Very little
is known about the distribution of Methanogenic archaea (methanogens) in the
sub-seafloor despite methane in marine methane hydrates being mostly of
microbial origin. Methanogens are the final step in anaerobic biodegradation of
organic matter in sediments, producing methane as a by-product. Understanding
the distribution of methanogens is therefore imperative to understand the methane
hydrate formation process. Isopranl glycerol ether lipids are unique to
arcahaea and have been used as biomarkers in many studies. One group, the
archaeal polar lipids, have also been used to show the presence of living
rather than fossil archaeal biomass. However, the reliability of this technique
had been called into question with compounds used at present being more stable
than originally thought. One potentially more reliable technique is using
archaeal lipids containing a tertiary –OH group. Due to the labile nature of
this tertiary alcohol it is thought to better represent recent archaeal
activity. Oba et. al. (2014) set out to investigate whether the use of
different archaeal lipids can more reliably estimate the distributions and
populations of methanogens in two sites in the Nankai Trench.
Cores were
collected from two borehole sites and samples were frozen immediately, the
outer 10mm was then removed to limit contamination. The zones of concentrated
methane hydrates were distributed >100m below the seafloor and only found in
the sandstone layers. The sediment core samples were then analysed for total organic carbon content
using a Yanako MT-5 CHN analyser and lipids were extracted with the Bligh and
Dyer method, separated and identified by gas-chromatography.
Hydroxyarchaeols
are specific to methanogens and anaerobic methanotropic bacteria (ANME) and
archaeol is ever-present in methanogens and widespread in other archaea. Their
co-occurrence therefore does not necessarily mean they are from same
biologically source. However, the results from this study strongly suggest that
the polar lipids in the marine sediment were derived from a common producer.
Furthermore, by comparing the carbon isotope values and depth profiles of
hydroxyarcaeols and archaeols it strongly suggests that methanogens are the
source and not ANMEs. Due to the strong correlations between two types of
hydroxyarchaeol the candidate clades of methanogens are believed to be Methanoloccales and Methanosarcinales. The use of these archaeal lipids is therefore
good potential contender for assessing the distribution of methanogens in
sub-seafloor sediments. However, considering the small proportion of
methanogens in prokaryotic communities (<1%) there seemed to be a higher
than expected concentration of sn-2-hydroxyarchaeol, suggesting detected levels
were mostly fossil. This questions the validity of this lipid as a biomarker,
however, the life expectancy of this lipid is thought to be very short relative
to geological timescale. This therefore means the depositional age of deep
sub-seafloor sediments is much older than that of the life expectancy of this
lipid, making this lipid a valid biomarker for in situ methanogens.
The use of
hydroxyarchaeols and archaeols has been demonstrated here to be an effective
tool as the use of a biomarker for methanogens in deep sub-seafloor sedmients.
This will allow further research to study the processes of methane formation,
such as insight into specific methongens involved and the causes of any fluxes
in methane production as seen in other methanogen communities. It also has some
relevance to global warming, increasing methane in the atmosphere is seen as a
cause for concern. With the marine ecosystem contributing around 10% to global
atmospheric methane concentrations, understanding the methanogen communities in
the deep sub-seafloor could become a valuable area of research. This paper is a
good starting block for potential research into the deep sub-seafloor
methanogen communities and provides some good preliminary insight. However, it
may be worth conducting analyses such as PCR or FISH on core samples as well as
the analysis carried out here to be more certain about the conclusions made.
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