Unveiling the Secrets of "Candidatus Nitrosocaldus islandicus"
Thriving at 73°C in Iceland's geothermal springs, this archaeon challenges our understanding of the nitrogen cycle
In the steaming hot springs of Iceland's Graendalur Valley, where water temperatures soar to 73°C, scientists have discovered a remarkable microscopic inhabitant that challenges our understanding of the nitrogen cycle. This organism, provisionally named "Candidatus Nitrosocaldus islandicus," represents a unique group of archaea that thrives where most life cannot survive.
As an obligately thermophilic ammonia-oxidizing archaeon (AOA), it performs the critical first step of nitrification—converting ammonia to nitrite—under conditions once thought too extreme for this process. The discovery and genomic analysis of this heat-loving microbe not only expands our knowledge of life's boundaries but also reveals unexpected metabolic capabilities that distinguish it from all other known ammonia oxidizers.
Grows exclusively between 50-70°C, representing true obligate thermophily among AOA.
Missing key genes present in all other AOA, suggesting novel metabolic pathways.
Ammonia-oxidizing archaea (AOA) within the phylum Thaumarchaeota are now recognized as essential players in global nitrogen cycles, particularly in extreme environments where their bacterial counterparts cannot compete 1 .
While Thaumarchaeota are found in virtually all oxic environments on Earth, from oceans to soils, the truly thermophilic varieties that flourish above 50°C are exceptionally rare. Before the discovery of "Ca. N. islandicus," only one other thermophilic AOA species growing above this temperature had been reported ("Candidatus Nitrosocaldus yellowstonensis") 2 .
What makes "Ca. N. islandicus" particularly fascinating is its obligate thermophily—it grows only within a temperature range of 50-70°C, representing a fundamentally different ecological niche compared to other characterized AOA.
The genomic analysis of "Ca. N. islandicus" has revealed several surprising features:
The isolation and characterization of "Ca. N. islandicus" required innovative approaches tailored to its extreme habitat preferences and slow growth requirements. Researchers initiated the enrichment process by inoculating sterile mineral medium containing ammonium chloride with a small sample (approximately 0.1g) of biofilm collected from a hot spring in Graendalur Valley, Iceland 3 .
The spring water at this location had a temperature of 73°C and a pH of 6.5, establishing the baseline conditions for cultivation.
Researchers found that when the pH dropped below 6, the culture ceased ammonia oxidation activity, but this could be restored by readjusting the pH to the optimal range—demonstrating the organism's sensitivity to environmental conditions.
Achieving a pure culture proved challenging due to thermophilic bacterial contaminants. Solutions included:
Through these methods, researchers obtained a highly enriched culture of approximately 85% "Ca. N. islandicus."
The obligate thermophilic nature of "Ca. N. islandicus" was confirmed through systematic growth experiments across a range of temperatures. The research demonstrated that this archaeon exhibits no ammonia-oxidizing activity outside the 50-70°C range, highlighting its specialized adaptation to high-temperature environments 4 .
| Organism | Temperature Minimum | Temperature Maximum | Classification |
|---|---|---|---|
| "Ca. N. islandicus" | 50°C | 70°C | Obligate thermophile |
| "Ca. N. yellowstonensis" | >50°C | >50°C | Thermophile |
| Other thermophilic AOA | Below 50°C | 50°C | Moderate thermophile |
Physiological experiments confirmed that "Ca. N. islandicus" is an aerobic chemolithoautotroph that stoichiometrically converts ammonia to nitrite. This means it uses ammonia as its energy source, oxygen as its electron acceptor, and carbon dioxide as its carbon source, typical of other documented AOA 5 .
NH3 + O2 → NO2- + Energy
Energy used for CO2 fixation and growthHowever, its mechanism for accomplishing this appears to involve some unique aspects, particularly regarding the surprising absence of the nirK gene.
Perhaps the most intriguing genomic finding was the absence of the nirK gene in "Ca. N. islandicus." This gene, which encodes a copper-containing nitrite reductase that produces nitric oxide (NO), is present in all other known AOA and has been considered essential for their ammonia oxidation pathway 6 .
The researchers demonstrated that despite this genetic absence, "Ca. N. islandicus" still appears to require NO for ammonia oxidation, as its activity was inhibited by the addition of an NO scavenger.
This paradox suggests several possibilities:
Comparative genomic analysis revealed that "Ca. N. islandicus" possesses metabolic capabilities not found in other AOA:
| Feature | Function | Significance |
|---|---|---|
| Indolepyruvate oxidoreductase (iorAB) | Aromatic amino acid fermentation | Not present in other AOA; may provide alternative energy source |
| Type 3b hydrogenase | Hydrogen metabolism | Potential for energy generation from hydrogen |
| Missing DNA polymerase D | DNA replication | Suggests different replication mechanism from other Thaumarchaeota |
The presence of these unique genetic features indicates that "Ca. N. islandicus" has metabolic versatility that may allow it to utilize alternative energy sources when ammonia is limited, potentially explaining its success in nutrient-fluctuating geothermal environments.
The cultivation and analysis of "Ca. N. islandicus" required specialized research solutions tailored to its extreme habitat and metabolic requirements:
| Reagent/Solution | Composition/Type | Function in Research |
|---|---|---|
| Mineral medium | Defined inorganic salts | Base growth medium mimicking natural conditions |
| NH₄Cl | Ammonium chloride | Ammonia source for energy generation |
| Nessler's reagent | K₂HgI₄ - KOH solution | Ammonium concentration detection |
| Nitrite/nitrate test strips | Merckoquant test strips | Nitrite formation monitoring |
| NaHCO₃ | Sodium bicarbonate | pH adjustment and carbon source |
| Spiramycin | Macrolide antibiotic | Selective inhibition of bacterial contaminants |
| FISH probes | 16S rRNA-targeted fluorescent probes | Microbial community composition monitoring |
The research employed sophisticated molecular techniques to characterize "Ca. N. islandicus," including:
Researchers tracked microbial growth and activity through:
The discovery and characterization of "Candidatus Nitrosocaldus islandicus" represents more than just the addition of another organism to the tree of life. It fundamentally challenges our understanding of how archaea perform essential biogeochemical processes under extreme conditions. The unique genomic features of this organism—from its missing nirK gene to its unusual DNA replication machinery—suggest that thermophilic AOA may have evolved distinct mechanisms for ammonia oxidation and cellular maintenance.
From an ecological perspective, "Ca. N. islandicus" and its relatives play crucial roles in nitrogen cycling in geothermal environments, where they likely influence the availability of nitrogen compounds for other microbial communities. Their ability to oxidize ammonia at high temperatures may also have biotechnological applications, particularly in wastewater treatment processes where thermophilic nitrification could offer advantages.
Perhaps most importantly, the study of "Ca. N. islandicus" reminds us that microbial diversity holds many surprises yet to be uncovered. As research continues on these enigmatic heat-loving nitrifiers, we can expect to gain deeper insights into the evolutionary adaptations that enable life to thrive in Earth's most extreme environments.