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Citation: Oliveira, M.F.; Figueredo,
C.C.; Maciel-Silva, A.S. Diversity and
Key Organisms in the Biocrust of a
Tropical Granite-Gneiss Rocky
Outcrop. Life 2025, 15, 759. https://
doi.org/10.3390/life15050759
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Article
Diversity and Key Organisms in the Biocrust of a Tropical
Granite-Gneiss Rocky Outcrop
Mateus Fernandes Oliveira 1,* , Cleber Cunha Figueredo 1,2 and Adaíses Simone Maciel-Silva 1,2
1 Programa de Pós-Graduação em Biologia Vegetal, Instituto de Ciências Biológicas, Universidade Federal de
Minas Gerais, Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte 31270-901, MG, Brazil;
cleberfigueredo@ufmg.br (C.C.F.); adaisesmaciel@ufmg.br (A.S.M.-S.)
2 Departamento de Botânica, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais,
Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte 31270-901, MG, Brazil
* Correspondence: deoliveira.mateusfernandes@gmail.com
Abstract: Rocky outcrops are harsh habitats that support specialized organisms and com-
munities, including biocrusts, which play roles in soil stabilization, water retention, and
nutrient cycling. Despite their importance, tropical biocrusts, particularly in granite-gneiss
formations, remain underexplored. This study examines biocrust composition in a granite-
gneiss outcrop in a rural landscape in Southeastern Brazil, identifying microhabitats and
analyzing co-occurrence patterns and community structure. We recorded eleven bryophyte
species and one diatom species, while six cyanobacteria, three charophytes, and two chloro-
phytes were identified at the genus level. They were found in shallow depressions, though
termite mounds also served as an important microhabitat. The cyanobacterium Scytonema
was the most prevalent taxon. The liverwort Riccia weinionis had the highest number of
positive co-occurrences, associating with cyanobacteria and algae. Network analysis based
on co-occurrence revealed that Scytonema and the mosses Anomobryum conicum and Bryum
argenteum were the most connected taxa, crucial for ecological network stability. The moss
Bryum atenense acted as a key intermediary, with the highest betweenness centrality—a
measure of its role in linking taxa. These findings provide insights into tropical rocky
outcrop biocrusts, shedding light on their composition and interactions. Furthermore,
the co-occurrence patterns and key taxa connectivity uncovered provide insights into
ecosystem stability and can guide ecological restoration strategies.
Keywords: biological soil crusts; photosynthetic communities; rocky ecosystem; tropical diversity
1. Introduction
Rocky outcrops are isolated geological formations shaped over millions of years by
erosion [1], wherein softer materials are gradually removed, leaving behind the more
resistant parent rock [2]. These formations occur on all continents and take various forms
depending on the type of bedrock, including sandstone escarpments, granite inselbergs,
limestone cliffs, and gneissic tors [3–5]. In Brazil, the most common types of outcrops are
granitegneiss, quartzite–sandstone, limestone, and ironstone [6], with granite formations
being the most abundant [7]. These unique formations create specialized habitats that
support a diverse array of endemic and highly adapted plant species [8–10], despite
challenging environmental conditions such as intense UV radiation, high daily temperature
fluctuations, low water retention, and nutrient-poor soils [9,11–13].
Although rocky outcrops are primarily known for their high diversity of vascular
plants and adaptations to harsh environments [8–10,12,14–17], they also serve as essential
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Life 2025, 15, 759 2 of 18
habitats for biocrusts [7,18–20]. These communities, composed of both heterotrophic and
autotrophic organisms, colonize the uppermost soil layer, particularly in arid and semi-arid
environments [21,22]. Biocrust-forming organisms include bacteria, cyanobacteria, algae
(chlorophytes, diatoms, and charophytes), bryophytes (mosses and liverworts), free-living
fungi, and lichens [21,22]. Additionally, they harbor a rich diversity of microfauna, such as
bacterivorous and predatory protozoans, nematodes, tardigrades, rotifers, mollusks, and
arthropods, particularly mites and collembolans [23].
Similar to vascular plants from rocky outcrops, the components of biocrusts have
developed a range of strategies to cope with extreme environmental conditions [24].
For instance, many cyanobacteria produce protective sheaths containing UV-absorbing
compounds [25], while lichens synthesize pigments such as zeaxanthin to mitigate light
stress [26]. Bryophytes, particularly mosses, develop specialized leaf structures such as
hyaline hairpoints, lamellae, papillae, and alar cells to enhance water retention, while
liverworts feature thickened cell walls, wall ornamentations, and dense rhizoidal mats
to increase water uptake and escape dry seasons by shortening their life cycles [27–30].
Furthermore, many cyanobacteria, algae, lichens, and bryophytes show high desiccation
tolerance and can survive being almost completely dried out for long periods [31–33].
In addition to their remarkable adaptations, biocrusts perform crucial ecological func-
tions, including carbon [34] and nitrogen fixation [35,36], soil stabilization [37], and water
retention [38], making them key components of outcrop ecosystems. However, most
research on their biodiversity and functional roles has focused on arid and semi-arid re-
gions [21], with tropical ecosystems remaining comparatively understudied. This historical
bias may explain why, despite studies on biocrusts in rocky outcrops worldwide—such
as in granite formations in Antarctica [18] and limestone outcrops in China [19,20]—their
diversity and distribution in tropical rocky outcrops, particularly in Brazil, are still poorly
understood [39]. In Brazil, attention has largely been paid to quartzite–sandstone, limestone,
and ironstone formations [7,40], while granite-gneiss outcrops remain underexplored.
Granite-gneiss outcrops are characterized by harsh environmental conditions and
plant communities adapted to a mosaic of microhabitats [41]. These formations often
feature shallow depressions with thin layers of substrate that support specialized vege-
tation, creating unique ecological niches [41] that may be highly favorable for biocrust
establishment. Biocrusts, which thrive in water-limited environments with minimal vas-
cular plant competition [20], are commonly found colonizing exposed soil surfaces across
various types of rocky outcrops [7]. Therefore, granite-gneiss outcrops represent promising
microhabitats for biocrust development and diversity yet remain critically understudied in
tropical regions.
The biodiversity of Brazilian granite-gneiss outcrops is increasingly threatened by
human activities, including quarrying, mining, grazing, goat herding, fires, biologi-
cal invasions, urban expansion, and the unsustainable extraction of ornamental plant
species [42–44]. These outcrops are often located in regions where 75% of the Brazilian
population resides [45], particularly in areas where urban development and agricultural
activities are rapidly expanding overnatural cover [46,47]. As a result, conflicts between
conservation efforts and human activities are becoming increasingly pronounced. More-
over, the lack of protected areas for granite-gneiss outcrops [7] highlights the urgent need
for comprehensive inventories of these formations, particularly those located on rural
landscapes. Surveys of species are essential, not only for advancing ecological, genetic, bio-
geographic, and evolutionary research but also for informing policies aimed at conserving
Brazil’s rocky outcrop ecosystems [48].
Despite the ecological importance of biocrusts, our understanding of their diversity,
structures, and interactions in tropical granite-gneiss outcrops remains limited. Within this
Life 2025, 15, 759 3 of 18
contextual framework, we conducted a study on biocrusts in a granite-gneiss rocky out-
crop situated in a rural landscape in Southeastern Brazil. To enhance our understanding of
the biocrust communities associated with these formations, we aimed to (I) determine the
microhabitats where biocrusts develop at the study site; (II) assess the taxonomic diversity
of biocrust components, focusing on bryophytes, cyanobacteria, and algae; (III) analyze co-
occurrence patterns among taxa to infer potential ecological interactions; and (IV) investigate
the structure and dynamics of species interactions within the biocrust community through
network analysis, identifying key species that play central roles in shaping ecological rela-
tionships. By focusing on understudied tropical biocrusts and integrating ecological network
analysis, we seek to fill key gaps in research on tropical biocrust biodiversity and function.
2. Materials and Methods
2.1. Study Area
Our research was conducted on a granite-gneiss rocky outcrop located on a private
property named Fazenda Pedra Grande in the Bocaina district, municipality of Cláudio,
Minas Gerais, Brazil (20◦20′25′′ S, 44◦52′02′′ W). The outcrop is bordered by two distinct
landscapes: on one side, a steep cliff faces extensive rural landscapes predominantly used
for maize and soybean cultivation, as well as grazing land for livestock (Figure 1A), while
the other side is surrounded by a semi-deciduous forest (Figure 1B). Mean annual minimum
and maximum temperatures are 22.0 ◦C and 23.5 ◦C, respectively, and the average annual
rainfall is 1411.6 mm (based on the most recent data provided by A564 (portal.inmet.gov.br)).
During the rainy season, several temporary ponds can be found on the top of the outcrop
(Figure 1C). The study site is located at the ecotone between the Atlantic Forest and the
Cerrado, comprising a rocky outcrop with an area of approximately 170,000 m2. There
is a notable abundance of biocrust components within the vegetation at the studied site
(Figure 1D–F).
Figure 1. Granite-gneiss outcrop within a rural landscape in Southeastern Brazil and the associated
biocrusts: (A) top of the outcrop, with a view of the surrounding pasture; (B) semi-deciduous forest
present on the outcrop; (C) temporary ponds on the summit of the outcrop; (D) biocrusts dominated
by cyanobacteria; (E) biocrusts with various species of mosses; and (F) biocrust dominated by a single
moss species.
portal.inmet.gov.br
Life 2025, 15, 759 4 of 18
2.2. Biocrust Survey
Sampling of biocrusts was carried out on 31 December 2024, using the random walk
method, which involves unrestricted movement along trails [49]. Thus, we walked along
the trails of the outcrop searching for morphologically distinct samples, aiming to collect
the greatest variety of species possible. Samples, each measuring approximately 100 cm2
and spaced at least 1 m apart, were collected from the topsoil by gently inserting a spatula
into the substrate. The samples typically formed a cohesive structure, with the surface
layer consisting of cryptogamic organisms, while the underlying soil was aggregated and
retained beneath this upper layer (Figure 2A–C). The samples were subsequently placed in
small paper bags, and the microhabitats from which they were collected were recorded.
Figure 2. Biocrusts from the studied granite-gneiss rocky outcrop and the microhabitats where they
can be found: (A) close-up of a biocrust dominated by cyanobacteria; (B) biocrust containing various
components; (C) moss-dominated biocrust, incorporating the underlying soil; (D) shallow depression
microhabitat; (E) biocrusts growing over a shallow depression; and (F) termite mounds atop shallow
depressions.
We sampled only two microhabitats hosting biocrusts: shallow depressions (Figure 2D,E)
and termite mounds (Figure 2F). These microhabitats were selected based on a study conducted
on granite-gneiss formations in Southeastern Brazil [41] and another study on different outcrop
formations with documented microhabitats supporting biocrusts [7]. Shallow crevices, ranging
from 0.8 to 5 m in radius, were the predominant microhabitat, with biocrusts mainly developing
along their edges (Figure 2E). Termite mounds, though located within crevices, were considered
a distinct microhabitat, according to the classification used in previous studies [7]. They were
relatively scarce, did not exceed 60 cm in height, and had low biocrust diversity.
In the laboratory, a decision tree approach [22] was applied to confirm that the collected
samples represented biocrusts. Each sample was first examined under a stereomicroscope
and then under a light microscope until all visible components were documented, with a
focus on identifying bryophytes, cyanobacteria, and algae. In samples with cyanobacterial
mats, a portion of the material was carefully removed using tweezers, and a slide was
prepared for microscopic observation. For samples containing bryophytes, a slide was
prepared to examine plant characteristics, while epiphytic algae and cyanobacteria were
also identified. Additionally, for each sample, soil aliquots (5 mL each) from the immediate
surface under the biocrust layer were diluted in 10 mL of deionized water. Using a pipette,
Life 2025, 15, 759 5 of 18
three drops of the suspension were placed on a slide and carefully examined for the
presence of algae and cyanobacteria.
All bryophytes, including mosses and liverworts, were identified up to the species level
using appropriate taxonomic references [50–54]. Cyanobacteria and algae were identified
up to the genus level, using specialized sources [55–59]. We opted for this less detailed
identification due to the limited literature available on identifying algae and cyanobacteria
in biocrusts, particularly in tropical regions [59]. However, each genus identified in the
samples appeared to correspond to a single species of these microorganisms. Voucher
specimens were deposited in the BHCB herbarium at the Instituto de Ciências Biológicas,
Universidade Federal de Minas Gerais (voucher numbers 223811 to 223848).
2.3. Statistical Analysis
We analyzed taxon co-occurrence patterns in biocrusts using Veech’s method [60],
implemented through the ‘cooccur’ package [61] in R 4.0.2 software [62]. To accomplish this,
we constructed a species coincidence matrix based on the presence or absence of taxa in
each biocrust sample from the studied rocky outcrop. This probabilistic approach excluded
from the analysis any co-occurrences expected to be less than 1, thereby filtering out
species pairs with a probability of sharing fewer than one sample. The probabilistic model
developed by Veech [60] applies combinatorial methods to assess whether the observed
frequency of co-occurrence is significantly greater than expected (positive co-occurrence),
significantly lower than expected (negative co-occurrence), or not significantly different
from expectations (random co-occurrence). Following this filtering process, only 82 out of
253 possible species pair combinations were analyzed.
In ecosystems, species form intricate interaction networks that shape community dy-
namics and ecological processes [63]. These networks, where species act as nodes connected
by their relationships,define ecological interactions and drive ecosystem functions [64].
While microbial community networks in biocrusts have been widely studied [65–68], other
biocrust components, such as bryophytes, remain underexplored in terms of their roles
within these communities. Thus, to better understand the structure and dynamics of taxon
interactions within the studied biocrust community, a biological co-occurrence network
was constructed using the ‘igraph’ package [69] in R 4.0.2 [62]. This analysis focused on key
parameters such as taxon connectivity, betweenness centrality, network density, community
structure, community detection, and modularity.
Taxon connectivity was evaluated through degree (degree function), which measures
the number of direct connections (edges) each taxon (node) has within the network [69].
Their taxa’s role as intermediaries was assessed using betweenness centrality (betweenness
function), which quantifies how often a taxon acts as a bridge between other taxa in a
network [69]. To evaluate overall network connectivity, network density (edge_density
function) was calculated, representing the proportion of observed connections relative to
the total possible connections in the network [69]. Community structure was analyzed
using modularity (modularity function), which detects groups of taxa that co-occur more
frequently with each other than with the rest of the network [69]. Community detection
(clusters function) was performed to determine the number of taxon groups (or modules)
containing more than two taxa, providing insight into how taxa are organized within
the network [69]. Network modularity was assessed through a permutation test, where
communities were randomly shuffled 1000 times, and modularity was calculated for each
permutation. The p-value was determined based on the proportion of permutations where
modularity was greater than or equal to the observed modularity [69].
Life 2025, 15, 759 6 of 18
3. Results
3.1. Biocrust Diversity in Granite-Gneiss Microhabitats
We collected 38 biocrust samples, each of which often contained multiple organisms.
In total, we analyzed 168 specimens, representing six phyla (Cyanobacteria, Chlorophyta,
Charophyta, Bacillariophyta, Bryophyta, and Marchantiophyta), 17 families, and 20 genera
(Table 1). Cyanobacteria was the most frequently recorded phylum, with 70 occurrences,
followed by Bryophyta, with 61 occurrences. Bacillariophyta was the least common, with
only four records. In terms of taxonomic diversity, Bryophyta exhibited the greatest richness,
comprising nine moss species distributed across four families, six genera, and nine species
(Table 1).
Table 1. Taxa recorded in biocrusts from the studied granite-gneiss outcrop and their specimen counts
(frequency).
Phylum Family Taxa Freq.
Cyanobacteria Cyanothecaceae Cyanothece Komárek 05
Microcystaceae Gloeothece Nägeli 03
Microcoleaceae Microcoleus Desmazières ex Gomont 11
Nostocaceae Nostoc Vaucher ex Bornet &Flahault 11
Scytonemataceae Scytonema C.Agardh ex Bornet
&Flahault 34
Stigonemataceae Stigonema C.Agardh ex Bornet &Flahault 06
Total 70
Chlorophyta Chlorococcaceae Chlorococcum Meneghini 07
Radiococcaceae Gloeocystis Nägeli 04
Total 11
Charophyta Desmidiaceae Actinotaenium (Nägeli) Teiling 04
Euastrum Ehrenberg ex Ralfs 01
Zygnemataceae Zygogonium Kützing 06
Total 11
Bacillariophyta Pinnulariaceae Pinnularia borealis Ehrenberg 04
Total 04
Bryophyta Bryaceae Anomobryum conicum (Hornsch.) Broth. 14
Bryum arachnoideum Müll. Hal. 03
Bryum argenteum Hedw. 17
Bryum atenense Williams 15
Bryum orthodontioides Müll.Hal. 01
Dicranaceae Campylopus lamellatus Mont. 07
Dicranella lindigiana (Hampe) Mit. 02
Fabroniaceae Fabronia ciliaris (Brid.) Brid. 01
Fissidentaceae Fissidens weirii Mitt. 01
Total 61
Marchantiophyta Cephaloziellaceae Cylindrocolea rhizantha (Mont.)
R.M.Schust. 01
Ricciaceae Riccia weinionis Steph. 08
Total 09
The mosses Bryum argenteum Hedw. (Figure 3A) and Bryum atenense Williams
(Figure 3B) were the most abundant among bryophytes. In contrast, some moss species
were recorded only once, such as Fabronia ciliaris (Brid.) Brid. The thallose liverwort
Riccia weinionis Steph. (Figure 3C) was also highly abundant in the biocrusts in the study
area. Among all the organisms identified, the most frequently occurring was the cyanobac-
terium Scytonema C. Agardh ex Bornet & Flahault (Figure 3D), recorded in 34 samples.
Other notable cyanobacteria included Microcoleus Desmazières ex Gomont (Figure 3E),
Nostoc Vaucher ex Bornet & Flahault (Figure 3F), Stigonema C. Agardh ex Bornet & Flahault
Life 2025, 15, 759 7 of 18
(Figure 3G), and Cyanothece Komárek (Figure 3H). Both Chlorophyta and Charophyta had
the same frequency in the studied biocrusts, but Chlorococcum Meneghini was the most
frequent green algae. Charophyta included both filamentous forms, such as Zygogonium
Kützing (Figure 3I), and unicellular representatives like Actinotaenium (Nägeli) Teiling
(Figure 3J) and Euastrum Ehrenberg ex Ralfs (Figure 3K). The only recorded diatom was
Pinnularia borealis Ehrenberg (Figure 3L).
Figure 3. Some representatives of mosses, liverworts, cyanobacteria, and algae identified within
the biocrusts of the studied granite-gneiss outcrop: (A) the silver moss Bryum argenteum; (B) Bryum
atenense on a termite mound with abundant sporophytes; (C) Riccia weinionis, a complex thallose
liverwort; (D) Scytonema sp., a nitrogen-fixing filamentous cyanobacterium; (E) Microcoleus sp., the most
common cyanobacterium in biocrusts; (F) Nostoc sp., forming mucilaginous colonies; (G) Stigonema sp.,
a pseudobranching cyanobacterium; (H) the filamentous algae Zygogonium sp.; (I) Actinotaenium sp.,
a basal desmid; (J) Euastrum sp., with distinct semi-cell symmetry; (K) Cyanothece sp., a unicellular
cyanobacterium; and (L) girdle view of P. borealis. The scale bars correspond to 50 µm.
In the studied granite-gneiss outcrop, shallow crevices were more prevalent than
termite mounds, with only four samples collected from the latter microhabitat. All these
samples contained the moss B. atenense, which co-occurred with various taxa, including
Life 2025, 15, 759 8 of 18
Stigonema sp., Chlorococcum sp. + Scytonema sp., Chlorococcum sp., and F. ciliaris + Scytonema
sp. Notably, F. ciliaris was the only species exclusive to termite mounds. Liverworts,
charophytes, diatoms, and several taxa of moss, chlorophytes, and cyanobacteria were
absent from this microhabitat.
3.2. Co-Occurrence Interactions
Out of the 82 pairs analyzed, the majority (73) co-occurred as expected based on the
random null model, while eight pairs co-occurred more frequently than expected, and
one pair co-occurred less frequently than expected (Figure 4A, Table 2). Interestingly,
R. weinionis was included in the greatest number of significant co-occurring pairs, with
positive associations with several cyanobacterial taxa, the most frequent being its co-
occurrence with Nostoc sp. (Table 2). Indeed, in most of the samples containing R. weinionis,
we observed several colonies of Nostoc sp. growing on the soil surrounding it (Figure 4B).
The moss B. argenteum was another bryophyte showing a positive co-occurrence with Nostoc
sp. The only negative co-occurrence relationship was observed between two bryophyte
species, the moss B. atenense and the liverwort R. weinionis.
Figure 4. Co-occurrence patterns among the taxa from the sampled biocrusts: (A) observed patterns,
including positive, negative, and random relationships; (B) Riccia weinionis (red arrow) and Nostoc
sp. (green arrow) in a sample, with the cyanobacterial colonies growing on the soil surrounding the
liverwort. The scale bar corresponds to 0.5 mm.
Table 2. Co-occurrence tests between biocrust taxa. The tests were conducted using the p_lt and p_gt
values, which represent the probability of observing a co-occurrence smaller or larger than expected
by chance. The p_lt values below 0.05 indicate a significantly smaller co-occurrence, while the p_gt
values below0.05 indicate a significantly larger co-occurrence, than expected by chance. Significant
values are highlighted in bold.
Taxa Pair p_lt p_gt
Riccia weinionis and Cyanothece 0.999 0.004
Riccia weinionis and Microcoleus 0.997 0.031
Riccia weinionis and Nostoc 1.000other biocrust
organisms [89,90]. Their presence across multiple outcrop types—granite-gneiss, quartzite–
sandstone, ironstone, and limestone [7]—highlights their role in soil stabilization and
nutrient cycling. In contrast, unicellular algae and cyanobacteria are rarely documented in
Brazilian biocrusts [39].
Microhabitats within the granite-gneiss outcrop share similarities with those in
quartzite–sandstone and ironstone formations while also being notably distinct from those
in limestone formations [7]. In fact, neither shallow depressions nor termite mounds
are found in limestone formations [7]. In contrast, both quartzite–sandstone and iron-
stone outcrops present soil islands, which closely resemble shallow depressions. These
soil islands are subject to intense sunlight and harsh environmental conditions, making
them areas where moss species like C. lamellatus, B. atenense, and B. argenteum are fre-
quently found [7,91,92]. These same species were found in shallow depressions within
the granite-gneiss outcrop, showcasing their ability to thrive in stressful environments.
Notable adaptations include the hyaline hairpoint and leaf lamellae in C. lamellatus and the
tightly overlapping leaves of B. argenteum, which aid in water absorption and retention and
minimizing water loss [24,28,93]. Additionally, the most frequently observed moss species
on termite mounds in these formations is B. atenense [7], as we observed at the studied site.
4.2. Insights into Biocrust Components’ Co-Occurrence
The positive association observed between bryophytes and cyanobacteria in the
granite-gneiss outcrop suggests potential beneficial interactions. Many bryophytes, includ-
ing mosses, hornworts, and liverworts, form symbiotic relationships with nitrogen-fixing
cyanobacteria [94–96]. The moss–cyanobacteria association occurs where nitrogen is scarce,
with N2-fixing cyanobacteria such as Nostoc growing epiphytically and providing nitrogen
in exchange for carbon and sulfur [97,98]. The positive co-occurrence between B. argenteum
moss and Nostoc may reflect a similar beneficial relationship, as other Bryum species show
similar interactions [99]. In contrast, only four liverwort genera establish symbioses with
cyanobacteria [95], and the Riccia genus found in the studied biocrusts does not appear
to form such associations. However, we observed Nostoc growing around R. weinionis,
which may indicate a potential interaction between these two taxa. This observation war-
Life 2025, 15, 759 12 of 18
rants further investigation to determine whether it represents a previously unreported
symbiotic relationship.
All the positive co-occurrence patterns observed between pairs of cyanobacteria or
between cyanobacteria and algae in the granite-gneiss outcrop biocrusts involved Nostoc.
When present in biocrusts, Nostoc can retain moisture [100,101] and increase nitrogen
levels in the crust [35,102], potentially influencing community composition. We suspect
that these effects drive the co-occurrence of Cyanothece with Nostoc. Since Cyanothece is
predominantly found in aquatic environments and is a nitrogen fixer [103], its presence in
biocrusts could be facilitated by the microclimatic buffering provided by Nostoc rather than
by nitrogen availability alone. The positive pattern between Microcoleus and Nostoc may be
explained by their roles in the early stages of biocrust formation, where they contribute
to the initial soil carbon and nitrogen inputs [35,89]. Similarly, the relationship between
Zygogonium, an alga that initiates biocrusts [89,104], and Nostoc may reflect a similar early
colonization stage.
Finally, the only negative co-occurrence pattern observed was between two bryophytes.
Liverworts, known for producing specialized metabolites in oil bodies [105,106], play a
role in allelopathy, inhibiting the growth of neighboring organisms [107]. This ecological
interaction allows one species to release chemical compounds that affect (negatively or
sometimes positively) the growth or survival of others, securing more resources [108,109].
The presence of allelopathic compounds in R. weinionis may explain the negative pattern
observed with B. atenense. However, bryophyte species coexistence does not appear to
be related to inter-specific competition [110], suggesting that other factors, such as differ-
ences in environmental preferences and niche requirements, could also influence species
distribution and co-occurrence [111]. Future studies should further investigate these two
possibilities—whether chemical interactions, like allelopathy, or ecological niche differenti-
ation better explain the observed pattern, considering the various environmental variables
that could be shaping this dynamic.
4.3. Lessons from Community Network Dynamics
In the biocrusts of the granite-gneiss outcrop, cyanobacteria were the most prevalent
group; however, mosses such as B. atenense, A. conicum, and C. lamellatus played a pivotal
role in shaping the community structure. Mosses contribute to the surface roughness
of biocrusts, providing an ideal substrate for the establishment of algae and cyanobacte-
ria [112], and there are numerous records of these organisms growing on the leaves of
mosses within biocrusts [7]—a phenomenon also observed in this study. Furthermore,
certain algae, like Chlorococcum, are commonly associated with bryophytes due to their
ability to retain water [89]. Despite these valuable insights, further research is necessary to
fully comprehend the complex interactions within biocrust communities, particularly the
role of bryophytes in influencing ecosystem functions and dynamics.
Regarding network modularity, a value of zero was observed, indicating the absence
of a clear subcommunity structure. This suggests thatrather than forming distinct clusters,
species interactions in the studied ecosystem may be randomly distributed [69,113]. The
permutation test p-value of 1 further supports this, showing no significant difference from
a random network. The lack of modularity points to random species interactions [113],
emphasizing the need for further exploration of these ecological dynamics in biocrusts
within tropical granite-gneiss outcrops.
Network analyses revealed key species within the community while also identifying
critical species that could aid in the restoration of granite outcrops, which face significant
anthropogenic threats [42–44]. Biocrusts perform essential ecosystem functions [21], and
their inoculation in various contexts, such as areas impacted by mining [114–116] and
Life 2025, 15, 759 13 of 18
desertification [117], has demonstrated environmental benefits. In fact, their application in
restoration programs is wellestablished [118–120], yet it remains largely unexplored in re-
gard to tropical rocky outcrops. Notably, recent studies suggest that inoculating terricolous
mosses in Brazilian quartzite–sandstone outcrops can promote biocrust formation and sup-
port vegetation succession around rocks within a few years [40]. However, fundamental
steps, such as selecting suitable species for these restoration practices, have yet to be taken.
From this perspective, the concept of interaction networks becomes particularly valu-
able. In our analysis, species such as B. atenense and C. lamellatus emerged as central
nodes, exhibiting high connectivity within the network. These species are known to
possess life-history traits associated with rapid reproduction and resilience in harsh envi-
ronments [50,54], suggesting that they may have potential for use in future restoration trials.
Similarly, cyanobacteria such as Nostoc and Scytonema showed strong associations with
other taxa, likely due to their capacity for nitrogen fixation and soil stabilization [86–90].
While our results do not directly test restoration outcomes, they point to a promising
avenue for future research: exploring whether the introduction of these highly connected
species could enhance ecological benefits, promote the recruitment of otherorganisms, and
contribute to greater biodiversity and ecosystem stability in biocrust restoration efforts.
5. Conclusions
Although this study investigates biocrust diversity at a single site and is based on a
limited number of samples, it contributes valuable insights into the diversity of biocrusts
on tropical rocky outcrops. It provides new species records and highlights the importance
of studying community dynamics in these unique ecosystems. For the first time, we have
identified diatoms within Brazilian biocrusts and highlighted the presence of bryophytes,
which are often overlooked. The observed positive co-occurrence between bryophytes and
cyanobacteria suggests beneficial ecological interactions, with mosses acquiring nitrogen
and cyanobacteria benefiting from carbon and sulfur. On the other hand, the negative
co-occurrence between liverworts and mosses raises the possibility of allelopathic effects
or ecological niche differentiation. The identification of central species within biocrust
networks enhances our understanding of community structure and dynamics, offering
valuable insights forrestoration strategies. Incorporating these key species into ecological
restoration projects can improve soil aggregation, enhance carbon and nitrogen fixation,
and foster greater biodiversity. We also acknowledge that the limited sampling efforts
and lack of spatial replication may affect the interpretation of ecological patterns. There-
fore, we emphasize the importance of conducting further studies on other granite-gneiss
outcrops using a larger number of samples to strengthen and expand upon the findings
presented here.
Author Contributions: Conceptualization, M.F.O.; methodology, M.F.O., C.C.F. and A.S.M.-S.; valida-
tion, M.F.O. and A.S.M.-S.; formal analysis, M.F.O.; investigation, M.F.O., C.C.F. and A.S.M.-S.; data
curation, M.F.O.; writing—original draft preparation, M.F.O.; writing—review and editing, C.C.F.
and A.S.M.-S.; visualization, M.F.O., C.C.F. and A.S.M.-S.; supervision, A.S.M.-S. All authors have
read and agreed to the published version of the manuscript.
Funding: Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, 8887.876207/2023-00).
Data Availability Statement: All data generated or analyzed during this study are included in this article.
Acknowledgments: We sincerely thank Maria Marli do Amaral Ferreira for allowing us to collect
samples on her property and Rinaldo Antônio Martins de Oliveira and Naiara Oliveira Paixão for
their crucial field assistance.
Conflicts of Interest: The authors declare no conflicts of interest.
Life 2025, 15, 759 14 of 18
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	Introduction 
	Materials and Methods 
	Study Area 
	Biocrust Survey 
	Statistical Analysis 
	Results 
	Biocrust Diversity in Granite-Gneiss Microhabitats 
	Co-Occurrence Interactions 
	Community Network 
	Discussion 
	Biocrusts of Brazilian Rocky Outcrops: A Comparative Perspective 
	Insights into Biocrust Components’ Co-Occurrence 
	Lessons from Community Network Dynamics 
	Conclusions 
	References