Biological Soil Crusts (BSC): Flaminkvlakte 111


Introduction

Biological Soil Crusts (BSC) occur in all biomes along the BIOTA-South transect except for the Fynbos. They can be classified into different types depending on the developmental time and the organism composition: three developmental stages of cyanobacterial crusts, mostly associated with green algae (ranging from initial to well developed crusts), lichen crusts (differentiated in cyanolichen and green algal lichen crusts), bryophyte and liverworth crusts, and the hypolithic crust type (community of photosynthetic organisms existing on sides and underneath translucent stones, e.g. quartz) occurring specifically in quartz gravel pavements (e.g. Knersvlakte - observatories Flaminkvlakte and Luiperskop, Namib Desert - observatory Wlotzkasbaken) but also underneath single quartz rocks of different sizes in other regions and biomes (e.g. observatory Ovitoto). Driving forces of such spatial variability of BSC types are climatic, topographic, pedogenetic and land use differences (as in observatory pairs such as Narais and Duruchaus) in different biomes along the transect.


 Dominant Crust Type

hypolithic crust (quartz pebbles) and lichen crust on soil

 Biological Soil Crust Biomass (chlorophyll)

high

BSC Biomass ranking (from BSC chlorophyll a+b values [mg/m2] )
very low <= 10
low = 11 to 50
medium = 51 to 100
high = 101 to 200
very high > 200


 Main cyanobacteria genera


 Pseudanabena
 Scytonema
 Tolypothrix
 Chroococcidiopsis
 Microcoleus
 Oscillatoria
 Schizothrix

Total Taxa No.: 11


 Main green algae genera


Apatococcus
Chlorococcum
Deasonia
Klebsormidium
Neospongiococcum
Oocystis
Tetracystis

Total Taxa No.: 7


 Implementation of molecular identification techniques - Cyanobacteria


Fig.A: Preliminary results of sequence data is shown in 1 of 6 most parsimonious trees based on 16S rDNA sequences. Asterisks indicate taxa found along the BIOTA South transect.(Please click on map)

The polyphasic approach (morphology and sequence data) is expected to give reliable results in detection of taxa diversity of cyanobacteria. However preliminary results of the molecular analysis of selected cyanobacteria strains (1 of 6 most parsimonious trees based on 16S rDNA sequences) indicate that most of the investigated genera (/Leptolyngbya, Pseudanabaena, Scytonema, Microcoleus/) are polyphyletic. These results are in concordance with findings e.g. by Casamatta et al. 2005. In a second approach, the more variable intertranscribed spacer (ITS) was amplified, which is expected to provide further information on species level.

Casamatta, D.A, Johansen, J.R., Vis, M.L., Broadwater, S.T., 2005,
Molecular and morphological characterization of ten polar and
near-polar strains within the oscillatoriales (cyanobacteria)
J. Phycol. 41, 421-438.


 Implementation of molecular identification techniques - Green algae

Regarding the algae in BSCs, a large fraction of the algal community appears to be non-culturable, but contributes considerably to the crust biodiversity. Recent studies demonstrate that the true taxonomic affiliations of microscopic desert green algae can only be established reliably with the aid of DNA sequence data, particularly 18S rDNA data. Thus molecuar techniques are required to complement the cultural approach in order to achieve a more reliable assessment of the biodiversity actually present in BSCs. SSU rRNA gene cloning and sequencing is the best established technique for assessing biodiversity from environmental samples with molecular techniques. A more rapid and easy to use technique is DGGE which in combination with cloning/sequences has been found a very useful method to investigate biodiversity of microalgae in and on rocks.
Maximum likelihood phylogenies demonstrating the high phylogenetic diversity of green algae obtained from Biological Crusts of 16 different sampling sites in Southern Africa.
A total of 279 green algal sequences were obtained from cultures (204) and clone banks (75). They are distributed on many independent lineages and clades comprising the green algal classes Chlorophyceae (Fig. B; 206 sequences) and Trebouxiophyceae (Fig. C; 73 sequences). Colour-coding is used to show the various origins of the groups of green algae. The origin of strains/clones is given in boxes with the number of studied strains/clones per locality. So far, in most cases only a rough genus assigment is possible and several yet unidentified lineages/clades may represent new genera and/or species. Green algae from the "cf. Deasonia", "Scenedesmus deserticola" clades and from a still unidentified chlorophyceaen clade are among the most frequently occuring algal species. There is a general trend with carotenoid-accumulating members of the Chlorophyceae being more abundant than non-carotenoid accumulating Trebouxiophyceae.

The phylogenies were obtained from the ARB SSU rDNA sequence database in which partial sequences (350-800 nts in length) from Biological Crust Green Algae were compared with corresponding full-length green algal sequences public databases and unpublished sources using a 50% position conservation filter. Groups of sequences with similarities among each other of 99% and above were collapsed into one triangle. Scale, 10% estimated genetic difference.

Fig.B
Fig. B: green algal sequences for class Chlorophyceae (206 sequences) (Please click on map)

Fig.C
Fig.C: green algal sequences for class Trebouxiophyceae (73 sequences) (Please click on map)


 Related Map(s)
Map2 - Diversity of BSC-biomass along the transect

Please click on map(s)


 Microclimate and weather as ecological regulators of BSCs and their response to selected microclimatic determinants

Since October 2004, meso- and microclimatic parameters are recorded on a bihourly basis in the Knersvlakte close to the observatory Flaminkvlakte (fig. 1, 2). Mesoclimatic parameters measured are: air humidity, air temperature, light (PAR), amount of rain and occurence of dew. To analyse the microclimatic conditions in their relevance for BSC establishment in the hypolithic habitat (under and at sites of quartz pebbles), the microclimatic sensors were installed at sites with and without hypolithic growth. The sensors installed comprise soil temperature and soil humidity sensors at different depth, soil temperature sensors below and in quartz pebbles, under differently sized quartz stones as well as light sensors under cleaned quartz pebbles (biological crust removed).

Selected results: During January 2005 (hottest month of the year) the hypolithic habitat was consistently cooler than at the quartz stone surface (fig. 3) and the temperature moderation effect was shown to vary with stone thickness (fig. 3). On the hottest summer day we observed distinct differences between temperature regimes during day and night. At night, temperature under stones was higher than at stone surface (reduced heat loss); the inverse occurred during daytime (fig. 4). Overall, the stone surface consistently reached the highest daytime temperatures during summer and lowest nighttime temperatures. It was concluded that external temperatures are moderated by quartz stones and thus moderate the hypolithic habitat.

From left to right: (Please click on figure)
Fig. 1: Microclimate station at observatory 26 Flaminkvlakte
Fig. 2: Subset of microclimatic setup with sensors measuring soil temperature, soil moisture and light in the hypolithic habitat
Fig. 3: January temperature differences in the hypolithic habitat (below different sized quartz stones and at stone surface)
Fig. 4: Temperatures at the surface and below thick and thin quartz stones during the warmest summer day


 Functional ecology of BSCs and their influence on nutrient cycling

In the hypolithic habitat of the Knersvlakte we classified quartz stones as colonised or uncolonised along a 300 m transect and found a high abundance of hypolithic growth (almost 70%), where thicker and heavier stones appeared to be preferably colonised (fig. 5).


Fig. 5: Colonised and un-colonised quartz pebbles in relation to their mass, thickness and depth of insertion into the soil
(Please click on figure)


In the hypolithic habitat as in observatories Flaminkvlakte and Luiperskop cyanobacteria species without UV- and light-protecting pigments such as Chroococcidiopsis are favoured and dominant due to reduced light intensities under the stones (less than 30% compared to surface).
The soil photosynthetic biomass (expressed as chlorophyll a content) at the same site in the Knersvlakte reached mean values of about 90 mg/m2, almost reaching values of dense biological crusts on soil in the region. The nitrogen content of the soil below the hypolithic habitat (10 cm depth) was found to be generally low but increases in the hypolithic crust. Analysis of the nitrogen content of Argyroderma delaetii living among the quartz pebbles (fig. 6) revealed the same range in nitrogen content, indicating that BSCs considerably contribute to the nitrogen pool, which might be used by higher plants.


Fig. 6: Argyroderma delaetii growing among quartz pebbles with hypolithic growth in the Knersvlakte
(Please click on figure)