Discussion
The primary goal of our study was to unveil how communities functionally respond to the combination of environmental factors typical of polar marine volcanoes. Our results show that regardless of proximity between fumaroles and glaciers on Deception Island, the community function is strongly driven by the combination of contrasting environmental factors, as occurred similar to what we previously observed for community composition and diversity (Bendia, Signori, et al., 2018). We detected some bacterial groups present in both glacier and fumarole sediments (most notably the phyla Proteobacteria, Firmicutes, and Bacteroidetes), despite the strong gradients in temperature, geochemistry and salinity. In addition, we observed specific groups that varied according to the environmental temperature: the hyperthermophilic members belonging to Crenarchaeota/Thermoprotei, Aquificae and Thermotoga phyla in the 98oC fumarole, Thaumarchaeota in <80oC fumaroles, and Acidobacteria and Verrucomicrobia in glaciers. These patterns are consistent with previous work carried out on Deception Island using the same sample set for diversity analysis (16S rRNA gene sequencing) (Bendia, Signori, et al., 2018), except for the Aquificae and Thermotogae phyla, which were not detected by that method. Furthermore, our taxonomic patterns were also consistent with a previous report that observed similar members along a temperature gradient ranging from 7.5 to 99 oC in geothermal areas in Canada and New Zealand (Sharp et al., 2014).
Surprisingly, our network analysis showed that the community interaction in the hottest fumarole (98 oC) was more complex and presented fewer positive interactions when compared to the lowest temperatures, in contrast to previous studies that showed that community complexity decreases with temperature increase (Cole et al., 2013; Merino et al., 2019; Sharp et al., 2014). Our results suggest that hyperthermophilic temperatures on Deception probably trigger ecological interactions between community members to modulate their resistance and resilience when facing strong environmental stressors. Similar patterns of community interaction have been previously observed in stressful conditions in the Atacama Desert (Mandakovic et al., 2018) and with increasing temperature in anaerobic digestion (Lin et al., 2016), although these environmental conditions are different from those found on Deception Island.
Correlation with environmental drivers varied among both taxonomic and functional groups. For example, groups positively influenced by temperature, sulfate, and sodium were those mainly abundant in fumaroles, while groups and functions prevalent in glaciers were positively correlated with ammonia. These results indicate that the mosaic of environmental parameters shapes both taxonomic and functional diversity of microbial communities. Indeed, we observed a partition of metabolic diversity among the steep environmental gradients on Deception Island. Unlike previous studies carried out at hydrothermal vents which pointed to metabolic functional redundancy at the community level (Galambos et al., 2019; Reveillaud et al., 2016), Deception communities showed metabolic heterogeneity across the sharp temperature gradient. The observation of functional redundancy despite the taxonomic variation has been observed in several environments such as venting fluids from the Mariana back-arc, cold subseafloor ecosystems, freshwater and gut microbiomes (Louca et al., 2016; Trembath-Reichert et al., 2019; Tully et al., 2018; Turnbaugh & Gordon, 2009; Várbíró et al., 2017). The metabolic heterogeneity observed in our results indicates that microbial communities on Deception harbor a remarkably diverse genetic content that reflects the strong selective pressures caused by a remarkable interaction between the volcanic activity, the marine environment, and the cryosphere.
The functional pattern clustered the samples by temperature, rather than by geographic location, and showed that microbial communities on Deception Island are grouped by 98 oC fumarole, <80 oC fumaroles and glaciers. The predominant metabolic potential in the hottest fumarole (98oC) was mostly associated with reductive pathways, such as sulfate reduction, ammonification, and dissimilatory nitrite reduction, and carbon fixation. We suggest that the hydrogen sulfide emissions and hyperthermophilic conditions of this fumarole (98oC) (Somoza et al., 2004) may decrease the dissolved oxygen even in the superficial sediment layers, creating a steep redox gradient and preferably selecting microorganisms with reductive and autotrophic pathways. In addition, communities from the hottest fumarole (98 oC) exhibited several genes related to different adaptive strategies, such as those associated with oxidative stress, specific archaeal heat-shock responses, base excision repair, recombination (recU ), reverse gyrase, protein biosynthesis, chemotaxis, and ABC transporters. This reflects its primaries stress factors, including the fumarolic production of hydrogen sulfide, which has a strong reductive power capable of causing oxidative stress, and hyperthermophilic temperature that induces disturbance to metabolic processes and cell-component denaturation (Hedlund et al., 2015; Merino et al., 2019). Enrichment in genes involved with chemotaxis was also observed in metagenomes from hydrothermal vents at Juan de Fuca Ridge (Xie et al., 2011), but different DNA repair mechanisms were found when compared to Deception metagenomes. Different types of ABC transporters were also detected in Ilheya hydrothermal fields (Wang & Sun, 2017); reverse gyrase and thermosome mechanisms have often been described in several groups of hyper(thermophilic) Archaea (Forterre et al., 2000; Lemmens et al., 2018; Lulchev & Klostermeier, 2014).
In contrast, <80 oC fumaroles were dominated by genes involved with different energetic and chemolithotrophic pathways: sulfur oxidation, ammonification, denitrification, nitrogen fixation, and dissimilatory nitrite reduction. This suggests a trend for both reductive and oxidative pathways, as well as metabolic versatility and complex biogeochemical processes at the local community level. Although genes related to sulfur and nitrogen pathways were detected in glaciers, the majority of potential pathways for glacier communities were related to carbon metabolism and heterotrophy. This lowest metabolic diversity can be explained by the decrease of marine and volcanic geochemicals (e.g. sulfate) towards glaciers (Supplementary Table 1), making these substrates unavailable for exploiting different energy sources, as occurs in fumaroles. The <80oC fumaroles and glacier communities harbored mechanisms for both heat and cold-shock genes, dormancy and sporulation functions, and DNA repair mechanisms through uvrABC complex,recA , and photolyase. Diverse survival strategies in <80 oC fumaroles and glaciers might be explained by community exposure to fluctuating temperatures and redox conditions that are more variable when compared to the stability of hottest fumarole, which maintains the hyperthermophilic temperatures and hydrogen sulfide emissions for long periods. Further, glacier communities exhibited more genes associated with osmotic stress, which reflects the low liquid water availability due to the predominant freezing conditions of the Antarctic ecosystems (Wei et al., 2016).
Although several studies have shown a quantitative decrease in microbial diversity as temperature increases in both geothermal and hydrothermal ecosystems (Cole et al., 2013; Sharp et al., 2014), little is known about how temperature affects ecosystem functioning due to inhibition of key metabolic enzymes or pathways (Hedlund et al., 2015). Despite the limitation of metagenomics in revealing the truly active microbial metabolic pathways, our results increase understanding of the potential temperature limits on different microbial metabolism at the community level and encourage more studies to elucidate the direct effect of temperature extremes on specific biogeochemical processes in Antarctic volcanic ecosystems.
The 159 MAGs recovered from the Deception Island volcano comprised a broad phylogenetic range of archaeal and bacterial phyla. The 11 MAGs selected for annotation included hyperthermophilic and thermophilic lineages, as well as lineages containing homologs of different predicted sulfur and nitrogen pathways, and archaeal groups underrepresented in genome data, such as Ca. Nitrosocaldus and Nanoarchaeota/Woesearchaeia. Since Ca. Nitrosocaldus was previously reported only in terrestrial geothermal environments (Abby et al., 2018; Daebeler et al., 2018; Torre et al., 2008), their presence on Deception fumaroles represents a novel outcome for the ecological distribution of thermophilic ammonia-oxidizing Archaea and encourages further investigation to better understand their role in marine volcanic ecosystems. Furthermore, the majority of our selected MAGs are equipped with gene-encoding proteins that protect cells against several stressful conditions, including cold and heat-shock, carbon starvation, oxidative and periplasmic stress, and DNA damage, likely enabling survival and adaptation of these microorganisms to a broad combination of extreme parameters. One of our MAGs was closely related to archaeon GW2011_AR20, which is an uncultivated and underrepresented Nanoarchaeota/Woesearchaeia member described previously in aquifer samples and appears to have a symbiotic or pathogenic lifestyle due to the small genome size and lack of some biosynthesis pathways (Castelle et al., 2015). The genome analysis of our Woesearchaeia MAG (archaeon GW2011_AR20, DI_MAG_00022) suggests a novel putative thermophilic lifestyle or at least a potential heat tolerance for this lineage due to the (i) lack of cold-shock genes, (these genes are mostly absent in the genomes of thermophilic archaea, while usually present in psychrophilic/mesophilic archaeal members (Cavicchioli, 2006; Giaquinto et al., 2007), and (ii) the presence of reverse gyrase, thermosome and other heat-shock genes (e.g. groES ) that are essentially related to (hyper)thermophiles and heat response. Although these heat-shock genes were also detected in some mesophilic archaeal lineages within Halobacteria, Thaumarchaeota, and Methanosarcina spp. (Lemmens et al., 2018), reverse gyrase is the only protein found ubiquitously in hyperthermophilic organisms, but absent in mesophiles (Catchpole & Forterre, 2019), pointing to this Woesearchaeia member as a likely thermophile or hyperthermophile.