Elemental content and stoichiometry of SAR11 chemoheterotrophic marine bacteria

We measured the carbon, nitrogen, and phosphorus content and production of cultured SAR11 cells in the genus Pelagibacter , from members of the 1a.1 and 1a.3 lineages, which are adapted to productive coastal waters and oligotrophic tropical/subtropical environments, respectively. The average growing SAR11 cell contained ~ 6.5 fg C, from which we calculated a global standing stock of 1.4 × 10 13  g C. Calculations that consider uncertainties in cell turnover rates and growth efficiencies indicate this stock could oxidize 6% to 37% of gross ocean primary production. We also found that SAR11 do not incorporate 3 H‐thymidine but do incorporate 3 H‐leucine. We estimate conversion factors of 0.74–1.51 kg C mol −1 leu, which are comparable to the low end of published leucine conversion factors for marine chemoheterotrophic bacterioplankton production. The molar ratio of elements C : N : P in growing cells was on average 25 : 6 : 1, significantly less than the mean (~ 50 : 10 : 1) for heterotrophic bacteria, indicating these strains are C and N poor relative to P.