Reply to Schouten et al.: Marine Group II planktonic Euryarchaeota are significant contributors to tetraether lipids in the ocean

Schouten et al. (1), referencing Lincoln et al. (2), question whether Marine Group II Euryarchaeota (MG-II) contribute significantly to glycerol dibiphytanyl glycerol tetraether (GDGT) lipids in the ocean. We respond below to what we believe are some mistaken claims and interpretations.

Schouten et al. (1) claim that “… according to Lincoln et al.’s definition, all SPM [suspended particulate matter] samples <100 m do not contain sufficient archaeal reads … to draw any conclusion.”

Archaeal DNA in our study (2) was never below detection limits. As expected, the proportion of archaeal DNA to total DNA was lower at shallow depths (tables S1 and S2 in ref. 2). Archaeal biomass at 83 m, however, where total cell numbers are greater and euryarchaea dominated (table 1 in ref. 2), was comparable to that in deeper waters, as indicated by GDGT yields (figure 2 in ref. 2).

Schouten et al. claim that “Core lipids do not occur as such in living cells, where they contain polar sugar and phospho head groups…. Thus, by definition core lipid GDGTs are derived from dead material…” (1) and “…monohexose GDGTs are also poor tracers of living archaeal cells… because they have a turnover time in the order of thousands of years (3), de facto also representing dead material” (1).

As constituents and products of living cells, core GDGTs are suitable tracers of archaeal biomass in the oxic water column. Recently, significant amounts of core GDGTs were found in living cultures of marine Thaumarchaeota (4). Moreover, the short turnover time (days) of cells in waters we sampled indicated that the lipids detected came from living cells (2).

To our knowledge, there are currently no published turnover times for tetraether lipids—intact polar lipid or core—in seawater. The relevance of deep biosphere processes (3) cited by Schouten et al. (1) for the oxic, sunlit surface waters of the North Pacific Subtropical Gyre that we studied (2) is questionable.

Even if GDGTs at 83 m did originate from “dead” material, what was their origin, if not planktonic euryarchaeal cells? Metagenomic data (table 1 and figure S6 in ref. 2) clearly showed that Marine Group II Euryarchaeota were consistently the dominant archaea in this habitat over a 5-y period.

Curiously, after questioning Lincoln et al. (2), Schouten et al. (1) reverse their stance citing their own work: “…members of the Marine Group III Euryarchaeota have been suggested to contribute to GDGTs 0–3… thus, members of the MG-II may potentially contribute to this pool of GDGTs as well.” Yet several earlier publications had already suggested planktonic Euryarchaeota may be a source of tetraether lipids (see ref. 5 and references therein), predating this reference cited by Schouten et al. (1).

“Absence of evidence is evidence of absence” is a risky truism if the search for evidence has not yet been conducted. In science, the jury is always out. All scientific hypotheses and interpretations are subject to reevaluation in light of new data. The data presented in Lincoln et al. (2) suggest that previous claims that marine Thaumarchaeota are the sole source of tetraether lipids including crenarchaeol in marine plankton need to be seriously reevaluated.