Across the globe surface and groundwater are impaired by extreme variations in pH, and metals outside of the toxicity tolerance limits of humans and native organisms. Often times these, and other similar locations are classified as "extreme" environments and the organisms that inhabit them are denoted as "extremophiles." However, these are the organisms that lay the groundwork for improvements and in the treatment of acidic and metal-rich waters.
Mine drainage originates underground with acidic, conditions that can dissolve metals such as; iron, copper, and aluminum in oxygen deplete conditions. These are favorable environmental conditions for "extremophilic" bacteria to survive and thrive. Although some argue about the lineage, timeline, and adaptations of these extremophilic organisms, the conserved gene lineage (preserved to certain bacterial groups) gives a stable background for their role throughout Earth's history. These organisms may represent some of the most primitive organisms to live on Earth, but are now restricted to these niche environments. The laws and regulations surrounding mine drainage treatment requires an extreme shift in chemistry which is essential for treatment to meet the legal requirements of a neutral or more normalized pH coupled with lower levels of dissolved metals. These changes are a direct result of the reduced anoxic system entering an oxidized environment. When this process occurs, even treatments not designed for biological parameters experience a variety of biological changes, and provide habitats for organisms which can survive in these extreme conditions.
These biological changes happen within inches of one another. The anoxic water is full of sulfate reducing bacteria, and organisms which are acidophilic (well-adapted to a low pH.) These organisms use metals as electron donors to generate energy and conduct the process of electron transfer. These organisms also survive in limited nutrients with limited carbon inputs, and very acidic conditions from which these organisms have to protect themselves. Between the formation of stronger cell walls and proton pumps these organisms are highly adaptable to even drastic changes in pH. However, these adaptations very energy intensive. Additionally, based on thermodynamics and kinetics, oxygen is a strong electron donor, and when it is available is the preferred electron donor for any organisms which can survive in its presence. The atmospheric oxygen also begins to react with the dissolved metals and begins oxidizing them on contact. As you move down the stratified layers of a treatment you will find small communities using alkalizing agents such as limestone to amend the chemistry for a community, allowing for a favorable electron transfer to obtain iron and other essential metals from the system.
In limestone ponds, bacterial communities are so reliant on the limestone that it forms biofilm scaling on the limestone surface, which is often either curved by the addition of compost or regular scouring. You can imagine that the limestone or other alkalizing agents work almost like a magnet, drawing the reduced metals in, and oxidizing them on the surface. Due to the raise in pH, the oxidized metals cannot redissolve into solution, and therefore precipitate out which is how the treatment works. Along these limestone substrates, the pH is raised often by 1000 to 100,000 fold, which is a large shift from the subcutaneous environment.
Biologically, this change is denoted by changes in the diversity of the populations. These populations change from sulfate reducing, acidophilic bacteria, to iron-oxidizing organisms and aerobic bacteria. The community shifts furthermore to the quick growing bacteria with fewer energy-intensive adaptations, and even to higher vascular plants at the tail end of the treatment. It is also important to note that even at the source of a pH 2.7 discharge cyanobacteria can be seen growing, as demonstrated in the picture above. Many treatment operators have experienced these growth patterns of algae and curse them out to high heaven for clogging the drainage systems. On the contrary, these organisms are essential to the required oxygenated environment. Therefore, these systems are replicating the same natural microbial processes that occur over miles of stream within a confined space. Thus allowing for better remediation of mine drainage. This means allowing more substrate for the biological growth, such as nutrient-rich compost additions, and it means that the photosynthetic organisms, although often overlooked may be crucial for improved success. Moreover, it is essential that the biological community is not only understood, but also utilized. These organisms help to complete intermediate processes, and biological treatments have been shown time and time again to improve the treatment efficiency overall, and reduce the maintenance requirements when spread throughout a compost substrate, which is especially beneficial when experiencing excessive metal loads.