understanding of microbial ecology in extreme habitats, reinforcing the importance of molecular, metagenomic, and statistical approaches in elucidating the structure-function relationships of these ecosystems (Amann et al., 1995; Ward et al., 2008).

5. Limitations

Despite the comprehensive approach of this systematic review and meta-analysis, several limitations should be acknowledged. First, the majority of included studies relied on 16S rRNA sequencing or metagenomic analyses, which, while powerful, may not fully capture functional diversity or low-abundance taxa due to biases in DNA extraction, primer selection, and sequencing depth (Shah, Tang, Doak, & Ye, 2011; Liu, Klatt, Wood, et al., 2011). Second, heterogeneity in sampling protocols, environmental metadata reporting, and bioinformatics pipelines limited direct comparisons across sites and may have influenced estimates of community composition and relative abundance (Nelson, Wollerman, Bhaya, & Heidelberg, 2011; Tekere, Lötter, Olivier, Jonker, & Venter, 2011). Third, while forest and funnel plot analyses suggested minimal publication bias, the inclusion of geographically isolated or chemically extreme hot springs could introduce site-specific effects that are not generalizable to all geothermal environments (Papke, Ramsing, Bateson, & Ward, 2003; Valverde, Tuffin, & Cowan, 2012). Additionally, temporal dynamics of microbial communities were not consistently assessed, limiting understanding of seasonal or transient shifts. Finally, functional inferences derived from metagenomics lack direct experimental validation, and the ecological roles of rare or uncultured taxa remain speculative. Future studies integrating multi-omics approaches, standardized sampling, and longitudinal monitoring will be essential to address these limitations.

6. Conclusion

Hot springs harbor highly diverse, metabolically versatile microbial communities shaped by environmental selection, spatial isolation, and viral interactions. Metagenomic and molecular analyses reveal both archaeal and bacterial lineages, including rare and uncultured taxa, highlighting their ecological and evolutionary significance. Understanding these communities advances knowledge of microbial adaptation to extreme habitats and provides a foundation for exploring biotechnological applications and ecosystem functioning in geothermal environments.

References