Habitat stress initiates changes in composition, CO2 gas exchange and C-allocation as life traits in biological soil crusts

Abstract

Biological soil crusts (BSC) are the dominant functional vegetation unit in some of the harshest habitats in the world. We assessed BSC response to stress through changes in biotic composition, CO2 gas exchange and carbon allocation in three lichen-dominated BSC from habitats with different stress levels, two more extreme sites in Antarctica and one moderate site in Germany. Maximal net photosynthesis (NP) was identical, whereas the water content to achieve maximal NP was substantially lower in the Antarctic sites, this apparently being achieved by changes in biomass allocation. Optimal NP temperatures reflected local climate. The Antarctic BSC allocated fixed carbon (tracked using (14)CO2) mostly to the alcohol soluble pool (low-molecular weight sugars, sugar alcohols), which has an important role in desiccation and freezing resistance and antioxidant protection. In contrast, BSC at the moderate site showed greater carbon allocation into the polysaccharide pool, indicating a tendency towards growth. The results indicate that the BSC of the more stressed Antarctic sites emphasise survival rather than growth. Changes in BSC are adaptive and at multiple levels and we identify benefits and risks attached to changing life traits, as well as describing the ecophysiological mechanisms that underlie them.

Figure 1

Organisms and map of the study sites: close-up pictures of the organisms are on the left site with (a) a lichen-dominated BSC with the lichen species Lecanora expectans and Caloplaca darbishirei from Garwood Valley, (c) the lichen-dominated BSC from Diamond Hill with Acarospora gwynii and (e) the lichen-dominated BSC from the research site near Würzburg with Psora decipiens. At top right, a map of western Ross Sea coastline showing the locations of the two research sites, Site Garwood, McMurdo Dry Valleys (Garwood Valley) and Site Darwin, Diamond Hill; the inset, top right shows the location of the Ross Sea in Antarctica (both maps downloaded from the SCAR Antarctic Digital Database). Pictures of the two research areas are in the middle of the figure. (b) Garwood Valley, view towards Garwood Lake, Joyce Glacier (background) and Péwé Peak (centre left), (d) Diamond Hill, view towards the Brown Hills (Antarctica New Zealand Pictorial Collection, Shulamit Gordon K002 04/05).At the bottom, centre of the figure is a map of Germany showing the location of the research site near Würzburg and, on the right, a map showing the position of Germany in Europe. A picture of the site is in the middle, (f) Nature reserve ‘Ruine Homburg'.

Figure 2

Cardinal points of photosynthesis of BSC from Site Darwin (D, black), Site Garwood (GW, light grey) and Site Homburg (H, dark grey); (a) range of optimal water content (mm precipitation), (b) mean light saturation points (μmol photons m⁻² s⁻¹; n=3), (c) mean light compensation point (μmol photons m⁻² s⁻¹; n=3), (d) mean maximum NP under optimal conditions (μmol CO₂ m⁻² s⁻¹; n=3), (e) mean maximum respiration (DR; μmol CO₂ m⁻² s⁻¹; n=3), (f) mean carbon use efficiency (NP/DR; n=3). In all cases, error bars are±s.d. Different letters indicate significant differences in the means between habitats (significance level P<0.05).

Figure 3

¹⁴C carbon uptake and allocation. The area of the circle indicates the total uptake of ¹⁴C during the 12 h incubation time. Left column 5 °C, right column 17 °C; (a) and (b) Darwin (D); (c) and (d) Garwood (GW); (e) and (f) Homburg (H). Methanol fraction (short-chained sugars) is indicated in black, the hot water fraction (polysaccharides) is indicated in light grey, the insoluble fraction is indicated in dark grey.

Figure 4

Metabolic pools and possible contents, possible functions and overall life traits for fixed carbon allocation in lichen-dominated BSC. Arrows indicate the likely flow directions of fixed carbon between the pools.