Over the last millennia, wetlands have been sequestering
carbon
from the atmosphere via photosynthesis at a higher rate than releasing
it and, therefore, have globally accumulated 550 × 10
15
g of carbon, which is equivalent to 73% of the atmospheric carbon
pool. The accumulation of organic carbon in wetlands is effectuated
by phenolic compounds, which suppress the degradation of soil organic
matter by inhibiting the activity of organic-matter-degrading enzymes.
The enzymatic removal of phenolic compounds by bacterial tyrosinases
has historically been blocked by anoxic conditions in wetland soils,
resulting from waterlogging. Bacterial tyrosinases are a subgroup
of oxidoreductases that oxidatively remove phenolic compounds, coupled
to the reduction of molecular oxygen to water. The biochemical properties
of bacterial tyrosinases have been investigated thoroughly in vitro
within recent decades, while investigations focused on carbon fluxes
in wetlands on a macroscopic level have remained a thriving yet separated
research area so far. In the wake of climate change, however, anoxic
conditions in wetland soils are threatened by reduced rainfall and
prolonged summer drought. This potentially allows tyrosinase enzymes
to reduce the concentration of phenolic compounds, which in turn will
increase the release of stored carbon back into the atmosphere. To
offer compelling evidence for the novel concept that bacterial tyrosinases
are among the key enzymes influencing carbon cycling in wetland ecosystems
first, bacterial organisms indigenous to wetland ecosystems that harbor
a TYR gene within their respective genome (
tyr
+
) have been identified, which revealed a phylogenetically
diverse community of
tyr
+
bacteria indigenous
to wetlands based on genomic sequencing data. Bacterial TYR host organisms
covering seven phyla (Acidobacteria, Actinobacteria, Bacteroidetes,
Firmicutes, Nitrospirae, Planctomycetes, and Proteobacteria) have
been identified within various wetland ecosystems (peatlands, marshes,
mangrove forests, bogs, and alkaline soda lakes) which cover a climatic
continuum ranging from high arctic to tropic ecosystems. Second, it
is demonstrated that (in vitro) bacterial TYR activity is commonly
observed at pH values characteristic for wetland ecosystems (ranging
from pH 3.5 in peatlands and freshwater swamps to pH 9.0 in soda lakes
and freshwater marshes) and toward phenolic compounds naturally present
within wetland environments (
p
-coumaric acid, gallic
acid, protocatechuic acid,
p
-hydroxybenzoic acid,
caffeic acid, catechin, and epicatechin). Third, analyzing the available
data confirmed that bacterial host organisms tend to exhibit in vitro
growth optima at pH values similar to their respective wetland habitats.
Based on these findings, it is concluded that, following increased
aeration of previously anoxic wetland soils due to climate change,
TYRs are among th...