Southeast Asia | EPFL Graph Search

Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainland China, east of the Indian subcontinent, and north-west of mainland Australia which is part of Oceania. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. Apart from the British Indian Ocean Territory and two out of 26 atolls of Maldives in South Asia, Maritime Southeast Asia is the only other subregion of Asia that lies partly within the Southern Hemisphere. Mainland Southeast Asia is completely in the Northern Hemisphere. East Timor and the southern portion of Indonesia are the only parts of Southeast Asia that are south of the Equator. The region lies near the intersection of geological plates, wit

Reclamation of low fertility soils using by-products and fast-growing trees

Marc Adami

The world population being increasing, a good energy production management is therefore necessary. Biomass production through short forest rotation by using fast-growing trees appears as a sensible way to limit fossil fuel uses. In Mediterranean regions of south-east Spain, soils present weakened ecosystem with low organic matter and nutrients content which are prone to irremediable degradation processes. The very low fertility of those soils usually makes them unfit to food production and so tree plantation with amendment addition seems to be a possible solution in order to use those soil and improve their qualities. In this study, a low fertility soil was exposed to the addition of two types of amendment (a compost of “Alperujo” and one of biosolid) and to the plantation of two different fast-growing trees (poplars and eucalyptus) during a time period of four months. The soil quality was determined by measuring different selected biochemical (β-glucosidase, phosphatase, dehydrogenase and urease activities), chemical (pH, electrical conductivity, organic matter, phosphate, potassium, water-soluble carbon, N-Kjeldahl, permanganate oxidizable carbon and metal extractible with CaCl2) and plants (nutrients, and trace elements) parameters. No significant differences between the different conditions were found. This was due to the very short time period, the too small doses of compost additions and the very high spatial variability due to the field conditions. It seems however that the composts had a positive effect on the chemical and biochemical parameters in the plots planted with poplars as the results presented systematically higher values for the amended plots than for the control plots (without amendment addition). This was attributed to the fact that all poplars were dead at the time of the study and so the soil was not confronted to any interference by the rhizosphere. Most of the parameters (chemical as well as biochemical) of the soil were improved in the eucalyptus plots compared to the poplar ones. This suggests the stimulation of the nutrient cycles by the tree roots, which improved the conditions for the microorganisms, increasing therefore their metabolic activities. The effect of composts in the eucalyptus plots were therefore clearly less obvious, probably due to the fact that the rhizosphere have had more time (tree planted in July 2011) to influence the soil properties than the compost (added in November 2011). Although both composts contained a non negligible quantity of micronutrients and trace elements, the extractible by CaCl2 0.01M quantities in soil (representing the available quantity for plants) did not present values too high to be toxic for plants and were all in normal ranges. The biomass development of the trees seems to have been improved by the compost of “Alperujo”, but presented a very high spatial variability. The compost of biosolids for its part did not show any difference as compared to the control. Next studies should reveal more significant differences between the plots, the soil having more time to react to the new conditions. It is however recommended to increase the addition doses of composts (double or more) in order to show better their effects

2012

Methylation of arsenic by single-species and soil-derived microbial cultures

Karen Elda Viacava Romo

Arsenic (As) is simultaneously a ubiquitous and a toxic element. Arsenic is subject to bio-transformations catalyzed by microorganisms constituting the As biogeochemical cycle. The primordial Earth was devoid of oxygen, exposing life to the highly mobile and toxic arsenite. This early contact with As is imprinted in the tree of life as arsenic resistance genes, e.g. arsenite efflux by transmembrane transporters. Arsenic bio-transformations includes As methylation catalyzed by the ArsM enzyme. ArsM attaches methyl groups to inorganic As, generating organic As in the trivalent (highly toxic species) and pentavalent (relatively innocuous species) forms. Worldwide concern about exposure to As has been raised due to the high As levels in groundwater in South-East Asia used for consumption and rice cultivation. Rice plants are grown under flooded conditions which are conducive to arsenite mobilization and uptake by roots, resulting in the presence of inorganic and methylated As in grains. Compared to inorganic As, methylarsenicals are more easily accumulated in rice grains and are implicated in rice plant disorders resulting in sterility. Methylated species are not synthesized by the plant but by soil microorganisms. To date, mainly aerobic microbial species able to methylate As as an apparent detoxification strategy, have been isolated from paddies. Thus, despite the fact that methylarsenical synthesis appears to be enhanced during anoxic conditions, the drivers and the physiological role of As methylation remain unknown in the absence of oxygen. The thesis focuses on the identification of active As-methylating microorganisms as single species and as members of anaerobic microbiomes from rice paddy soils. In the first part, five bacterial strains and two methanogenic archaea, belonging to genera identified in paddy soils, were evaluated for active arsenite methylation. While aerobic bacterial strains were sensitive to increasing As concentrations, they efficiently methylated As. Conversely, anaerobic bacterial strains were resistant to increasing As concentrations but methylated poorly. Suspecting that As detoxification was occurring through As efflux and outcompeting methylation under anaerobic conditions, we proceeded with the deletion of the arsenite transmembrane transporter in the anaerobic strain Clostridium pasteurianum. The As efflux was disrupted in the mutant leading to higher intracellular concentrations, higher arsM transcription, and increase in the methylation efficiency. Based on the results, we hypothesize that under anoxic conditions, the efficient arsenite efflux systems from anaerobic microorganisms preclude efficient As methylation. In the second part, meta-omic approaches (metagenomics, metatranscriptomics and metaproteomics) were used to identify active As methylators in two soil-derived microbiomes shown to methylate arsenite. The metagenomic data was used to reconstruct the microbial genomes present as metagenome-assembled genomes (MAG). The metabolic capacity for each MAG was identified by the functional annotation of genes while active metabolisms were deciphered from mRNA transcripts (metatranscriptome) and protein expression (metaproteome). Annotation and expression of arsenic resistance genes pointed to fermenting microorganisms as the main drivers of As methylation in both microbiomes. This work is a contribution to the understanding of the linkages between microbial diversity and arsenic bio-transformation.

EPFL2020

Methylation of arsenic by single-species and soil-derived microbial cultures

Karen Elda Viacava Romo

EPFL2020

Reclamation of low fertility soils using by-products and fast-growing trees

Marc Adami

2012