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University of Queensland (2010)

Cyanobacteria in the Australian mulga lands:diversity, function and ecological role in high stress environments

Wendy Williams

Titre : Cyanobacteria in the Australian mulga lands:diversity, function and ecological role in high stress environments

Auteur : Wendy Williams

Université de soutenance : University of Queensland

Grade : Doctor of Philosophy (PhD) 2010

Cyanobacteria form a unique group of prokaryotes whose tolerance to low water potentials affords them a competitive advantage in situations where most other organisms would die. Cyanobacterial soil crusts consist of cyanobacteria, lichens, bryophytes, bacteria, fungi and algae in varying proportions. These dynamic ecosystems occur as a protective, nutrient rich surficial cover, broadly distributed in arid and semi-arid environments worldwide. Soil chemistry, precipitation, light and temperature strongly influence crust diversity. Understanding the diversity and eco-physiology of cyanobacterial communities is fundamental to assessing their role in the landscape. This thesis addresses the lack of data and basic knowledge on cyanobacterial soil crusts in Australia. The investigation was based in the Mulga Lands, a bioregion covering 251,640 km2, characterised by woodlands and extensive perennial grasslands. Twelve individual Mulga Land sites were used for a cyanobacterial crust diversity study. A further four studies were based at the main research site at Glencoban. Since the late 1800s this site was settled by pastoralists and grazed by sheep or cattle with bore drains used as a stock water source. Following 70 years of disturbance and droughts the Glencoban bore drain was closed and replaced with watering points. At Glencoban, research was focused on analysing the ecological role of cyanobacterial crusts in the context of land use through natural and grazing-induced disturbances. These studies spanned 2002-2005 throughout four distinct climatic phases : pre-drought, drought, early post-drought and seasonal drought. The first detailed botanical descriptions based on morphological features of the key terrestrial cyanobacteria found across the Mulga Lands are presented. Floristic surveys of cyanobacterial crusts from twelve sites showed Porphyrosiphon notarisii was dominant or co-dominant with nitrogen fixing species Scytonema and Stigonema. Diversity totalled 63 taxa, comprising 23 cyanobacteria, 18 chlorolichens, 7 cyanolichens, 6 liverworts, 6 mosses, 2 eukaryotic algae and 1 micro-fungi. viii Cyanobacteria found at Glencoban accounted for more than 50% of the 23 species recorded throughout the Mulga Lands. Seasonal precipitation strongly influenced crust cover and the responses and abundance of individual cyanobacterial crust taxa. During drought-cycles following a loss of vegetation cover, extreme irradiation and high soil surface temperatures resulted in a significant decline in crust cover. This decline was mainly attributed to cyanobacterial crust burial by sand. Porphyrosiphon notarisii returned to pre-drought levels of abundance in the year following floods and continued to increase in the seasonal drought. De-stocking during drought, motility and reproductive strategies were contributory factors to its success. Schizothrix and Stigonema were heavily impacted by drought and their recovery was retarded by the seasonal drought. During, and in the post-drought period, the bioavailability of nitrogen associated with the cyanobacterial crusts was significantly altered by sand burial. During drought, sand–covered crusts had up to three times more mineral nitrogen and twice the mineralisable nitrogen than sand–free samples. This increase was independent of the abundance and survival of nitrogen-fixing cyanobacteria. It was evident that the length of time and the depth of burial of non-motile cyanobacteria like Stigonema resulted in the disintegration and lysis of cell material. Minor precipitation events during drought contributed to both the survival of motile cyanobacteria and the break down of exopolysaccharides of those cyanobacteria buried at depth. Stock trampling exacerbated the effects of drought on cyanobacterial crust communities. Cyanobacterial cover improved markedly at distances greater than 200 m from stock water points. The diversity of crust species expanded significantly as well with distance from water, and was greatest in two years after drought. Cyanobacterial crusts within 100 m of stock water points before the drought had not recovered to pre-drought levels two years after drought. Excessive damage by stock trampling had reduced their capacity to recover in the post-drought period. In contrast, seven years after the bore drain closure the crusts had recolonised previously long-degraded bare ground. This recovery was triggered by good annual precipitation and the removal of stock pressures from the bore drain region. There were no longer any gradients found in cyanobacterial crust cover, biomass or species richness. However, there was a significant threshold of change in the crust composition at about 100 m ix from the closed bore drain. Early colonising cyanobacterial taxa accounted for over half the differences and were only found within 100 m of the bore drain, representative of a time-distance gradient. These studies have clearly shown that cyanobacteria are adapted to drought but certain species can be adversely affected by its severity. Furthermore, precipitation timing and frequency are overriding influences in crust survival and composition. Domestic stock exacerbates the effects of drought with ongoing damage to cyanobacterial soil crusts potentially lowering bioavailable nitrogen in the long-term. These studies have demonstrated that the integrity of cyanobacterial soil crusts in the Mulga Lands is closely integrated with landscape function. Cyanobacterial function and interaction withi

Mots Clés : Cyanobacterial soil crusts, biological crusts, drought, nitrogen, morphology, Mulga Lands, rangeland management, sand burial, grazing gradients, bore drains, arid and semi-arid environment


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