Novel hydrocarbon-utilizing soil mycobacteria synthesize unique mycocerosic acids at a Sicilian everlasting fire

Soil bacteria rank among the most diverse groups
of organisms on Earth and actively impact global processes
of carbon cycling, especially in the emission of greenhouse
gases like methane, CO2 and higher gaseous hydrocarbons.
An abundant group of soil bacteria are the mycobacteria,
which colonize various terrestrial, marine and anthropogenic
environments due to their impermeable cell envelope that
contains remarkable lipids. These bacteria have been found
to be highly abundant at petroleum and gas seep areas, where
they might utilize the released hydrocarbons. However, the
function and the lipid biomarker inventory of these soil mycobacteria
are poorly studied. Here, soils from the Fuoco
di Censo seep, an everlasting fire (gas seep) in Sicily, Italy,
were investigated for the presence of mycobacteria via 16S
rRNA gene sequencing and fatty acid profiling. The soils
contained high relative abundances (up to 34% of reads assigned)
of mycobacteria, phylogenetically close to the Mycobacterium
simiae complex and more distant from the wellstudied
M. tuberculosis and hydrocarbon-utilizing M. paraffinicum.
The soils showed decreasing abundances of mycocerosic
acids (MAs), fatty acids unique for mycobacteria,
with increasing distance from the seep. The major MAs at
this seep were tentatively identified as 2,4,6,8-tetramethyl
tetracosanoic acid and 2,4,6,8,10-pentamethyl hexacosanoic
acid. Unusual MAs with mid-chain methyl branches at positions
C-12 and C-16 (i.e., 2,12-dimethyl eicosanoic acid
and 2,4,6,8,16-pentamethyl tetracosanoic acid) were also
present. The molecular structures of the Fuoco di Censo MAs
are different from those of the well-studied mycobacteria like
M. tuberculosis or M. bovis and have relatively 13C-depleted
values (􀀀38‰ to 􀀀48 ‰), suggesting a direct or indirect
utilization of the released seep gases like methane or ethane.
The structurally unique MAs in combination with their depleted
13C values identified at the Fuoco di Censo seep offer
a new tool to study the role of soil mycobacteria as hydrocarbon
gas consumers in the carbon cycle.