To date, the biological soil crusts (BSCs) of southern Africa are thought to be dominated mainly by cyanobacteria, with the exception of the lichen
fields of the Namib Desert. Because soil microorganisms can physically modify, maintain or create habitat for other organisms – including soil
biota and plants – they have been considered ecosystem engineers. Therefore, the presence of BSCs may be a good indicator of ecosystem
resilience. Although BSCs are found throughout the world, recent work has suggested that the absence of BSCs in the fynbos of South Africa may be as
a result of the inherent acidity of soils. We surveyed one area within the fynbos biome for the presence of BSCs and determined the relative cover
of vegetation and different crust types. We found a widespread presence (up to 80% of surface soil) of BSC communities in fynbos soils. We conclude
that soil acidity may not be a constraining factor in the development of BSCs in fynbos soils and that previous reports on the absence of BSCs in
fynbos soils may have been based on insufficient field observations. We encourage future studies in this region in order to determine the currently
unexplored spatial distribution of soil microbial communities and the taxonomic composition of microorganisms in fynbos soils.
Biological soil crusts (BSCs) are formed by an association of soil mineral particles and microorganisms which live in the top few millimetres of the
soil.1 The presence, distribution and characteristics of BSCs are controlled by the interactions between climate, geology, vegetation
and the disturbance impact of livestock and game.2,3 BSC formation is often initiated by
filamentous cyanobacteria, such as
Microcoleus spp., during episodic events of available moisture with the subsequent entrapment of mineral particles by a matrix of
extracellular polysaccharides.4 If undisturbed, the development of an appropriate substrate by filamentous
cyanobacteria may lead to the
establishment of fungal, lichen and moss populations, characterised by a slower growth rate.5 Crust organisms
have low moisture
requirements and tolerate a wide range of temperatures, which enables them to exist even when moisture deficit limits vascular plant
growth.6 Once crust organisms have colonised gaps, the characteristics of the crust is then influenced by edaphic
factors such as soil
texture and topography.7
BSCs are found in almost every habitat in the world, including hot regions (e.g. Mojave Desert8), cool or
semi-arid drylands (e.g.
Colorado Plateau6), beneath rocks (hypolithic crusts9), continental and oceanic
landscapes of the Arctic to Antarctic and the
Polar desert,10 savanna woodlands,11 sub-humid
regions,12 subalpine and alpine areas13 and sand dunes
(e.g. Kalahari14). Although BSCs have colonised almost all soil types, finer textured soils tend to have higher
BSC cover than
unconsolidated sand and are found in areas with the lowest impact from wind forces, such as concave
A recent study16 described the diversity and distribution patterns of BSCs from the
Namibian–Angolan border down south to the
Cape Peninsula (South Africa), reporting BSCs in six out of the seven different biomes covered along the transect. In the hyper-arid Namib Desert,
BSCs are mostly lichen-dominated,16,17 cover vast areas devoid of vascular plants and take
most of their available moisture from fog.
In the dry savannas of southern Africa, including on Kalahari Sand soils, BSCs are dominated by
et al.16 reported that BSC formation was absent in the fynbos because of the acidity of the soil.
Fynbos vegetation occurs within the Cape Floristic Region (CFR) at the south-western tip of Africa, which is recognised as a global biodiversity
hotspot.19 Although the fynbos includes a range of soil types (e.g. Regosol, Podzol and Arenosol), which are
typically acid to neutral
(ranging from pH 4 to 7) and nutrient-poor,20 the presence of microbial soil communities in these soil types remains poorly understood.
Most studies in the fynbos have focused on soil factors (i.e. soil nutrients) that determine the distribution of
vascular plants21 or
the diversity of microbial communities in soils.1 We report the occurrence of BSCs in the CFR where previously reported as
We surveyed an area within Table Mountain National Park, between October and December 2010. The site selected (34.14340 S, 18.24177 E) was located
at Olifantsbosch in the Cape of Good Hope Nature Reserve portion of the Table Mountain National Park. The annual rainfall is about 650 mm, with
maximum and minimum temperatures of 24 ºC and 9 ºC, respectively. The landscape (60 m to 80 m above sea level) is flat to
moderately sloping (Figure 1a) on light-grey quartzite Table Mountain Sandstone. This site was selected based on the availability of its climatic,
edaphic and botanical information, and because BSC formation was recently reported as absent.16
Different biological soil crust (BSC) formations in fynbos soils. BSCs
were found in (a) flat to moderately undulating slopes at Olifantsbosch in Table
Mountain National Park. Type 2 crusts formed (b) small coppices whilst (c) well-
developed type 3 crusts were found in open spaces. (d) A combination of crust
types 1 and 2 was usually found in plant interspaces (with Dilatris pillansii in
the centre and Serruria villosa in the lower left corner). (e) The presence of
coppices within the vegetation was also recorded.
Four types of BSCs were used in this study from the classification given in Dougill and Thomas23, and were
selected based on crust form
and morphology: unconsolidated, type 1, type 2 and type 3. This classification has previously enabled the study of different developmental stages of
the BSC community within crust types.24 To determine BSC distribution, the percentage of BSC cover was recorded on a 50 m x 50 m
plot (divided into 2500 1 m x 1 m quadrats) and every plant species within the plot was identified. The diversity of vegetation was determined
to correlate the spatial distribution of BSCs with soil nutrient levels and the presence of vegetation (data not shown here). The spatial
arrangement of plant species and BSC was recorded on each 1 m x 1 m quadrat. BSCs were collected with a spade and carefully placed in Petri
dishes between two layers of cotton wool to avoid rupture of the crust. Samples were transported back to the laboratory to determine pH levels and
soil community composition. Soil pH was measured in a 1:2.5 solution of soil : water as suggested
by Anderson and Ingram25.
Identification of soil microorganisms was done by mounting portions of the BSC for microscopic examination as described by Alef and
Nannipieri26 at a magnification of 115 times with an Auto-Montage microscope (Leica MZ16A, Leica Microsystems,
BSCs covered between 5% and 80% of the surface (when considering 1 m x 1 m quadrats). BSCs varied from a weakly consolidated crust with no
obvious surface colouration to a dark well-consolidated surface with microtopography (Figure 1c and 1d), with crust types 1 and 2 as the most
dominant types. Well-established BSCs were common and could be removed in large pieces with algal filaments visible within the sheath. The sheath is
thought to be mostly composed of carbohydrates.14 BSCs were also observed in plant interspaces and were most commonly found away from
walking trails. The formation of small coppices (30 cm to 50 cm in diameter and up to 10 cm high) was commonly observed among wiry vegetation,
composed mostly of crust type 2 (Figure 1b and 1e). These small coppices have, to our knowledge, never been described before. A widespread presence
of lichens on rocks was also observed in the reserve. Algal patches (greening of the surface) were observed on the soil surface at several sites
after summer rainfall showers or periods of increased humidity. Based on light microscopy, BSCs from the observed sites were mainly dominated by
cyanobacteria and algae, but the identification of species was not possible at this magnification.
Soil texture was predominantly characterised by medium sand, with a pH ranging from 4.5 to 5.5. Plant cover ranged between 5% and 100% of the
surface. The vegetation forms part of the Peninsula Sandstone Fynbos, dominated by a wide range of genera of Asteraceae, Ericaceae, Fabaceae,
Proteaceae and Restionaceae. BSCs were commonly found under the protection of small shrubs such as Metalasia and were present despite the
high cover of fynbos vegetation such as Elegia cuspidata.
Soil microbial diversity and relative abundance are generally characterised after isolation of DNA followed by an analysis of the 16S rDNA sequence
by amplification through polymerase chain reactions22; however, this procedure is expensive and time-consuming.
Here, we report only on the presence of BSC formations on soils in the fynbos with low pH.
Among soil properties, soil pH is important for the establishment and diversity of microorganisms.27
Büdel et al.16
suggested that the absence of BSC formation, in particular of filamentous cyanobacteria, in the Fynbos Biome can be attributed to the low soil pH
(below pH 4). The absence of BSCs on such acidic soils would thus be related to the physiology of soil microorganisms. In general, green algae seem
to favour more acidic soils, whereas cyanobacteria are found preferentially on alkaline soils and are considered intolerant to low pH
conditions,28 and lichens seem to grow at pH levels across the gradient.1 However, there are some apparently contradictory
findings on the effect of soil pH on BSC distribution and on crusts dominated by mosses, lichens or cyanobacteria. For example, a positive
correlation between lichens and soil pH was recently found in the Zapotitlán drylands of Mexico,29 but no
correlation was detected
between soil pH and crusts dominated by mosses and lichens in the Mojave Desert.8 Although infrequent, there are also scattered records
of cyanobacteria in acidic environments at pH values just above 4,30 which suggests that cyanobacteria can tolerate
The reported absence of BSCs in the fynbos could also be related to a combination of several biotic and abiotic factors that affect the development
of BSCs. BSCs can be present under all conditions of soil moisture and their cover is generally not reduced during droughts, as microorganisms can
remain dormant throughout long drought periods.3 The spatial and temporal distribution of BSCs can also change with time, for example,
depending on the level of disturbance.3
We conclude that the previously reported16 absence of BSCs within Table Mountain National Park may not have been
related to low soil pH
levels, but rather could have been based on insufficient field observations. BSCs are not always visible and can be difficult to identify in the
field as the diversity of microorganisms in the crust can also induce different crust morphology and colouration at the surface. Given the
widespread presence of BSCs found, we suggest that future studies should include extensive analyses of topsoil parameters and microbial diversity.
We thank SANParks Conservation Board for allowing access to the sites and Suzaan Kritzinger-Klopper for helping in the identification of plant
We declare that we have no financial or personal relationships which may have inappropriately influenced us in writing this article.
D.M.M. was responsible for the experimental and project design. D.M.M. and C.H. wrote the manuscript.
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