Revealing the microalgal response to rare earth elements

Rare earth elements (REEs) are metals used in many products of modern technology, such as illuminated screens, magnets for electric motors and computers, high-performance metal alloys, or lasers. Surging demand for high-tech products has led to increased waste containing REEs, particularly electronic waste, mine tailings and mine drainage. Thus, REEs are an emerging contaminant in soil and water, posing a threat to environmental and human health. There is a need for sustainable technologies to recover REEs from waste streams and incorporate this resource into a circular economy. This may be achieved by utilizing microalgae. They are unicellular organisms that perform photosynthesis and can be found in almost any natural body of water, including extreme environments exposed to high temperatures (50-60C) and acidity (pH as low as 0). Organisms that thrive under such conditions are called extremophilic. The potential of extremophilic microalgae to recover toxic metals from waste, such as cadmium or lead, has been recognized, and can be partly explained by their intrinsic mechanisms to withstand harsh conditions. Yet, the processes by which microalgae accumulate REEs, as well as the physiological impact of these metals, are still poorly understood. In particular, the impact of a planktonic (suspended, freely floating) or a sessile (attached to a solid surface as a biofilm) lifestyle on the response to REEs is not known. Notably, REEs had been thought to lack any biological function in living organisms, until their importance for the metabolism of soil-dwelling bacteria was discovered a mere decade ago. The scope of this project is to elucidate the mechanisms of transport, accumulation, and detoxification of REEs as well as the stress response to these metals in microalgae. The extremophilic red alga Galdieria sulphuraria will serve as a model organism. Algal cultures, both planktonic and sessile, will be exposed varying concentrations of REEs to establish a dose-response relationship under the different growth conditions. Transcriptomics and proteomics, showing which genes are expressed into which proteins, are at the core of the analytical toolbox used in this project. The gained insights will not only extend our knowledge of the impact of REEs on microalgae in the environment but facilitate the development of sustainable processes for the recovery of these critical metals from waste.

Rare earth elements, the so-called lanthanides, are crucial components of modern technologies such as batteries, wind turbines, smartphones, or catalysts. To make the production of these metals more sustainable, methods are being sought to recycle lanthanides from aqueous waste streams. This project aimed to investigate the role that microalgae - microscopic organisms that utilize light and carbon dioxide to grow - can play in this process, and what considerations must be taken into account during experiments on this topic. Acid-loving (so-called acidophilic) microalgae thrive in extreme conditions. The environment in which they grow is as acidic as lemon juice and often contains very high concentrations of dissolved toxic metals. Could this resilience be beneficial in sorbing and concentrating lanthanides? The results of this project show that acidophilic microalgae can indeed thrive at high concentrations of rare earth elements, but under optimal growth conditions, they sorb minimal amounts of lanthanides. These do not even enter into the cell but are rejected at the outer cell membrane. For efficient sorption, the pH level must be increased, reducing the acidity, so that the charge of the cell wall and the metals are opposite, causing stronger attraction between them. Although microalgae also grow at an increased pH level, under these conditions, insoluble lanthanide phosphate salts can form. This could lead to uncontrolled loss of metals or an overestimation of the actual uptake by the cells. It is therefore important to determine whether lanthanide phosphates are soluble under the given experimental conditions. This project demonstrated that some solution components commonly found in experiments with microorganisms and lanthanides can lead to an oversaturation of lanthanide phosphates. This means that the solution is not stable, and the phosphate salts only precipitate as solids when a crystallization seed is present. This results in inaccurate conclusions about metal uptake efficiency. Consequently, an approach for control experiments based on simulations and laboratory trials was outlined, which can make studies on the interactions between lanthanides and microorganisms more accurate. The results of this project are significant for the development of processes for the recovery of rare earth elements and, more broadly, for research on lanthanides in biological systems.