The next source of eco-friendly fertiliser: purple bacteria?

By Oliver Morrison

- Last updated on GMT

Purple bacteria are found in a variety of shallow environments such as estuaries, salt marshes and hypersaline salterns. Image: Getty/wallix
Purple bacteria are found in a variety of shallow environments such as estuaries, salt marshes and hypersaline salterns. Image: Getty/wallix

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Biomass made from a species of purple bacteria typically found in marine sediments, seawater pools and mud flats is an ‘excellent nitrogen fertiliser’, according to research published in the journal Sustainable Agriculture.

Increasing agricultural output using nitrogen-rich fertilisers is a common practice that is expected to increase in the coming decades as global food demands rise. As this happens, the ecologically harmful side effects of overusing common inorganic nitrogen fertilisers are also expected to skyrocket, including substantial greenhouse gas emissions, contaminated ground water, and poor soil quality.

On the other hand, organic fertilisers such as compost and manure have much less nitrogen and must therefore be used in greater amounts for the same effect on plant growth. This leads to excessively salty soil, which is toxic and stunts plant growth, as well as more carbon dioxide and nitrous oxide byproducts in the long run.

Led by the RIKEN Center for Sustainable Resource Science (CSRS) and Kyoto University, a new study shows​ that biomass made from purple bacteria is as effective as common inorganic synthetic fertilisers but avoids several side effects that harm the environment.

The Biomacromolecules Research Team at CSRS had been searching for a natural source of nitrogen that can replace ammonia-based synthetic fertilisers and help prevent a future crisis. Purple non-sulfur bacteria (PNSBs) are known to have enzymes that help them take nitrogen from the atmosphere and incorporate it into proteins, but until now, no one has tested their effectiveness as fertilisers. To create a PNSB fertiliser for the new study, the team mashed up the purple photosynthetic marine bacterium Rhodovulum sulfidophilum and generated dried biomass from the released cellular material.

Purple bacteria are a group of photosynthetic microorganisms characterised by their ability to produce their own food through photosynthesis, but unlike plants and cyanobacteria, they perform anoxygenic photosynthesis, which does not produce oxygen.

Purple bacteria are of particular interest to scientists studying photosynthesis due to their diverse molecular characteristics and their ability to perform photosynthesis in unique ways compared to plants and algae.

Japanese mustard spinach test

Analysis showed that the nitrogen content of this fertiliser was 11% by weight, which is much higher than what is found in other organic fertilisers, including biomass made from other microbes or microalgae.

The researchers then compared how well Japanese mustard spinach grew when assisted by either inorganic fertilisers or the new PNSB biomass fertiliser.

The first major finding was that the mustard spinach could indeed absorb nitrogen from the dried biomass. Further experiments showed that the biomass fertiliser boosted plant growth just as well as the nitrogen-rich inorganic fertilisers did, at both cool and warm temperatures.

On top of that, even when the biomass fertiliser contained up to four times the amount of nitrogen, soil pH and salinity remained normal, similar to soil fertilised without any nitrogen.
The PNSB biomass fertiliser has a low carbon to nitrogen ratio and the nitrogen is released for plant use relatively slowly compared with inorganic fertilisers—about 60% in 30 days. Although this means that twice as much biomass fertiliser will be needed for similar crop growth, the big upside is that carbon dioxide and nitrous oxide emissions will be lower, and less nitrogen will be dumped into the environment through leaching. “In the long term this could revolutionise agriculture and mitigate its negative impact on the environment,” claimed Morey-Yagi, one of the study’s lead authors.

Read more: Why the hype surrounding RNAi for crop protection?

Although the basic experiments prove the effectiveness of the biomass fertiliser, the authors stress that results are preliminary and other factors will eventually need to be taken into consideration. “A life-cycle assessment of this fertiliser will be essential for evaluating its environmental footprint across production, storage, application, transport, and disposal,” said lead author Keiji Numata. Additionally, how to scale-up the biomass production process must be considered, and shelf-life must also be established.
The researchers are optimistic that these challenges will be met and that their discovery will help make fertilisers more eco-friendly also help prevent future problems with supply-chain distribution of inorganic fertilisers. Because the biomass fertiliser is produced using carbon dioxide and nitrogen from the air, they have named it Air Fertiliser and have registered it for use as an organic fertiliser in Japan as a product marketed by Symbiobe Inc.

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