Phosphorus is a vital nutrient for all living organisms, but it is also a major pollutant that causes harmful algal blooms in water bodies around the world.
To prevent this, wastewater treatment plants often use chemical and energy-intensive methods to remove phosphorus before discharging the treated water.
However, this also means that valuable phosphorus is wasted and not recycled for agricultural use.
A team of researchers from the US has discovered a potential solution to this problem: using microalgae that can store large amounts of phosphorus as polyphosphate, a polymer of phosphate molecules.
These algal strains can be grown in revolving algal biofilm (RAB) systems, which are an emerging technology for wastewater treatment that use partially submerged rotating discs covered with algae.
The algae can take up and store phosphorus from the wastewater as they grow, and then be harvested and dried for use as fertilizer.
Isolating phosphorus-hyperaccumulating microalgae from RAB systems
DANIEL SLIM/AFP via Getty Images
The researchers isolated and characterized 101 microalgae strains from active RAB systems across the US Midwest, including 82 green algae, 9 diatoms, and 10 cyanobacteria.
They identified the strains by microscopy and DNA sequencing, and screened them for their phosphorus content.
Based on their research, they found that seven strains had at least 50% more polyphosphate by cell dry weight than a microalgae consortium from a RAB system, with the top strain accumulating nearly three times more polyphosphate.
These top phosphorus-hyperaccumulating strains include the green alga Chlamydomonas pulvinata TCF-48 g and the diatoms Eolimna minima TCF-3d and Craticula molestiformis TCF-8d, which had 11.4, 12.7, and 14.0% polyphosphate by cell dry weight, respectively.
The researchers also tested the ability of Chlamydomonas pulvinata TCF-48 g to recolonize a bench-scale RAB system containing a synthetic medium with phosphorus.
They found that the strain successfully grew on the system and recovered twice as much phosphorus from the medium as a microalgae consortium from a RAB system treating municipal wastewater.
It is a biological process that does not require chemicals or high energy input.
It can also remove other pollutants from wastewater, such as nitrogen, carbon, and metals.
It can produce valuable biomass that can be used as fertilizer or biofuel.
It can help mitigate climate change by reducing greenhouse gas emissions from wastewater treatment and fertilizer production.
However, there are also some challenges and limitations that need to be addressed. For example, the optimal conditions for algal growth and phosphorus uptake may vary depending on the strain, the wastewater composition, and the environmental factors.
The algal biomass may also contain contaminants or pathogens that need to be removed or inactivated before use. The algal biofilm may also be susceptible to grazing or infection by predators or parasites.
Moreover, the economic feasibility and scalability of using microalgae for phosphorus recovery and reuse need to be evaluated and compared with other alternatives.
Future directions and implications
The researchers suggested that the isolated phosphorus-hyperaccumulating microalgae may have broad applications in resource recovery from various waste streams, including improving phosphorus removal from wastewater.
They also proposed that further studies are needed to optimize the RAB system design and operation, to evaluate the performance of the algal strains under different conditions, and to explore the potential uses and benefits of the algal biomass.
The discovery of new algal strains that can store large amounts of phosphorus as polyphosphate could help fix the broken phosphorus cycle and reduce the environmental impact of wastewater treatment.
By using microalgae to recover and reuse phosphorus from waste streams, we could also conserve the finite and unevenly distributed phosphorus reserves and ensure food security for the future.
