K

4.3 ± 0.7

7.5 ± 1.2

1.9 ± 0.3

Ca

1.1 ± 0.7

2.7 ± 1.0

2.8 ± 0.2

Mn

0.2 ± 0.1

0.5 ± 0.1

49.7 ± 4.0

Fe

0.5 ± 0.3

6.3 ± 1.3

15.5 ± 0.9

Others

6.8 ± 0.5

10.0 ± 2.4

3.4 ± 0.3

a Serrania Paru 3 samples, 1-2 analyses each. b Inselbergs Orinoco 8 samples, 4-13 analyses each. c Riverbed Orinoco 1 sample, 10 analyses.

Values are x ± SE for the averages of the individual analyses of the three and eight samples in a and b respectively, and for the ten analyses of the sample in c. One sample ofb had 82% Os.

a Serrania Paru 3 samples, 1-2 analyses each. b Inselbergs Orinoco 8 samples, 4-13 analyses each. c Riverbed Orinoco 1 sample, 10 analyses.

Values are x ± SE for the averages of the individual analyses of the three and eight samples in a and b respectively, and for the ten analyses of the sample in c. One sample ofb had 82% Os.

11.2.1.2 Success on Bare Substratum

The role of cyanobacteria in cryptogamic soil crusts of deserts and other dry habitats has recently received much attention (Lange et al. 1992; Evans and Ehlehringer 1993; Jeffries et al. 1993a,b; Belnap and Lange 2001). Very little is known, however, about the ecophysiology of cyanobacteria on rocks in the tropics, and in view of the enormous distribution of terrestrial cyanobacteria, which cover almost any surface not occupied by other vegetation, this is quite astonishing. The major reasons for the success of cyanobacteria on the bare substratum of rocks appear to be:

• a potential to adapt to high light intensities,

• the ability to fix atmospheric dinitrogen, and

• desiccation tolerance.

High light intensities bring about considerable heating up of the rock surfaces with cyanobacterial crusts. In response various heat shock proteins (hsp) are synthesised in the cyanobacteria (Adhikary 2003). Survival of high light intensities in the extremely sun exposed habitat of the inselbergs is also sustained by the production of effective sunblocking pigments, such as the indol-alkaloid scytonemin (Garcia-Pichel and Castenholz 1991), which occurs in cyanobacteria in light-exposed environments (Budel et al. 1997b; Budel 1999). In exposed epilithic cyanobacteria of Venezuela and French Guiana, scytonemin, with its absorption maximum at 380 nm, is found in such high concentrations that irradiance up to 500 nm is reduced inside the cells. Furthermore, intracellular carotenoids such as zeaxanthin (Demmig-Adams et al. 1990) and canthaxanthin (Albrecht et al. 2001; Lakatos et al. 2001) may prevent photodamage. The formation of these pigments is slow and takes place over days adapting the cyanobacterial cells to high irradiance.

Thus, the responses of photosynthetis of cyanobacteria to high light intensities depend greatly on the irradiance experienced during growth. Cells grown at 50|mol photonsm-2s-1 (at X = 400 — 700 nm) or below are already photoin-hibited at 250|molm—2s—1 and strongly affected at still higher light intensities (Samuelsson et al. 1985; Luttge et al. 1995). However, inselberg rocks usually receive full sunlight unless clouds and rain quench exposure. Figure 11.12 shows that in an inselberg sample from Ivory Coast, a transfer from 480 to 1,200 |mol photons m—2s—1 did not affect fluorescence yield and photochemical fluorescence quenching, indicating unimpaired photochemical and carbon-assimilatory activity,

Fig. 11.12 Optimal quantum yield of photosystem II (Fv/Fm), effective quantum yield (a F/F^) and photochemical quenching (qp) of cyanobacterial crusts of an inselberg near Seguela, Ivory Coast (07° 42r N, 06° 43r W) in drying (arrow drying above the graphs) and rewetting (arrow H2O above the graphs) cycles and during transfers between lower and higher light intensities (arrows with numbers above the graphs giving light intensities at X = 400 — 700 nm in | mol photons m—2 s—1). Dark and white bars above the graphs indicate dark and light periods respectively (Luttge et al. 1995)

Fig. 11.12 Optimal quantum yield of photosystem II (Fv/Fm), effective quantum yield (a F/F^) and photochemical quenching (qp) of cyanobacterial crusts of an inselberg near Seguela, Ivory Coast (07° 42r N, 06° 43r W) in drying (arrow drying above the graphs) and rewetting (arrow H2O above the graphs) cycles and during transfers between lower and higher light intensities (arrows with numbers above the graphs giving light intensities at X = 400 — 700 nm in | mol photons m—2 s—1). Dark and white bars above the graphs indicate dark and light periods respectively (Luttge et al. 1995)

while a transfer from 240 to 1,280 | mol photons m-2s-1 resulted in a slight and rapidly reversible inhibition. (The drying and wetting cycles shown in Fig. 11.12 will be discussed in Sect. 11.4.) As in higher plants (Schreiber and Bilger 1993), potential quantum yield of photosystem II after dark adaptation (Fv/Fm), effective quantum yield (AF/F^) and photochemical fluorescence quenching (qp) (see Sect. 4.1.7, Box 4.6) decrease with increasing light intensity in cyanobacterial crusts (Fig. 11.13). (In higher plants Fv/Fm is close to 0.8 in non-photoinhibited samples (Sect. 4.1.7) and we note that in comparison in the cyanobacteria in Figs. 11.12 and 11.13 Fv/Fm is rather low even in the early morning. This is an intrinsic property

Fig. 11.13 Potential quantum yield of photosystem II after dark adaptation ( Fv / Fm), effective quantum yield (a F/Fm), photochemical quenching (qp) and relative photosynthetic electron transport rates (a F/F^ x PPFD) related to photosynthetic photon fluence density (PPFD) as determined in the laboratory with tropical rock samples. A and closed symbols in C sample of a granitic rock from the eastern slope of the coastal mountain range in the SE of Madagascar (24° 49' S, 46° 57' E, 100 m a.s.l.) with Stigonema minu-tum and Gloeocapsa magma B and open symbols in C sample of rock outcrops in Menagesha State Forest near Addis Ababa, Ehtiopia (09° 04' N, 38° 22' E, 2,800 m a.s.l.) with Gloeocapsa sanguinea (Luttge et al. 1995)

of the prokaryotic cells due to the particular structure of their photosynthetic membranes (Lüttge 1997) and does not imply that these cyanobacteria were under very severe chronic photoinhibition.) The multiplication of (A F/F^) by photosynthetic photon fluence density (PPFD) gives relative photosynthetic electron transport rates, which saturated at the high intensities of 1,000 ^ mol photons m-2s-1 or above in the inselberg sample from Madagascar and the rock outcrop sample from Ethiopia measured in the experiments of Fig. 11.13. Thus, although even cyanobacteria crusts grown under full sun exposure may be subject to partial photoinhibition, it is quite clear, that cyanobacteria can adapt very well to very high irradiance (Lüttge et al. 1995).

Cyanobacterial communities show conspicuous zonations and niche occupation across furrows running down the rocks of inselbergs (Fig. 11.14). On an inselberg at Les Nouragues in French Guiana a community in the centre of such furrows was found to be dominated by compact growth forms, like the unicellular, colony-building Gloeocapsa sanguínea (Fig. 11.10C) and the short branching species Stigonema mamillosum. The growth form appears important because during and after rainfall the cyanobacterial mats are covered by water up to a few centimetres in depth, occasionally with strong current. The lateral slopes of the furrows are covered by a different community dominated by a thick layer of Stigonema ocel-latum (Fig. 11.10A). A third community dominated by Scytonema myochrous was found at some distance (normally > 50 cm) from the furrows in the more or less horizontal rock areas covering more than 80% of the whole rock surface (Rascher

Fig. 11.14 Drainage furrow running down from a small vegetation island on the inselberg Pedra Grande at Atibaia, SP, Brazil

et al. 2003). After rainfall before drying again the mats in the centre of the furrows and at the lateral slopes are covered for longer times by water and films of water than the mats on the horizontal rocks, and therefore, their photosynthesis is more pronouncedly limited by diffusion of CO2 and HCO- in the liquid phase. To counterbalance the liquid diffusion limited carbon supply cyanobacteria have evolved an inorganic carbon concentrating mechanism or a CO2/HCO- pump, where transport mechanisms and carbonic anhydrase catalyzing the CO2/HCO- equilibrium are involved (Skleryk et al. 1997; Sultemeyer et al. 1997). Diffusion limitation is reflected in the stable isotope ratios 813C of the cyanobacteria. They are performing C3-photosynthesis and the enzyme of primary CO2-fixation ribulose-bis-phosphate carboxylase/oxygenase (RuBISCO) has a 13C-discrimination of +27%c. The 8 13C-values of cyanobacteria of inselbergs, however, are much less negative than -27%

Fig. 11.15A, B Carbon isotope ratios (8 13C, %) of cyanobacteria in a transect across a seepage furrow of the inselberg at Galipero (A) and distribution of 8 13C-values among 17 samples of cyanobacterial crusts and mats from the area around Caicara - Puerto Ayacucho along the Orinoco river, Venezuela (B). (Luttge 1997; Ziegler and Luttge 1998)

Fig. 11.15A, B Carbon isotope ratios (8 13C, %) of cyanobacteria in a transect across a seepage furrow of the inselberg at Galipero (A) and distribution of 8 13C-values among 17 samples of cyanobacterial crusts and mats from the area around Caicara - Puerto Ayacucho along the Orinoco river, Venezuela (B). (Luttge 1997; Ziegler and Luttge 1998)

which would be obtained if RuBISCO were maily determining 13C-discrimination during photosynthesis (Ziegler and Luttge 1998; Fig. 11.15). This is due to the lower 13C-discrimination of dissolution of CO2 ( - 0.9%c) and diffusion of CO2 and HCO- in water (0.0%) determining CO2 delivery to RuBISCO, and as expected then, there is also a gradient from less negative to more negative 813C values from the centre via the lateral slopes to the open rock surfaces of the inselbergs (Fig. 11.15). The mats in the centre of the furrows also have a somewhat lower effective quantum use efficiency (AF/Fm') and apparent electrom transport rate (ETR) of photosynthesis than the mats at the slope and outside the furrows (Fig. 11.16).

The second important trait of cyanobacterial rock crusts as highlighted above is N2-fixation. By the possession of heterocytes they are characterized as N2-fixing organisms, because heterocytes are special cells in the coenobial colonies

Fig. 11.16A, B Apparent effective quantum yield (A F/F^) (A) and apparent rates of electron transport (ETR) (B) recorded over several days while the cyanobacteria communities across furrows of the inselberg at Les Nouragues, French Guiana, were wetted after rainfall. Lines indicate linear regressions (A) or fitted exponential growth to maximum (B). Filled circles and continuous lines, cyanobacterial community in the centre of the furrows, open squares and broken lines with long dashes, cyanobacterial community at the sides of the furrows, open triangles and broken lines with short dashes, cyanobacterial community on the vertical rocks beside the furrows. (Rascher et al. 2003)

Fig. 11.16A, B Apparent effective quantum yield (A F/F^) (A) and apparent rates of electron transport (ETR) (B) recorded over several days while the cyanobacteria communities across furrows of the inselberg at Les Nouragues, French Guiana, were wetted after rainfall. Lines indicate linear regressions (A) or fitted exponential growth to maximum (B). Filled circles and continuous lines, cyanobacterial community in the centre of the furrows, open squares and broken lines with long dashes, cyanobacterial community at the sides of the furrows, open triangles and broken lines with short dashes, cyanobacterial community on the vertical rocks beside the furrows. (Rascher et al. 2003)

Fig. 11.17 Distribution of inselbergs at the upper Orinoco river. Black areas, largely soil free granitic inselbergs with a slope of at least 15% (from Groger 1995; see Luttge 1997)

Fig. 11.17 Distribution of inselbergs at the upper Orinoco river. Black areas, largely soil free granitic inselbergs with a slope of at least 15% (from Groger 1995; see Luttge 1997)

or filaments bearing the enzymatic machiney for the reduction of atmospheric N2 (Sects. 10.2.3.2.1 and 10.2.3.2.2). Evidently N2-fixation of the cyanobacteria is important primarily for their own nutrition and growth. However, by leachates cyanobacterial crusts and mats on the rock habitat probably provide an essential starting point for succession and possibly also contribute considerably to the N-input into the ecosystems of the inselbergs themselves and via run-off into the surrounding savannas or forests. In fact the nitrogen content of the soil 10 m from the base of inselbergs in savannas was three times (1.40gN/kg soil) that measured more than 30 m from the base (0.45gN/kg soil) (E. Medina in Budel et al. 1997a,b). On the basis of a map of the inselbergs along the upper Orinoco river (Fig. 11.17) presented by Groger (1995) one can estimate the total savanna area with inselbergs as ca. 3,425 km2 (342.5 x 103ha) and the area of the inselbergs as ca. 480 km2 (48 x 103 ha) or 14%. The N2-fixation of cyanobacterial mats in tropical savannas was estimated as 60 g N/ha per day (Medina 1993) so that with assuming 300 good days per year as Groger (1995) gives the number of dry months in the area at only one to three and a total coverage of the inselbergs with cyanobacteria, the fixation of the inselbergs would be ca. 18 kg N/ha per year. This appears small in comparison to N-fertilisation in high-technology agriculture (several 10s to 200 kg N/ha; see Luttge 1997) but is substantial for the nutrient poor savanna ecosystem.

The third of the traits highlighted above which allow cyanobacteria survival on bare rocks, i.e. desiccation tolerance, appears to be the most important one and is discussed in Sect. 11.4.

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Renewable Energy 101

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