Bacteria exhibit an astonishing metabolic diversity, which exceeds that of all animals, plants, fungi and higher organisms. Their invisible, and often overlooked activity, sustains all of the life that we can see as bacteria contribute, on a global scale, to all of the Earth’s life-sustaining natural chemical cycles.
The Winogradsky column is a simple device for culturing environmental bacteria and is an elegant means of demonstrating their vast diversity and complex interactions. Invented in the 1880s by Sergei Winogradsky, the device comprises, a column of pond mud that has been fortified with a carbon source and a sulphur source. The column is exposed to sunlight for a period of months to years, during which aerobic/anaerobic, and sulphur gradients form. All of the bacteria in the mud column are present initially in low numbers and are thus not visible to the human eye. However, during the incubation, different types of microorganism will come to occupy distinct zones where the oxygen and sulphur gradients generate specific environmental conditions, and niches, that favour their particular growth requirements and specific activities. In these zones, particular bacteria proliferate massively to form visible and brightly coloured communities.
The large quantity of the carbon source added to the column initially promotes rapid microbial growth, which consumes oxygen, and soon depletes this gas in the sediment . The only organisms that can now grow under these newly generated anaerobic conditions are those that can ferment organic matter and those that perform anaerobic respiration, and thus grow in the absence of oxygen. One type of such bacteria are the Clostridium species which start to grow when oxygen is depleted. To grow, they must breakdown the added carbon source to gain energy, and when they do this, as a by-product of metabolism, they also produce a range of simple organic compounds (e.g. ethanol, acetic acid). Subsequently, sulphur-reducing bacteria, towards the bottom of the column will then use these simple fermentation products to facilitate their own growth. In order to do this they though, they must also use the added sulphur compound which enables them to respire under anaerobic conditions. In this process sulphur reducing bacteria generate large amounts of hydrogen sulphide and some of this gas will diffuse through the column where it allows certain kinds of anaerobic photosynthetic bacteria to grow. These bacteria called green and purple sulphur bacteria gain energy from light reactions and produce their cellular materials from carbon dioxide in much the same way as plants do. However, there is one key difference, they do not generate oxygen during photosynthesis because they do not use water a reductant for this process and instead use hydrogen sulphide. Distinct and separate zones of purple and green sulphur bacteria form because green sulphur bacteria are more tolerant of hydrogen sulphide and can grow closer to the source of production as more tolerant that purples. The sulphur compounds produced this type of photosynthesis is then recycled into hydrogen sulphide again by sulphur-reducing bacteria. In other parts of the column where the concentration of hydrogen sulphide is low, another group of photosynthetic bacteria are able to grow. These are the purple-non sulphur, which are not tolerant to hydrogen sulphide but also gain their energy from light reactions and grow in zones devoid of hydrogen sulphide. Towards the top of the column, bacteria with a more familiar type of photosynthesis grown can be found. These bacteria, called cyanobacteria are the only bacteria that possess oxygen-evolving photosynthesis like that seen in plants. In fact, there is very strong evidence that chloroplasts, the organelles in plant cells which bring about photosynthesis of plants originated from cyanobacteria. Once these bacteria begin to grow, they produce oxygen via their photosynthesis and oxygenate the top of the column. Finally, any hydrogen sulphide that diffuses, from the bottom of the column, into this aerobic zone will be used by sulphide-oxidizing bacteria allowing them to grow, and to form bright orange zones.
Whilst it is a striking and visual demonstration of bacterial diversity and activity, the Winogradsky column, is also microcosm which harbours many of the vital natural processes that bacteria carry out, and which can additional be used to predict or model the effect that anthropogenic factors, such as global warming, might have on delicate sustaining ecosystems. Immortal Worlds?, is a collaborative project between artist Jac Scott and myself, with our initial investigations being funded by an A-N New Collaboration Bursary. The focus of the project is on mapping the unseen, but vitally important world of bacteria and, particularly how climate change will impact on these organisms, which underpin all of the Earth’s many diverse and living ecosystems. We aim to create innovative and collaborative studies that will not only experimentally and critically engage art and science, but will also spark debate about our rapidly changing world. Our initial explorations have been to replicate natural microbial ecosystems from important environments like salt marshes, wilderness areas, and various water courses, and then to mimic the predicted effects of global warming, like increased temperature, in the laboratory, and finally to observe the outcome. These images are from microbial ecosystems that have been established from a salt marsh in Blakeney, Norfolk. They are nearly a year old now but still continue to develop. One set of ecosystems has been incubated at temperatures that we might encounter today, and the others at a higher temperature that might be the outcome of global warming. The differences in the health and diversity of the ecologies is both striking and frightening. The low temperature systems are vibrant and dynamic, and have all of the wonderful complexity of the Windogasky column as describe above.
However, in the high temperature system this complex ecology has been dramatically disturbed, as the higher temperatures inhibit the growth of vital bacterial species. This system is far less complex, and is dominated by anaerobic and hydrogen sulphide producing life. In this dark ecology, there is no sink for the hydrogen sulphide, as it remains unused by the photosynthetic sulphur bacteria, and so it reacts with any iron in the column to form the black and noxious compound hydrogen sulphide.