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.