Response of desert biological soil crusts to alterations in precipitation frequency

Biological soil crusts, a community of cyanobacteria, lichens, and mosses that live on the soil surface, occur in deserts throughout the world. They are a critical component of desert ecosystems, as they are important contributors to soil fertility and stability. Future climate scenarios predict alteration of the timing and amount of precipitation in desert environments. Because biological soil crust organisms are only metabolically active when wet, and as soil surfaces dry quickly in deserts during late spring, summer, and early fall, the amount and timing of precipitation is likely to have significant impacts on the physiological functioning of these communities. Using the three dominant soil crust types found in the western United States, we applied three levels of precipitation frequency (50% below-average, average, and 50% above-average) while maintaining average precipitation amount (therefore changing both timing and size of applied events). We measured the impact of these treatments on photosynthetic performance (as indicated by dark-adapted quantum yield and chlorophyll a concentrations), nitrogenase activity, and the ability of these organisms to maintain concentrations of radiation-protective pigments (scytonemin, beta-carotene, echinenone, xanthophylls, and canthaxanthin). Increased precipitation frequency produced little response after 2.5 months exposure during spring (1 April–15 June) or summer (15 June–31 August). In contrast, most of the above variables had a large, negative response after exposure to increased precipitation frequency for 6 months spring–fall (1 April–31 October) treatment. The crusts dominated by the soil lichen Collema, being dark and protruding above the surface, dried the most rapidly, followed by the dark surface cyanobacterial crusts (Nostoc-Scytonema-Microcoleus), and then by the light cyanobacterial crusts (Microcoleus). This order reflected the magnitude of the observed response: crusts dominated by the lichen Collema showed the largest decline in quantum yield, chlorophyll a, and protective pigments; crusts dominated by Nostoc-Scytonema-Microcoleus showed an intermediate decline in these variables; and the crusts dominated by Microcoleus showed the least negative response. Most previous studies of crust response to radiation stress have been short-term laboratory studies, where organisms were watered and kept under moderate temperatures. Such conditions would give crust organisms access to ample carbon to respond to imposed stresses (e.g., production of UV-protective pigments, replacement of degraded chlorophyll). In contrast, our longer-term study showed that under field conditions of high air temperatures and frequent, small precipitation events, crust organisms appear unable to produce protective pigments in response to radiation stress, as they likely dried more quickly than when they received larger, less frequent events. Reduced activity time likely resulted in less carbon available to produce or repair chlorophyll a and/or protective pigments. Our findings may partially explain the global observation that soil lichen cover and richness declines as the frequency of summer rainfall increases.     

UV‐B impairs growth and gas exchange in grapevines grown in high altitude

We previously demonstrated that solar ultraviolet‐B (UV‐B) radiation levels in high altitude vineyards improve berry quality in Vitis vinifera cv. Malbec, but also reduce berry size and yield, possibly as a consequence of increased oxidative damage and growth reductions (lower photosynthesis). The defense mechanisms toward UV‐B signal and/or evoked damage promote production of antioxidant secondary metabolites instead of primary metabolites. Purportedly, the UV‐B effects will depend on tissues developmental stage and interplay with other environmental conditions, especially stressful situations. In this work, grapevines were exposed to high solar UV‐B (+UV‐B) and reduced (by filtering) UV‐B (?UV‐B) treatments during three consecutive seasons, and the effects of UV‐B, developmental stages and seasons on the physiology were studied, i.e. growth, tissues morphology, photosynthesis, photoprotective pigments, proline content and antioxidant capacity of leaves. The +UV‐B reduced photosynthesis and stomatal conductance, mainly through limitation in gas exchange, reducing plant's leaf area, net carbon fixation and growth. The +UV‐B augmented leaf thickness, and also the amounts of photoprotective pigments and proline, thereby increasing the antioxidant capacity of leaves. The defense mechanisms triggered by + UV‐B reduced lipid peroxidation, but they were insufficient to protect the photosynthetic pigments per leaf dry weight basis. The +UV‐B effects depend on tissues developmental stage and interplay with other environmental conditions such as total radiation and air temperatures.     

Plant Interactions with Changes in Coverage of Biological Soil Crusts and Water Regime in Mu Us Sandland,China

Plant interactions greatly affect plant community structure. Dryland ecosystems are characterized by low amounts of unpredictable precipitation as well as by often having biological soil crusts (BSCs) on the soil surface. In dryland plant communities, plants interact mostly as they compete for water resources, and the direction and intensity of plant interaction varies as a function of the temporal fluctuation in water availability. Since BSCs influence water redistribution to some extent, a greenhouse experiment was conducted to test the hypothesis that the intensity and direction of plant interactions in a dryland plant community can be modified by BSCs. In the experiment, 14 combinations of four plant species (Artemisia ordosica, Artemisia sphaerocephala, Chloris virgata and Setaria viridis) were subjected to three levels of coverage of BSCs and three levels of water supply. The results show that: 1) BSCs affected plant interaction intensity for the four plant species: a 100% coverage of BSCs significantly reduced the intensity of competition between neighboring plants, while it was highest with a 50% coverage of BSCs in combination with the target species of A. sphaerocephala and C. virgata; 2) effects of the coverage of BSCs on plant interactions were modified by water regime when the target species were C. virgata and S. viridis; 3) plant interactions were species-specific. In conclusion, the percent coverage of BSCs affected plant interactions, and the effects were species-specific and could be modified by water regimes. Further studies should focus on effects of the coverage of BSCs on plant-soil hydrological processes.     

Nitrogenase activity by biological soil crusts in cold sagebrush steppe ecosystems

In drylands worldwide, biological soil crusts (BSC) form a thin photosynthetic cover across landscapes, and provide vital benefits in terms of stabilizing soil and fixing nitrogen (N) and carbon (C). Numerous studies have examined the effects of climate and disturbance on BSC functions; however, few have characterized these responses in rolling BSCs typical of northern ecosystems in the Intermountain West, US. With temperature increases and shifts in precipitation projected, it is unclear how BSCs in this region will respond to climate change, and how the response could affect their capacity to perform key ecosystem functions, such as providing ‘new’ N through biological N₂ fixation. To address this important knowledge gap, we examined nitrogenase activity (NA) associated with rolling BSCs along a climatic gradient in southwestern Idaho, US, and quantified how acetylene reduction rates changed as a function of climate, grazing (using exclosures), and shrub-canopy association. Results show that warmer, drier climates at lower elevations hosted greater cover of late successional BSC communities (e.g., mosses and lichens), and higher NA compared with colder, wetter climates at higher elevations. Highest NA (0.5–29.3 µmol C₂H₄ m^?2 h^?1) occurred during the early summer/spring, when water was more available than in late summer/autumn. Activity was strongly associated with soil characteristics including pH and ammonium concentrations suggesting these characteristics as potentially strong controls on NA in BSCs. The relationship between grazing and NA varied with elevation. Specifically, lower elevation sites had lower NA at grazed locations, whereas higher elevation sites had higher NA with grazing. At both low and high ends of the elevation gradient, shrub-canopy associated BSCs maintained two to three times higher NA compared to BSCs in the interspace among shrubs. Taken together, our findings indicate that the controls and rates of NA in BSCs vary seasonally and strongly with climate in the Intermountain West, and that drier springs are likely to influence rates of NA more than warmer summers.     

腾格里沙漠东南缘生物土壤结皮的固氮潜力

以腾格里沙漠东南缘广泛分布的3类典型生物土壤结皮(藻类结皮、地衣结皮和藓类结皮)为研究对象,在野外环境下连续一年(2010年6月至2011年5月)测定了不同类型结皮的固氮潜力、季节变化,及其对环境因子的响应特征.结果表明：整个试验期间,藻类、地衣和藓类结皮的固氮活性分别为14～133、20～101和4～28 mol·m^-2·h^-1,差异显著,这种差异主要是由结皮种类组成的差异所致.3类结皮固氮活性对环境因子的响应特征基本一致.3类结皮固氮活性与降水量关系不显著,但与试验前3天小于3 mm的降水量均呈显著的正相关关系,说明该地区3类结皮在3 mm降水条件下即可达到最大固氮速率.3类结皮固氮活性与试验期温度均呈显著的二次函数关系,随气温升高均呈先上升后下降的趋势,藻类和地衣结皮的固氮活性在超过30 ℃后即迅速下降,而藓类结皮的固氮活性在超过25 ℃后即开始下降,说明不同类型结皮具有不同的固氮适宜温度区间.3类结皮固氮活性的季节变化均表现为秋季>春季>夏季>冬季,夏季高温和冬季低温抑制了结皮固氮酶活性,而春末秋初适宜的水热条件促进了其固氮活性的提高,结皮固氮活性的季节变化主要受水热因子的共同调控.温带荒漠区生物土壤结皮在湿润条件下全年均具有固氮能力,环境因子对其氮固定的控制作用层次分明,水分是影响其固氮速率和持续时间的关键因子,在水分和碳源充足的条件下,温度则是制约其固氮能力的主要因子.     

Dynamics of cover, UV-protective pigments, and quantum yield in biological soil crust communities of an undisturbed Mojave Desert shrubland

Biological soil crusts are an integral part of dryland ecosystems. We monitored the cover of lichens and mosses, cyanobacterial biomass, concentrations of UV-protective pigments in both free-living and lichenized cyanobacteria, and quantum yield in the soil lichen species Collema in an undisturbed Mojave Desert shrubland. During our sampling time, the site received historically high and low levels of precipitation, whereas temperatures were close to normal. Lichen cover, dominated by Collema tenax and C. coccophorum, and moss cover, dominated by Syntrichia caninervis, responded to both increases and decreases in precipitation. This finding for Collema spp. at a hot Mojave Desert site is in contrast to a similar study conducted at a cool desert site on the Colorado Plateau in SE Utah, USA, where Collema spp. cover dropped in response to elevated temperatures, but did not respond to changes in rainfall. The concentrations of UV-protective pigments in free-living cyanobacteria at the Mojave Desert site were also strongly and positively related to rainfall received between sampling times (R² values ranged from 0.78 to 0.99). However, pigment levels in the lichenized cyanobacteria showed little correlation with rainfall. Quantum yield in Collema spp. was closely correlated with rainfall. Climate models in this region predict a 3.5–4.0 °C rise in temperature and a 15–20% decline in winter precipitation by 2099. Based on our data, this rise in temperature is unlikely to have a strong effect on the dominant species of the soil crusts. However, the predicted drop in precipitation will likely lead to a decrease in soil lichen and moss cover, and high stress or mortality in soil cyanobacteria as levels of UV-protective pigments decline. In addition, surface-disturbing activities (e.g., recreation, military activities, fire) are rapidly increasing in the Mojave Desert, and these disturbances quickly remove soil lichens and mosses. These stresses combined are likely to lead to shifts in species composition and the local extirpation of some lichen or moss species. As these organisms are critical components of nutrient cycling, soil fertility, and soil stability, such changes are likely to reverberate throughout these ecosystems.     

E113 is required for the efficient photoisomerization of the unprotonated chromophore in a UV-absorbing visual pigment

Protonation of the retinal Schiff base chromophore is responsible for the absorption of visible light and is stabilized by the counterion residue E113 in vertebrate visual pigments. However, this residue is also conserved in vertebrate UV-absorbing visual pigments (UV pigments) which have an unprotonated Schiff base chromophore. To elucidate the role played by this residue in the photoisomerization of the unprotonated chromophore in UV pigments, we measured the quantum yield of the E113Q mutant of the mouse UV cone pigment (mouse UV). The quantum yield of the mutant was much lower than that of the wild type, indicating that E113 is required for the efficient photoisomerization of the unprotonated chromophore in mouse UV. Introduction of the E113Q mutation into the chicken violet cone pigment (chicken violet), which has a protonated chromophore, caused deprotonation of the chromophore and a reduction in the quantum yield. On the other hand, the S90C mutation in chicken violet, which deprotonated the chromophore with E113 remaining intact, did not significantly affect the quantum yield. These results suggest that E113 facilitates photoisomerization in both UV-absorbing and visible light-absorbing visual pigments and provide a possible explanation for the complete conservation of E113 among vertebrate UV pigments.     

Photoinhibition by visible and ultraviolet radiation in the red macroalga Porphyra umbilicalis grown in the laboratory

Photosynthetic oxygen production and PAM fluorescence measurements were used to follow photoinhibition in the red macroalga Porphyra umbilicalis. Exposure to simulated solar radiation caused inhibition of the effective photosynthetic quantum yield from which the thalli partially recovered in the shade in subsequent hours. There were no significant differences between samples exposed to unfiltered radiation and those exposed to radiation from which increasing portions of UV radiation had been removed indicating that the thalli are well adapted to current levels of solar PAR and UV radiation. This notion was supported by the finding of high concentrations of UV screening pigments which were even enhanced by exposure to increased UV radiation. However, when exposed to (only) UV radiation about 50% higher than that encountered by the organisms in their natural habitat, the photosynthetic yield decreased slowly and did not show any recovery even when the degree of inhibition did not exceed 10%.     

Adapting management to a changing world: Warm temperatures,dry soil,and interannual variability limit restoration success of a dominant woody shrub in temperate drylands

Restoration and rehabilitation of native vegetation in dryland ecosystems, which encompass over 40% of terrestrial ecosystems, is a common challenge that continues to grow as wildfire and biological invasions transform dryland plant communities. The difficulty in part stems from low and variable precipitation, combined with limited understanding about how weather conditions influence restoration outcomes, and increasing recognition that one‐time seeding approaches can fail if they do not occur during appropriate plant establishment conditions. The sagebrush biome, which once covered over 620,000 km² of western North America, is a prime example of a pressing dryland restoration challenge for which restoration success has been variable. We analyzed field data on Artemisia tridentata (big sagebrush) restoration collected at 771 plots in 177 wildfire sites across its western range, and used process‐based ecohydrological modeling to identify factors leading to its establishment. Our results indicate big sagebrush occurrence is most strongly associated with relatively cool temperatures and wet soils in the first spring after seeding. In particular, the amount of winter snowpack, but not total precipitation, helped explain the availability of spring soil moisture and restoration success. We also find considerable interannual variability in the probability of sagebrush establishment. Adaptive management strategies that target seeding during cool, wet years or mitigate effects of variability through repeated seeding may improve the likelihood of successful restoration in dryland ecosystems. Given consistent projections of increasing temperatures, declining snowpack, and increasing weather variability throughout midlatitude drylands, weather‐centric adaptive management approaches to restoration will be increasingly important for dryland restoration success.     

Photoisomerization efficiency in UV-absorbing visual pigments: protein-directed isomerization of an unprotonated retinal Schiff base

A visual pigment consists of an opsin protein and a chromophore, 11-cis-retinal, which binds to a specific lysine residue of opsin via a Schiff base linkage. The Schiff base chromophore is protonated in pigments that absorb visible light, whereas it is unprotonated in ultraviolet-absorbing visual pigments (UV pigments). To investigate whether an unprotonated Schiff base can undergo photoisomerization as efficiently as a protonated Schiff base in the opsin environment, we measured the quantum yields of the bovine rhodopsin E113Q mutant, in which the Schiff base is unprotonated at alkaline pH, and the mouse UV pigment (mouse UV). Photosensitivities of UV pigments were measured by irradiation of the pigments followed by chromophore extraction and HPLC analysis. Extinction coefficients were estimated by comparing the maximum absorbances of the original pigments and their acid-denatured states. The quantum yield of the bovine rhodopsin E113Q mutant at pH 8.2, where the Schiff base is unprotonated, was significantly lower than that of wild-type rhodopsin, whereas the mutant gave a quantum yield almost identical to that of the wild type at pH 5.5, where the Schiff base is protonated. These results suggest that Schiff base protonation plays a role in increasing quantum yield. The quantum yield of mouse UV, which has an unprotonated Schiff base chromophore, was significantly higher than that of the unprotonated form of the rhodopsin E113Q mutant, although it was still lower than the visible-absorbing pigments. These results suggest that the mouse UV pigment has a specific mechanism for the efficient photoisomerization of its unprotonated Schiff base chromophore.     

外源输入氮素在藓类生物土壤结皮中各氮组分的分配特征与归趋途径

针对黄土高原典型藓类生物结皮,以无结皮裸地为对照,通过野外原位同位素标记试验,在标记后1~30 d连续取样测定,示踪外源添加¹⁵N在生物结皮中各氮组分的分配特征,并分析¹⁵N在土壤-微生物-藓植株中的归趋途径,对比揭示生物结皮对土壤氮循环的影响。结果表明: 1)生物结皮的全氮、微生物生物量氮、可溶性有机氮中¹⁵N含量分别平均比裸地高2.9、17.5、9.0倍,且藓植株固定的¹⁵N含量高达4.73 mg·kg^-1。2)生物结皮的¹⁵N残留率平均为13.0%,其氮固持能力是裸地(3.3%)的4.0倍;生物结皮中各组分¹⁵N占全氮的比率依次为微生物生物量氮(54.3%)>藓植株氮(22.5%)>可溶性有机氮(6.2%),而裸地则为微生物生物量氮(11.5%)>可溶性有机氮(2.6%),显示生物结皮中微生物和藓植株的氮固持能力合计比裸地高65.3%。3)生物结皮中微生物生物量¹⁵N的转移量和库容量分别比裸地高17.2和20.5倍,但其周转率为每月5.8次,低于裸地的每月7.2次,其周转期是裸地的1.2倍。综上,与无结皮裸地相比,生物结皮具有更高的氮固持能力,同时能够改变土壤各氮组分的分配率,因此,在干旱生态系统土壤氮循环过程中具有重要作用。     

Effects of ultraviolet (UV) exclusion on the seasonal concentration of photosynthetic and UV-screening pigments in Scots pine needles

The effects of ultraviolet (UV) radiation on the photosynthetic and UV‐screening pigments in needles of Scots pine (Pinus sylvestris L.) saplings were studied in a UV‐exclusion field chamber experiment in northern Finland (67°N) during 2001–2002. The chambers held filters that excluded both UVB and UVA, only UVB, transmitted all UV, or lacked filters. Analyses of control needles (no filter and polyethene filter) showed that the first changes to occur in spring (end of April) was an abrupt increase in the epoxidation state (EPS) of the xanthophyll cycle pigments, likely in relation with the beginning of the photosynthetic activity. The concentration of chlorophyll, lutein, neoxanthin, α‐carotene, β‐carotene, and the size of the xanthophyll cycle pool (violaxanthin+antheraxanthin+zeaxanthin=VAZ) changed only later when needles reached their summer photosynthesis state. Exclusion of UV radiation significantly affected the xanthophyll cycle but not the other photosynthetic pigments analysed. Interestingly, the effects on xanthophylls were dependent on the sampling date. Under UVA/B‐exclusion, the EPS was increased and VAZ pool size was unchanged in April, whereas EPS remained unchanged and the VAZ pool size was reduced in May and June. The existence of two sustained and active antenna modes during winter and summer could be an explanation for the specific UV‐exclusion effect in the different season. A high‐performance liquid chromatography analysis of soluble phenolics showed that the exclusion of UVA/B radiation caused a significant effect on five compounds out of 46 studied, without affecting the concentration of the total soluble phenolics. Under UVA/B‐exclusion, the concentration of three of them (secoisolariciresinol‐glucopyranoside, two unknown) was reduced while the concentration of dicoumaroyl‐astragalin and pinosylvin monomethylether was increased compared with both controls separately. In general, the exclusion of UVA/B caused a stronger effect than the exclusion of UVB on both photosynthetic and UV screening pigments. The effects of UV radiation on xanthophyll cycle pigments were season‐specific and detectable only under stressful spring conditions (freezing temperatures and high irradiance due to snow reflection). The effect on the xanthophyll cycle could be a direct consequence of UV treatments, or an indirect consequence of the changed flavonoid composition, or a combination of both.     

Warming reduces the growth and diversity of biological soil crusts in a semi-arid environment: implications for ecosystem structure and functioning

Biological soil crusts (BSCs) are key biotic components of dryland ecosystems worldwide that control many functional processes, including carbon and nitrogen cycling, soil stabilization and infiltration. Regardless of their ecological importance and prevalence in drylands, very few studies have explicitly evaluated how climate change will affect the structure and composition of BSCs, and the functioning of their constituents. Using a manipulative experiment conducted over 3 years in a semi-arid site from central Spain, we evaluated how the composition, structure and performance of lichen-dominated BSCs respond to a 2.4°C increase in temperature, and to an approximately 30 per cent reduction of total annual rainfall. In areas with well-developed BSCs, warming promoted a significant decrease in the richness and diversity of the whole BSC community. This was accompanied by important compositional changes, as the cover of lichens suffered a substantial decrease with warming (from 70 to 40% on average), while that of mosses increased slightly (from 0.3 to 7% on average). The physiological performance of the BSC community, evaluated using chlorophyll fluorescence, increased with warming during the first year of the experiment, but did not respond to rainfall reduction. Our results indicate that ongoing climate change will strongly affect the diversity and composition of BSC communities, as well as their recovery after disturbances. The expected changes in richness and composition under warming could reduce or even reverse the positive effects of BSCs on important soil processes. Thus, these changes are likely to promote an overall reduction in ecosystem processes that sustain and control nutrient cycling, soil stabilization and water dynamics.     

Effects of high temperature on nodulation and nitrogen fixation by Phaseolus vulgaris L.

Screening of Rhizobium leguminosarum bv. phaseoli strains showed some that were able to nodulate common beans (Phaseolus vulgaris L.) at high temperatures (35 and 38°C/8 h/day). The nodulation ability was not related to the capability to grow or produce melanin-like pigment in culture media at high temperatures. However, nodules formed at high temperatures were ineffective and plants did not accumulate N in shoots. Two thermal shocks of 40°C/8 h/day at flowering time drastically decreased nitrogenase activity and nodule relative efficiency of plants otherwise grown at 28°C. Recovery of nitrogenase activity began only after seven days, when new nodules formed; total incorporation of N in tops did not recover for 2 weeks. Non-inoculated beans receiving mineral N were not affected by the thermal shock, and when growing continuously at 35 or 38°C had total N accumulated in shoots reduced by only 18%.     

NONSYMBIOTIC AND SYMBIOTIC NITROGEN FIXATION IN A WEAKLY MINEROTROPHIC PEATLAND

The acetylene reduction assay was used to measure nonsymbiotic and symbiotic nitrogen fixation in a weakly minerotrophic peatland throughout the ice-free season. Nonsymbiotic nitrogen fixation was found in surface materials and subsurface peat. In surface materials, nitrogenase activity measured in the field contributed about 0.6 kg N ha^-1 yr^-1, was closely associated with Sphagnum, but was not correlated with temperature between 12 and 27 C. No cyanobacteria were found in association with Sphagnum. In subsurface peat, nitrogenase activity measured in situ contributed no more than 0.4 kg N ha^-1 yr^-1 and was closely correlated with temperature between 7 and 21 C. There were uncertainites in these measurements due to presence of ethylene oxidizing activity and a long time lag. Symbiotic nitrogen fixation was found only in actinomycete-induced root nodules of Myrica gale L. Legumes were absent and the few lichens present lacked nitrogenase activity. Based on acetylene reduction assays, Myrica gale fixed about 35 kg N ha^-1 yr^-1. Nitrogenase activity in Myrica gale showed a strong seasonal pattern which varied little during three consecutive years even though water levels varied substantially. Nitrogen input to the peatland from nonsymbiotic nitrogen fixation was only 15% the amount contributed by bulk precipitation. Symbiotic fixation, in contrast, contributed approximately six times the amount in bulk precipitation.     

Modulation of physiological performance by temperature and salinity in the sugar kelp Saccharina latissima

The sugar kelp Saccharina latissima experiences a wide range of environmental conditions along its geographical and vertical distribution range. Temperature and salinity are two critical drivers influencing growth, photosynthesis and biochemical composition. Moreover, interactive effects might modify the results described for single effects. In shallow water coastal systems, exposure to rising temperatures and low salinity are expected as consequence of global warming, increased precipitation and coastal run‐off. To understand the acclimation mechanisms of S. latissima to changes in temperature and salinity and their interactions, we performed a mechanistic laboratory experiment in which juvenile sporophytes from Brittany, France were exposed to a combination of three temperatures (0, 8 and 15°C) and two salinity levels (20 and 30 psu (practical salinity units)). After a temperature acclimation of 7 days, sporophytes were exposed to low salinity (20 psu) for a period of 11 days. Growth, and maximal quantum yield of photosystem II (Fv/Fm), pigments, mannitol content and C:N ratio were measured over time. We report for the first time in S. latissima a fivefold increase in the osmolyte mannitol in response to low temperature (0°C) compared to 8 and 15°C that may have ecological and economic implications. Low temperatures significantly affected all parameters, mostly in a negative way. Chlorophyll a, the accessory pigment pool, growth and Fv/Fm were significantly lower at 0°C, while the de‐epoxidation state of the xanthophyll cycle was increased at both 0 and 8°C compared to 15°C. Mannitol content and growth decreased with decreased salinity; in contrast, pigment content and Fv/Fm were to a large extent irresponsive to salinity. In comparison to S. latissima originating from an Arctic population, despite some reported differences, this study reveals a remarkably similar impact of temperature and salinity variation, reflecting the large degree of adaptability in this species.     

Effects of N supply on the accumulation of photosynthetic pigments and photoprotectors in <Emphasis Type="Italic">Gracilaria tenuistipitata</Emphasis> (Rhodophyta) cultured under UV radiation

We have studied the effects of nitrate supply under photosynthetic active radiation (PAR) plus ultraviolet radiation (UVR) exposure on photosynthetic pigments (chlorophyll a and carotenoids), photoprotective UV screen mycosporine-like amino acids (MAAs), and photosynthetic parameters, including the maximum quantum yield (F _v/F _m) and electron transport rate (ETR) on the red agarophyte Gracilaria tenuistipitata. Apical tips of G. tenuistipitata were cultivated under ten different concentrations of NO₃⁻ for 7 days. It has been shown that G. tenuistipitata cultured under laboratory conditions has the ability to accumulate high amounts of MAAs following a nitrate concentration-dependent manner under PAR + UVR. Two MAAs were identified, shinorine and porphyra-334. The relative concentration of the first increased under high concentrations of nitrate, while the second one decreased. The presence of antheraxanthin is reported for the first time in this macroalgae, which also contains zeaxanthin, lutein, and β-carotene. The accumulation of pigments, photoprotective compounds, and photosynthetic parameters of G. tenuistipitata is directly related to N availability. All variables decreased under low N supplies and reached constant maximum values with supplements higher than 0.5 mM NO₃⁻. Our results suggest a high potential to acclimation and photoprotection against stress factors (including high PAR and UVR) directly related to N availability for G. tenuistipitata.     

Southern African Biological Soil Crusts are Ubiquitous and Highly Diverse in Drylands,Being Restricted by Rainfall Frequency

Biological soil crusts (BSCs) are found in all dryland regions of the world, including the polar regions. They are also known to occur in the southern African region. Although there were a number of case studies on BSCs from that region, we did not know if they are a normal part of the vegetation cover or just a phenomenon that occasionally occurs here and there. In order to investigate diversity, distribution patterns, and the driving factors of both, we followed a random sampling system of observatories along a transect, stretching from the Namibian-Angolan border down south to the Cape Peninsula, covering seven different major biomes. Biological soil crusts were found to occur in six out of seven biomes. Despite the fact that soil-dwelling algae occurred in the Fynbos biome, crust formation was not observed for hitherto unknown reasons. Seven BSC types were distinguished on the basis of morphology and taxonomic composition: three of them were cyanobacteria-dominated, one with additional chlorolichens, two with bryophytes, one hypolithic type restricted to quartz gravel pavements, and the unique lichen fields of the Namib Desert. Besides 29 green algal species in 21 genera, one heterokont alga, 12 cyanolichens, 14 chlorolichens, two genera of liverworts, and three genera of mosses, these crusts are positioned among the most diverse BSCs worldwide mainly because of the unusual high cyanobacterial species richness comprising 58 species in 21 genera. They contribute considerably to the biodiversity of arid and semi-arid bioregions. Taxonomic diversity of cyanobacteria was significantly higher in the winter rain zone than in the summer rain zone (54 versus 32 species). The soil photosynthetic biomass (chlorophylla/m2), the carbon content of the soil and the number of BSC types were significantly higher in the winter rain zone (U27, 29=215.0, p=0.004 [chla]; U21, 21=135.0, p=0.031 [C]; U27, 29=261.5, p=0.028 [BSC types]; excluding the fog-dominated Namib biome). The winter rain zone is characterized by a lower precipitation amount, but a higher rain frequency with the number of rainy days more evenly distributed over the year. The dry period is significantly shorter per year in the winter rain zone (U8, 9=5.0, p=0.003). We conclude that rain frequency and duration of dry periods rather than the precipitation amount is the main factor for BSC growth and succession. Nitrogen content of the soils along the transect was generally very low and correlated with soil carbon content. There was a weak trend that an increasing proportion of silt and clay (<0.63 mm) in the soil is associated with higher values of BSC chlorophyll content (Pearson correlation coefficient=0.314, p=0.237). Furthermore, we found a significant positive correlation between silt and clay and the number of BSC types (Pearson correlation coefficient=0.519, p=0.039), suggesting that fine grain-size promotes BSC succession and their biomass content. Lichens and bryophytes occurred in BSCs with lower disturbance frequencies (e.g. trampling) only. Crust thickness and chlorophyll content increased significantly from crusts of the early successional type to the late successional crust types. From our results, we conclude that BSCs are a normal and frequent element of the vegetation in arid and semi-arid southwestern Africa, and that rain frequency and duration of dry periods rather than the precipitation amount are the key factors for the development, differentiation and composition of BSCs.     

Diurnal variations of chlorophyll fluorescence and CO2 exchange of biological soil crusts in different successional stages in the Gurbantunggut Desert of northwestern China

Biological soil crusts (BSCs) formed by different combinations of photosynthetic algae, cyanobacteria, lichens and mosses are well-developed in the Gurbantunggut Desert of northwestern China. To investigate the different responses of BSCs to environmental factors, the diurnal variations of chlorophyll fluorescence and CO₂ exchange of BSCs in different successional stages were measured following artificial rehydration in the field. Results showed that the maximum potential quantum efficiency of PSII (F _v/F _m), the actual PSII efficiency (Φ_PSII) and the relative rate of electron transport as well as net photosynthesis of the different successional BSCs varied similarly and changed markedly with diurnal fluctuations in light and temperature. Further analyses indicated that CO₂ exchange and photosynthetic pigment content of chlorophyll (Chl) a, Chl b and carotenoids increased with the developmental level of BSCs, from cyanobacterial crust to lichen crust to moss crust. The differences in responses of BSCs to environmental factors and photosynthetic pigment content may be partially attributed to differences in species composition and morphological characteristics of the various BSCs. Overall, moss crust is better adapted to a wide range of irradiance and higher temperatures than lichen and cyanobacterial crusts. Therefore, BSCs in a later successional stage are expected to play a more important role in desertification control than those of the earlier stages.     

Nitrogenase activity of the antarctic cyanobacteriumNostoc commune: Influence of temperature

Nitrogenase activity (C₂H₂ reduction) ofNostoc commune isolated from Schirmacher Oasis (Antarctica) was compared withNostoc muscorum, N. calcicola, Anabaena doliolum andGloeocapsa sp. The temperature profile of acetylene reduction (5–30 °C) forN. commune revealed (a) that the highest rate of nitrogenase activity was at 25±1 °C, (b) that it was low (69 %) in comparison withN. muscorum, and (c) that nitrogenase activity continued at lower temperatures, which was not evident for other cyanobacteria. The results suggest thatN. commune is adapted to lower temperatures in terms of nitrogen fixation.