Nitrogen plays a crucial role in forming proteins, amino acids, and DNA, making it essential for cellular function and reproduction. Although nitrogen is prevalent in the atmosphere as nitrogen gas, plants and animals cannot directly utilize it. Instead, we depend on bioavailable forms like:
ammonia
and
nitrate
.
Specialized microorganisms convert atmospheric nitrogen into ammonia through a process known as
biological nitrogen fixation
. This transformation is facilitated by nitrogen-fixing soil bacteria through a unique protein complex called the
nitrogenase complex
, which is activated by a specific group of genes known as the
nif gene
.
Leguminous plants such as peanuts, peas, and beans host nitrogen-fixing bacteria in their root zones, known as the
rhizosphere
. This forms a
symbiotic relationship
, where legumes receive essential nitrogen while providing organic acids for energy to the bacteria, thus supplying most of the bioavailable nitrogen necessary for life on Earth.
Bacteria attached to plant roots “feed” nitrogen to the plant through the soil and receive organic acids for energy from the photosynthetic plant. Source: Wikimedia Commons
An intriguing aspect of the nitrogen cycle lies in the impact of soil viruses on nitrogen transformations.
Nitrogen cycle
refers to the processes by which nitrogen is converted between its various chemical forms. Viruses, which are microscopic entities made up of genetic material and proteins, cannot replicate independently and require a host cell for reproduction. Their classification teeters between living and nonliving entities.
To reproduce, a virus locates and infects a host cell, introducing its genetic material, which commandeers the host’s cellular machinery to produce new virus particles. During this process, viruses can incorporate genetic material from their host cells, resulting in a transfer of
auxiliary metabolic genes
(AMGs). This means that viruses have a mobile genetic potential that can influence ecological fitness.
Researchers from China, Spain, and the Czech Republic have concluded that viruses associated with legume roots can enhance local soil nitrogen fixation by expressing
niff
AMGs or transferring
niff
genes to nearby bacteria. Their study, which analyzed approximately 8.6 million viral genomes from diverse locations, revealed that only 0.003% of these viruses carried nitrogen-fixing genes while clustering with nitrogen-fixing bacteria.
The team identified three predominant virus families associated with nitrogen-fixing genes: Cyanoviridae, Nudiviridae, and Bronfenbrennerviridae. Notably, Cyanoviridae viruses contain the
niff
gene, which was validated as a functional genetic element using a computational tool called DRAM-v.
Soil samples taken from a cowpea field in Nanjing, China, were analyzed for nitrogen-fixing genes to ascertain whether cowpea roots had an influence on virus populations. The study compared soil from the cowpea’s rhizosphere with non-crop soil. Using a method known as
metatranscriptomics
, the researchers identified active viral and bacterial RNA, which indicated that viral
niff
genes were expressed more vigorously in the cowpea rhizosphere than in soil from non-cropped areas. Among these, approximately 96% of the
niff
gene expression was attributed to bacteria, while viruses accounted for about 4%.
Subsequently, the researchers examined whether rhizosphere viruses influenced nitrogen fixation. They established small sealed containers with sterile soil supplemented with either bacteria alone or a combination of bacteria and viruses from the cowpea rhizosphere. The nitrogen content in soil containing viruses was significantly higher (36 mg/kg) compared to that without viruses (17 mg/kg).
Finally, each container’s atmosphere was adjusted to feature different forms of nitrogen gas, with lighter nitrogen-14 and heavier nitrogen-15 isotopes. By exposing nitrogen fixers to nitrogen-15, researchers could identify which organisms contributed to nitrogen fixation through a process known as
nitrogen-15 stable isotope exploration
. Over time, nitrogen fixers incorporate heavier nitrogen-15 into their biomass, allowing for tracking and analysis of nitrogen-fixation activity.
After cultivating the bacteria and viruses in nitrogen-15 for 35 days, stable isotope probes facilitated the isolation of nitrogen fixers, detecting the virus with
niff
in the heavier biomass, which confirmed that rhizosphere viruses are capable of fixing nitrogen.
The researchers believe that rhizosphere viruses and viruses harboring
niff
AMGs could enhance nitrogen fixation in cowpea soil. Their findings indicate that while nitrogen-fixing viruses are rare, they might significantly influence soil nitrogen cycling and recommend further research to elucidate the role of these viruses in nitrogen fixation through controlled infection experiments.
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Source: sciworthy.com
