Maxime Gommeaux

Biological soil crusts in continental Antarctica: Garwood Valley, southern Victoria Land, and Diamond Hill, Darwin Mountains region

Abstract Biological soil crusts are associations of lichens, mosses, algae, cyanobacteria, microfungi and bacteria in different proportions forming a thin veneer within the top centimetres of soil surfaces. They occur in all biomes, but particularly in arid and semi-arid regions, even in the most extreme climates. They carry out crucial ecosystem functions, such as soil stabilization, influencing water and nutrient cycles, and contribute to the formation of microniches for heterotrophic life. In continental Antarctica especially, these roles are essential because no higher plants provide such ecosystem services. We provide a detailed description of biological soil crusts from Garwood Valley, McMurdo Dry Valleys region (78°S) and Diamond Hill (80°S) in the Darwin Mountains region. The coverage was low at 3.3% and 0.8% of the soil surface. At Garwood Valley the crusts were composed of green algal lichens, cyanobacteria, several species of green algae and the moss Hennediella heimii (Hedw.) R.H. Zander. Diamond Hill crusts appear to be unique in not having any species of cyanobacteria. Major parts are embedded in the soil, and their thickness correlates with higher chlorophyll contents, higher soil organic carbon and nitrogen, which are fundamental components of this species poor cold desert zone.

Mineral and Bacterial Diversities of Desert Sand Grains from South-East Morocco

Mineralogy and microbiology of sand from Merzouga (Morocco) were simultaneously characterized, with the purpose of contributing to a better understanding of the geomicrobiology of deserts. In spite of very low measured bacterial biomass, bacterial diversity on each of the five defined mineralogical classes, was found high. An original grain by grain cultivation method enabled to obtain bacterial isolates with an unusually high recovery rate. The results of this study show that the genus Arthrobacter is well adapted to this environment with a preference for grains other than the dominant mineral quartz, and that the genera Chelatococcus and Saccharotrix are strongly attached to the grains.

Assessment of new protective treatments for porous limestone combining water-repellency and anti-colonization properties

Water is known to be the main deteriorating agent of building geomaterials. Owing to the presence of water, the alteration phenomena observed particularly on limestones are transportation of soluble salts, dissolution of calcite and development of micro-organisms. In restoration works, several chemical treatments can be used, such as biocides to remove existing biofouling and water-repellents to prevent the ingress of water and its deteriorating effects. The aim here was to combine these two properties in one treatment that could be applied at the end of the restoration work and to develop treatments based on natural products and that are safer for the users and the environment. Products with water-repellent and/or anti-colonization properties were developed and tested on a bioclastic limestone both in laboratory and outdoor conditions. They were compared with commercial water-repellents and a preventive anti-colonization treatment. Samples were exposed for 2 years in a forest environment, where the hydrophobicity and the colonization were evaluated every 6 months. In the laboratory, anti-colonization effects were determined through algae growth measurements. The product that confirmed an efficient hydrophobicity and anti-colonization effect will be applied on a restored site to test it under real conditions.

Lead distribution in soils impacted by a secondary lead smelter: Experimental and modelling approaches

Smelting activities are one of the most common sources of trace elements in the environment. The aim of this study was to determine the lead distribution in upper horizons (0-5 and 5-10cm) of acidic soils in the vicinity of a lead-acid battery recycling plant in northern France. The combination of chemical methods (sequential extractions), physical methods (Raman microspectroscopy and scanning electron microscopy with an energy dispersive spectrometer) and multi-surface complexation modelling enabled an assessment of the behaviour of Pb. Regardless of the studied soil, none of the Pb-bearing phases commonly identified in similarly polluted environments (e.g., anglesite) were observed. Lead was mainly associated with organic matter and manganese oxides. The association of Pb with these soil constituents can be interpreted as evidence of Pb redistribution in the studied soils following smelter particle deposition.

Influence of dissolved organic matter and manganese oxides on metal speciation in soil solution: A modelling approach.

Trace element (TE) speciation modelling in soil solution is controlled by the assumptions made about the soil solution composition. To evaluate this influence, different assumptions using Visual MINTEQ were tested and compared to measurements of free TE concentrations. The soil column Donnan membrane technique (SC-DMT) was used to estimate the free TE (Cd, Cu, Ni, Pb and Zn) concentrations in six acidic soil solutions. A batch technique using DAX-8 resin was used to fractionate the dissolved organic matter (DOM) into four fractions: humic acids (HA), fulvic acids (FA), hydrophilic acids (Hy) and hydrophobic neutral organic matter (HON). To model TE speciation, particular attention was focused on the hydrous manganese oxides (HMO) and the Hy fraction, ligands not considered in most of the TE speciation modelling studies in soil solution. In this work, the model predictions of free ion activities agree with the experimental results. The knowledge of the FA fraction seems to be very useful, especially in the case of high DOM content, for more accurately representing experimental data. Finally, the role of the manganese oxides and of the Hy fraction on TE speciation was identified and, depending on the physicochemical conditions of the soil solution, should be considered in future studies.

Response of bacterial communities to Pb smelter pollution in contrasting soils

Anthropogenic inputs of trace elements (TE) into soils constitute a major public and environmental health problem. Bioavailability of TE is strongly related to the soil physicochemical parameters and thus to the ecosystem type. In order to test whether soil parameters influence the response of the bacterial community to TE pollution, we collected soil samples across contrasting ecosystems (hardwood, coniferous and hydromorphic soils), which have been contaminated in TE and especially lead (Pb) over several decades due to nearby industrial smelting activities. Bacterial community composition was analysed using high throughput amplicon sequencing and compared to the soil physicochemical parameters. Multivariate analyses of the pedological and biological data revealed that the bacterial community composition was affected by ecosystem type in the first place. An influence of the contamination level was also evidenced within each ecosystem. Despite the important variability in bacterial community structure, we found that specific bacterial groups such as γ-Proteobacteria, Verrucomicrobia and Chlamydiae showed a consistent response to Pb content across contrasting ecosystems. Verrucomicrobia were less abundant at high contamination level whereas Chlamydiae and γ-Proteobacteria were more abundant. We conclude that such groups and ratios thereof can be considered as relevant bioindicators of Pb contamination.

Bacterial Adaptation to Hot and Dry Deserts

The analysis of microbial diversity highlights the idea that tolerance of desiccation is more important than tolerance of temperature as an adaptive microbial trait for desert conditions. The culture-independent analysis of this microbial diversity in deserts revealed that 70–80 % of 16S-rDNA sequences do not match with described bacterial species, suggesting the presence of a majority of ‘new’ species and genera to be described and highlighting the necessity to make this biodiversity culturable, and for that we propose the use of the grain-by-grain cultivation as an efficient technique allowing the isolation of bacteria strongly attached to a sand grain surface. In Sahara samples, most of the bacterial isolates or clone sequences belong to phyla and genera for which the mechanisms of adaptation to desiccation have been described (sporulation in Firmicutes and Actinobacteria, DNA repair in Deinococcus) or remain to be unravelled (most Proteobacteria). A novel mechanism of desiccation tolerance was found for the betaproteobacterium Ramlibacter tataouinensis and involves the ability to divide as a desiccation-tolerant cyst. Surprisingly, the very low density of bacteria in desert sand is not due to a much lower diversity but to a very low number of bacteria per taxon. The main conclusion is that numerous bacterial species have been able to adapt in response to extreme conditions prevailing in hot and dry deserts.

Chlorophyll Fluorescence and Colorimetric Analysis for Monitoring the Algal Development on Biocide-Treated Stone

The present study used two non destructive technics, chlorophyll fluorescence and colorimetry to assess the Chlorella vulgaris growth on stone surface treated by experimental treatments for preventing biofouling. A protocol for laboratory tests was set up, consisting of inoculating treated slabs stone with a suspension of an algae culture (Chlorella vulgaris), that is a well known monument colonizing organism. The biofouling test was carried out under fluorescent lights for four weeks at room temperature which was 20°C and monitored by chlorophyll a fluorescence and colorimetry analysis. These techniques are rapid, non-invasive, and reliable. Treatments were introduced by mixing a tetraethoxysilane base with different ingredients: chitosan and/or silver nitrate for biocide effect and/or hydrophobic silica for water repellency. Results revealed four different patterns of algal development. The stones treated with the product containing chitosan and silver nitrate did not show any difference in their fluorescence signal as compared with the untreated stones (control). The stones treated with the product containing only silver nitrate showed biocide effect two weeks after the start of the experiment. The product containing silver nitrate, chitosan and hydrophobic silica at low concentrations completely inhibited algal development in the long term, the similar product with the same compounds at a higher concentration only delayed algal development by 1-2 weeks. This suggests that low concentrations of the products have a synergistic effect, that is lost if they occur in excess. In the present study, chlorophyll a fluorescence proved to be a valuable tool in detecting damage in the photosynthetic system of organisms and as a useful complement to other conventional measurements such as colorimetry.