Featured image caption: Fungi, like Penicillium, produce spores in order to reproduce. Here, they are visualized under a scanning electron microscope. Credit: EMSL; Source: Flickr
Original Paper: Xu, X., Hao, R., Xu, H. and Lu, A., 2020. Removal mechanism of Pb (II) by Penicillium polonicum: immobilization, absorption, and bioaccumulation. Scientific Reports, 10(1), pp. 1-12. Link to article
You may be familiar with the dreaded mold, Penicillium, whose blue, fuzzy growth can render food inedible. But Penicillium species, such as P. polonicum, are not pure villains that are solely determined to wreak havoc on your rations! This charismatic fungus actually has remarkable properties, such as the ability to remove toxic heavy metals from its environment. A team of researchers from China recently cast a spotlight on a strain of Penicillium polonicum to determine efficiency and exact mechanisms of lead removal. The ability of this fungus to reduce the amount of lead in its immediate surroundings makes it a key contender for bioremediation, the process of using organisms or their natural products to reduce environmental contaminants. Lead (Pb(II)) pollution in the environment results from a variety of sources, but contamination into soil, air, and unfortunately, even our drinking water, is mostly a result of industrial processes and leaks. Air and water movement result in far-reaching consequences for people even far away from the point of contamination. According to the EPA, lead exposure, especially over time, can have serious health consequences for both children and adults.
The story of P. polonicum recounted by this scientific paper begins as the researchers exposed this fungus to a solution containing lead nitrate. This is a stressful environment for the fungus, so it is very keen to remove the lead as quickly and efficiently as possible. The scientists tracked the concentration of lead in the solution over time to determine how much the fungus was eliminating. In addition, they took it a step further by using microscopy to investigate how the physical structure of the fungus changed due to the environmental stress. Finally, three mechanisms of Pb(II) removal by P. polonicum were explored in detail.
Lead Induced Some Changes in Physical Characteristics of Penicillium polonicum
Upon encountering this stressful environment filled with lead ions, P. polonicum got right down to business. After 12 days, the fungus had removed 90.3% of the lead from the solution! However, it was not without some costs that became apparent as the days passed. When compared to the control fungus that was not exposed to lead, the fungus exposed to lead had slightly higher dried biomass at the beginning of the experiment. This has been reported previously; heavy metals initially stimulate some fungi to grow as they actively work to remove the threat. However, as time progressed, P. polonicum with the lead treatment had consistently lower dried biomass than the control. This signals that the toxic metal did have an adverse effect on the fungal growth rate. In addition, microscopy of the hyphae (branching strands by which the fungus grows) displayed a smaller diameter when P. polonicum was exposed to lead. The scientists also noted that there were lead-containing minerals intertwined in the hyphae of the treated fungus, presumably negatively impacting growth and development.
The Three Mechanisms of Lead Detoxification by Penicillium polonicum
Imagine being able to sweat specialized chemicals to break down environmental contaminants. This is similar to what is occurring in P. polonicum exposed to lead. The fungus begins to excrete organic acids outside of its cells, and through complex processes, the chemicals convert lead to less toxic lead-containing compounds. The reduction of toxic heavy metals in the immediate environment enables the fungus to survive and grow.
In addition, the fungus has defense if lead ions reach the cell wall. The cell wall contains negatively charged groups that, under normal circumstances, will bind to positively charged ions such as calcium or magnesium. However, if lead is present in the solution, the fungus will bind this toxic metal to the cell wall instead of calcium/magnesium. The scientists found that the reduction in calcium uptake when exposed to lead is staggering: P. polonicum treated with lead absorbed over 3 times less (3 mg/g) calcium than the fungal culture without lead (10.9 mg/g). The fungus will precipitate the bonded lead in the form of minerals, and the toxic ions are then no longer attached to the cell wall. This strategy does have a cost, however. P. polonicum needs both calcium and magnesium for essential biological processes, so binding lead instead will result in a delay or temporary halt of these crucial activities.
There is one more facet to this intriguing story. P. polonicum has evolved a mechanism whereby it tolerates some build-up of lead *inside* the cells too. The scientists continued their use of precise instrumentation and photography to visualize lead-containing minerals within the cell. They theorized that the fungus could sequester lead ions inside the cell and quickly reduce these ions to less toxic forms.
Now, let’s summarize what this fungus can accomplish when exposed to lead. It can continue to grow! It can excrete specialized organic acids to neutralize the threat! It can bind the toxic ion to its cell wall, can tolerate build-up inside its cells, and can create and precipitate entirely new compounds! Based on these mechanisms to confront lead contamination, P. polonicum seems like a prime candidate for use in bioremediation. However, like with all things in life, there are some drawbacks. To grow and survive, fungi need moisture, nutrients, and sometimes very precise environmental conditions. If one or more of these components are missing, bioremediation may not take place. Additionally, this process can be slow. Recall that there was some damage to the fungal hyphae because of lead exposure. If the lead present at the contamination site exceeds P. polonicum’s tolerance levels, the fungal cells may begin to die rapidly, and removal efficiency will be dramatically decreased. Balancing these drawbacks are some advantages, including cost-efficiency and environmental friendliness. Initial large masses of fungal growth can be acquired in laboratories with simple media, and additionally, this type of bioremediation will not require substantial inputs of energy to operate. Representatives from Kingdom Fungi are an excellent way to use nature itself to clean the environment, and I hope you will give that pesky blue mold a slight amount of appreciation next time you happen upon it.