Thawing permafrost: Another step towards assessing the consequences

Thawing permafrost: the hidden threat

Climate change is a pressing global issue with far-reaching consequences, the extent of which we are still far from understanding. One such effect is the thawing of permafrost. Permafrost is a layer of soil that has remained frozen for at least two consecutive years and is found in high latitude regions, such as the Arctic. It acts as a massive carbon sink, storing an estimated 50% of the world’s soil carbon^​1,2​, which is almost double the amount of carbon in the atmosphere^​3​. The effects of thawing permafrost are far-reaching, affecting the Arctic ecosystems as well as the global climate. As global temperatures rise, permafrost begins to thaw, releasing stored carbon that is decomposed by resident microbial communities into carbon dioxide and methane; potent greenhouse gases that further accelerate global warming^​4,5​.

Enzyme latch hypothesis: Polyphenols controlling the carbon cycle in peatlands

The enzyme latch hypothesis suggests that when there is insufficient oxygen in wetlands, the enzymes that break down organic matter are slowed down and thus carbon is stored. Polyphenols are a chemically diverse and abundant group^​6​ which are decomposed by oxygen-requiring phenol oxidases^​7,8​. Saturated soil conditions following permafrost thaw would render phenol oxidases inactive and ultimately stop microbial carbon decomposition to stabilise soil carbon. Several studies have found conflicting support for parts of the enzyme latch hypothesis^​9–12​ and the possibility of alternative microbial metabolic strategies is so far unexplored.

Beyond phenol oxidases

There is some evidence against phenol oxidases being the only enzymes that degrade polyphenols. Plants have been producing polyphenols for hundreds of millions of years^​13,14​, and microbes that are frequently exposed to them have probably evolved different mechanisms to utilise or resist them^​15​. Moreover, in the human gut other microbial enzymes have been discovered that degrade various polyphenols even in the absence of oxygen^​16​. 

New insights from Stordalen Mire 

A group of researchers from the USA and Australia led by Kelly C. Wrighton at Colorado State University, USA, have revisited the basic ideas of the enzyme latch hypothesis and investigated how microbes in peatland soils transform polyphenols^​17​. Stordalen Mire is an Arctic peatland, where natural thaw has created three different habitats: intact permafrost palsa, partially thawed bog, and fully thawed fen (see figure). The research team obtained paired genome-resolved metatranscriptome, metabolite and geochemical data to provide a new perspective on polyphenols in the microbially catalysed carbon cycle in thawing permafrost.

For the metabolomic data, water-soluble metabolites were extracted from peat and analysed by untargeted UHPLC-MS/MS. Metabolite annotation was initially performed using an internal database and extended using SIRIUS^​18​, CSI:FingerID^​19​ and CANOPUS^​20​.

Seamless Integration of Spectral Library Search

In SIRIUS 6 we offer smooth integration of your long-established annotation workflow of in-house suspect libraries with comprehensive structure annotation by SIRIUS. No more tedious manual merging – we’ve streamlined the process to enhance your workflow. Identification results will be ranked by CSI:FingerID score and spectral library hits will be added to provide you with a comprehensive view of your data.

The talented microbiome

Summarising the extensive results of the study, it is clear above all that the enzyme latch theory is not a suitable description for the dynamics of carbon processing in the Stordalen Mire. The microbial community has adapted its gene expression strategies to the habitat and the oxygen conditions prevailing there. Phenol oxidases alone do not control the fate of polyphenols and the presence or absence of oxygen is not an on/off switch for polyphenol degradation. Rather, polyphenols are integrated into a larger carbon degradation network, including a variety of polyphenol metabolic processes. The decomposition of polyphenols into smaller phenols can even boost carbon dioxide and carbon monoxide production.

Assessing the consequences

The biggest question arising from this study is how these findings will affect carbon storage. We cannot assume that carbon in thawing permafrost will remain stored due to a lack of oxygen. Addressing the challenges posed by thawing permafrost requires a concerted effort to mitigate climate change. While the situation is complex, understanding the dynamics of permafrost and its relationship with climate change is essential for developing strategies to cope with its effects.