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Bio-strategies of mineral nutrient extraction from silicates by selected microorganisms

Periodic Reporting for period 1 - Bio-Strategies (Bio-strategies of mineral nutrient extraction from silicates by selected microorganisms)

Reporting period: 2015-09-01 to 2017-08-31

Microorganisms are by far the most important group of organisms introducing metal nutrients into the life cycle by extracting them from minerals. After the microorganisms assimilate the metals or dissolve them, other organisms can also have access to them. The Bio-Strategies project addresses two important aspects of these processes:

1) How do microorganisms adapt to the mineral environment where they live in order to retrieve the mineral nutrients they need? As minerals have different chemical composition and resilience to chemical attack, do microorganisms use different strategies in different environments to attack the minerals and obtain the desired metals? Particularly, how do the strategies change as the minerals become progressively depleted of metal nutrients?

2) How does the release of metal nutrients by microorganisms help to introduce these metals into the life cycle? Plants are obvious beneficiaries from the activity of microorganisms. How much of the metals released by microorganisms become immediately available to plants?

These questions are central to understanding how metal nutrients are incorporated and circulate in the food chain, which is necessary in order to maintain healthy natural habitats and productive agricultural soils. For this end, it is required to progress in the understanding of the influence of minerals on microbial activity and colonization patterns, to monitor and model soil genesis and to understand the symbiosis of plants and microorganisms. With this knowledge, it will be possible to use low-cost and efficient environmental, agricultural and industrial processes based on mineral-microbe interaction and plant-fungus symbiosis.

The objectives of the project are:

1) To reveal he microbial habitability of a series of silicate minerals of different metal nutrient content, which represent soils in different stages of degradation.
2) To investigate the microbial strategies for chemical attack and extraction of metal nutrients by a bacterium (Bacillus subtillis) and microbial fungus (Aspergillus niger), both of which are abundant in soils.
3) To determine the availability of the metal nutrients released by the microorganisms to plants, particularly liverworts, with and without symbiotic fungi.

The conclusions of the action cannot be yet described completely as some aspects are still under evaluation. The mineral substrate had a large effect in the activity of the microorganisms, greater for the fungus than the bacterium. This effect is observed in the intensity of the attack on the minerals, and the type of attack is now under evaluation. Metals released by the microorganisms were available to the plants, but in different degrees depending on mineral and microorganism, which affected plant growth.
All experimental work has been completed: experiments of Aspergillus niger (fungus) and Bacillus subtillis (bacterium) on lava, biotite, vermiculite and kaolinite, with media containing no metal nutrients; chemical analysis of the media after the experiments for metals and organic substances that dissolve and stabilise metals in solution; electron microscopy investigation of dissolution patterns of the minerals; chemical analyses of the biomass; experiments of plants (liverworts) with and without a fungus symbiont growing on the media from the mineral dissolution tests; chemical analysis of the plants and measurement of their development.

The results have shown that the mineral substrate can have a tremendous effect on the intensity of the chemical attack of microorganisms. Aspergillus niger (fungus) caused severe leaching of vermiculite, in which certain areas of the mineral were depleted of all metals except silicon. Pending further analysis of the results, this severe attack is considered to be due to the low potassium content in vermiculite, an important metal nutrient. Other minerals caused a more moderate attack due to greater availability of metals, or microbial inactivity due to the lack of metal nutrients in the mineral necessary for the proper development of the microorganisms. Aspergillus niger (fungus) always proved more aggressive than Bacillus subtillis (bacterium). Further analysis is aiming at investigating whether the type of attack also changed in the several conditions.

Metal nutrients were typically available to the plants after the microbial attack, although in different proportion and allowing different plant development.

So far, the results have been disseminated as follows.
Professional audiences:
Oral contribution in the International Clay Conference held in Granada, Spain, 17-21 July 2017. “Experimental investigation of mineral control on the microbial mode of attack on mineral surfaces for the obtention of inorganic nutrients”.
Talk at IMIDRA (Institute for Farm and Food Research and Development), Alcalá de Henares, Madrid, Spain. 21 of June 2017. “Bio-Strategies”.
Seminar at the Natural History Museum: “Microorganisms as extractors of nutrients: from clays to plant“. 28 of June 2017.

Public audiences:

Interactive talk of the Nature Live cycle at the Natural History Museum. “Microbes munching minerals”. 3 of February 2017.

Scientific articles are in preparation.
The investigation has produced a novel and surprising result: the very aggressive attack of Aspergillus niger (fungus) to vermiculite resulting on a complete depletion of all metals except silicon. In spite of the extent of the metal leaching, however, the mineral did not show visual signs of breaking down, from where it is assumed that the leaching was rather superficial. This is a form of microbial attack not yet reported. Further analysis is being carried out to describe and quantify this process fully.

Ongoing work is quantifying the results described above.

The results from this project contribute to the fundamental understanding of the acquisition of metal nutrients by microorganisms and their immediate bio-availability. Such knowledge is essential in order to develop applications in relation to the preservation of natural habitats, efficient and sustainable agriculture and new, low-cost industrial processes that involve mineral-microorganism interaction for mining, transformation or environmental remediation. The development of such applications were not within the scope of this project but they will obviously benefit from this project’s results.
Electron microscopy image of a vermiculite flake with bacterial biofilm
Bacillus subtillis growing on kaolinite
Aspergillus niger growing on vermiculite
Electron microscopy image of crystals generated by Aspergillus niger on vermiculite