The results presented in this case study constitute the largest record for the country of free-living nematodes in Antarctica. Four genera (Plectus, Calcaridorylaimus, Eudorylaimus, and Coomansus) of nematodes from soil samples, associated with vascular plants and mosses, from different areas with anthropic intervention and pristine areas, were molecularly determined. These genera could be linked, thanks to bioinformatics analyses, to genomic sequences of other Antarctic nematode specimens.
Regarding Coomansus 25 de Mayo, a 100% bootstrap value was obtained for the clade formed by three sequences of the species Coomansus gerlachei (KM092523; LC457639/LC457655) cited by Elshishka et al. (2015) and Kagoshima et al. (2019), respectively. This genus of predatory nematodes was found only at the sites furthest away from the pristine zone, at points PZ1 and PZ2. As no associated microfauna were prospected, it was not possible to determine the diet of this cp4 nematode, although it has been cited as a predator of other nematodes (Devi and George 2018; Khan and Kim 2007). Due to the high abundance of Plectus observed in the same area, further analyses may be conducted to establish if there is an association between this nematode and the Coomansus diet.
For the specimens of Calcaridorylaimus 25 de Mayo, the genomic analyses showed a 96% bootstrap value with the species C. signatus (LC457648.1) described by Kagoshima et al. (2019). In this study, this cp4 nematode belonging to the trophic group of omnivores was the second most abundant in the samplings carried out and was found in the pristine site and in the anthropic zone furthest from the station (AZ5). Molecular determination of Calcaridorylaimus is necessary to avoid misinterpretations of the results, since this nematode, when classified according to morphological data, is synonymized with the genera Mesodorylaimus and Aporcelaimus (Velasco-Castrillón et al. 2014).
Eudorylaimus 25 de Mayo was 97% related to GenBank sequences of the genus Eudorylaimus sp. (HQ270134.1) determined within 25 de Mayo/King George Island, confirming its identification with this genus (Raymond et al. 2014). It was 22% related with the clade formed by the species Enchodeloides signyensis (KY881720), also isolated on 25 de Mayo/King George Island, and with E. coniceps (LC457645/ LC457662) (Elshishka et al. 2017; Kagoshima et al. 2019). Due to their sharing a low bootstrap value, a clear phylogenetic relationship with the species of this group could not be determined. In the present study, Eudorylaimus 25 de Mayo was found in low abundance, agreeing with other results of this nematode on the island (Raymond et al. 2014). In turn, it was only isolated in pristine sites, so its absence in sites with anthropic intervention could be considered indicative of sensitivity to anthropic disturbance (Ayres et al. 2008). Eudorylaimus was also reported on Isla 25 de Mayo/King George Island in freshwater and soil samples (Janiec 1993; Ileva-Makulec et al. 2009).
The phylogenetic analysis of Plectus 25 de Mayo through 18S showed a 100% bootstrap value for the clade of species determined in Antarctica of P. murrayi, P. antarcticus, P. belgicae and P. cf. meridianus (Kagoshima et al. 2012; Raymond et al. 2014; Kagoshima et al. 2019). However, the lack of differentiation between the specimens found and those belonging to the NCBI database suggests that the 18S small subunit and the primers used may not be entirely effective for determining species within Plectus. Therefore, future genomic studies using different set of primers should be performed with new isolated specimens. Plectus 25 de Mayo was the most abundant nematode and the most distributed at all sampling points. Its feeding habits and life cycle position it as an opportunistic pioneer nematode (cp2), with the capacity to colonize extreme environments such as the Antarctic soil (Borgmeier et al. 2022). This nematode has also been dominant in other studies within 25 de Mayo/King George Island and other Antarctic sites (Janiec 1993; Ileva-Makulec et al. 2009). In this case study, the mere presence of Plectus would not suffice to make inferences about the degree of disturbance in pristine and anthropic soils due to its uniform distribution throughout the sampled sites.
The specimens of the Rhabditidae family found at Carlini Station could not be morphologically identified at the genus level because only juvenile stages were found. However, its high abundance would reflect the degree of colonization and permanence of these opportunistic cp1 organisms in disturbed sites (Mokuah et al. 2023).
The indices of maturity and structure of the nematode community allow a description of its trophic state, reflecting the health of the habitat in which these organisms are found. MI is an index that decreases with environmental contamination (heavy metals, waste or treated water, fuel) (Bongers 1990). In this work, the lowest MI values (< 2) were observed in the areas with the highest anthropic activity AZ1, AZ2, and AZ3 (fuel storage area, water intake box, and water treatment plant, respectively). These results of MI values match the descriptions of other authors in the maturity analysis of nematode communities on 25 de Mayo Island/King George Island (Gheller et al. 2022) and what has been described in other works from Antarctica where a weak organization in the nematode community structure was observed (Ileva-Makulec et al. 2009). In contrast, the highest values of MI (2–4) were found in the PZ2, PZ3 and PZ4 sites where the anthropic presence is practically nil.
The results of EI were congruent with the maturity values obtained from the trophic webs in the nematode community. The AZ1 site presents the highest degree of anthropic activity in the station, due to circulation on foot and with heavy machinery, and because it is the fuel exchange site. The highest enrichment value (EI = 100) was found in this site due to the exclusive presence of an abundant population of cp1 nematodes belonging to the Rhabditidae family, known to be very apt colonizers of disturbed soils (Salas and Achinelly 2020, Du Preez et al. 2022). The other anthropic sites where enrichment values were obtained were AZ2 and AZ3, where cp2 nematodes were found, represented by Plectus populations. The presence of these nematodes in these sites where wastewater and treated water are handled could indicate a certain degree of environmental response that should be studied in greater depth in future samplings within the Carlini Station.
The equilibrium of an edaphic community is given by the balance within the structure of the trophic web where the organisms in its highest levels control the growing populations of the primary producers. In this context, the SI is used to see the current state of the assemblage of nematodes in the soils studied. The AZ5 anthropic site was the furthest and with the least (almost nil) presence of people moving in the area and was the only one in the station that presented structure values due to the presence of the cp4 nematode Calcaridorylaimus. Within the pristine sites, the highest SI value was found in PZ2 due to the presence of this genus, together with the predatory nematode Coomansus. The omnivorous nematode Calcaridorylaimus was previously isolated from the Antarctic maritime sector on the Galíndez and Signy islands (Maslen and Convey 2006; Velasco-Castrillón et al. 2014, Kagoshima et al. 2019), and from continental soils (Peneva et al. 2009). In our work, the sequences of Calcaridorylaimus 25 de Mayo (found in the continental area) were closely related to C. signatus (LC457648) (Kagoshima et al. 2019) from the Antarctic maritime sector of Signy Island. Calcaridorylaimus 25 de Mayo was exclusively associated with mosses and with the vascular plant Deschampsia antarctica (site AZ5.), in agreement with Peneva et al. (2009), although these authors did not perform molecular analyses, and therefore we do not know whether they are phylogenetically related to our specimens. Further sampling should be carried out to determine the distribution of these nematodes sensitive to disturbances, considered as one of the key pieces within the structure of the edaphic community.
Conducting research and gathering data on nematodes in Antarctica is crucial for comprehending the biodiversity of this exceptional environment, discovering how organisms can adjust to extreme conditions, and examining how human activities affect the ecosystem. In this study, it was possible to observe through the analysis of the nematode assemblage that the pristine sites showed less disturbances than the anthropic sites. Future studies at Carlini Station and 25 de Mayo/King George Island should continue investigating the nematode community, placing emphasis on obtaining physicochemical data to establish more precise distinctions between environments that have been disturbed by human activity and those that remain pristine.