60th Annual Report on Research 2015

This project investigates the role of the coenzyme F430 in methane oxidation by anaerobic Archaea in anoxic marine sediments.  The coenzyme initiates methane generate by related methanogenic archaea, and this work tests the hypothesis that F430 is also a biochemical gateway to methane oxidation.  The study has focused on cores from the Santa Monica Basin, and from Hydrate Ridge, to document the distribution patterns of coenzyme F430 and Archaeal lipids in association with microbiological and geochemical data documenting methane oxidtion. ANME-2 was the most dominant ANME group in the Santa Monic sediment unlike, and ANME-1 taxa are most prominent in Hydrate Ridge samples.  Large differences in ¹³C/¹²C patterns were observed between the two sites, with F430 from the Santa Monic sediment much more depleted in ¹³C (-69‰ to -90‰) than at Hydrate Ridge.  At both sites, F430 is enriched in ¹³C than archaeol and hydroxy-archaeol. The collective evidence supports our interpretation that F430 is associated with methane oxidation, although the carbon in this structure is also derived from dissolved inorganic carbon (DIC).   We estimated the proportion of carbon sources assimilated into lipids and F430 from methane or DIC (f_CH4, f_carb) using isotopic mass balance calculations, after first solving for fractionation during biosynthesis (α_F430/CH4, α_F430/carb; ε= [α -1]).

d¹³C _F430 = f_CH4(d¹³C _CH4 α_F430/CH4 + ε_F430/CH4) + (1-f_CH4)(d¹³C _carb α_F430/carb+ ε_{F430/ carb})

Following up the natural abundance study, we next set up a series of incubation experiments using live mud and ¹³C labeled DIC and CH₄. Four different experiments were set up with the mud; ¹³C labeled methane and unlabeled DIC; unlabeled Methane and ¹³C labeled DIC; ¹³C labeled acetate with an N₂ CO₂ headspace to stimulate methanogenesis; ¹³C labeled DIC with no additional methane.  The experiments set up in 2015 using Santa Monic basin mud, complimented a previous set of experiments with Hydrate Ridge sediment.  The results of these experiments indicate mixed importance of methane and inorganic carbon uptake at the two sites, highlight differences between ANME-1 and -2 taxa, and are consistent with the natural abundance isotope data.

Results from the Santa Monica core and the mass balance model were presented at IMOG 2015 in Prague.  We anticipate at least two manuscripts on research supported by this award will be submitted during summer, 2016.

Presentation

Bird, L.R., Fulton, J.F., Dawson, K., Orphan,V.J. & Freeman, K.H 2015. Carbon Isotopic heterogeneity between ANME biomolecules. 27^th International meeting on organic geochemistry, Prague, Czech Republic.

Figure 1 (below) Resent results from the Santa Monic Basin. F430 is more depleted in ¹³C than the previous sample site (Hydrate Ridge) but are still more enriched than Archaeol and Hydroxy-Archaeol