Elemental content and stoichiometry of SAR11 chemoheterotrophic marine bacteria

Abstract

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 × 1013 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 3H‐thymidine but do incorporate 3H‐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.

Expanding the diversity of bacterioplankton isolates and modeling isolation efficacy with large scale dilution-to-extinction cultivation

“…These span wider growth rates than observed for axenic cultures of SAR11 (0.4-1.2 day -1 ), but isolate-specific growth ranges within that spread are much more constrained (29, 36, 49, 131, 132)). Conversion factors for determining production from 3 H-leucine incorporation appear to be accurate for at least Ia subclade members of SAR11 (133), so variations in growth rate estimates from microradiography experiments likely have other explanations. It is possible that different strains of SAR11 simply have variations in growth rate not captured by existing isolates.…”

Pelagibacter metabolism of diatom‐derived volatile organic compounds imposes an energetic tax on photosynthetic carbon fixation

“…This estimate is subject to large uncertainties, and likely depends on many unexplored factors, including growth conditions and community composition, but it establishes an approximate magnitude for the process we observed. Unexpectedly large fluxes of VOCs from phytoplankton and specialization in the oxidation of labile low‐molecular‐weight compounds, including VOCs, by SAR11 may partially explain how small, simple SAR11 cells capture an estimated 6%–37% of gross primary production (White et al ., ).…”

“…Microphytoplankton, a pigment‐based classification primarily composed of diatoms, is estimated to carry out about 32% of net primary production in the ocean (Uitz et al ., ). Pelagibacter are estimated to oxidize 6% to 37% of marine gross primary production (White et al ., ). The flow of carbon between these two groups of organisms is therefore a potentially significant component of the marine carbon cycle.…”

Isolation of SAR11 marine bacteria from cryopreserved seawater

“…We reasoned that, since existing SAR11 isolates could be cryopreserved in the presence of 10% glycerol (e.g. 20,33,34), then there was no a priori reason to believe that natural populations of SAR11 cells could not similarly be preserved. One of many unknown variables, however, was whether or not the process of cryopreservation would result in significant cell loss and thus affect cultivation efficiency.…”

“…The preservation of cultivated bacterial strains by freezing in the presence of cryoprotectants such as glycerol or dimethyl sulfoxide (DMSO) has proved an effective method for preserving viable cells over long time periods (i.e. years), including cultivated strains of SAR11 (20,33,34). However, to our knowledge it has not previously been tested for its efficacy in preserving raw seawater for subsequent use as inoculum for highthroughput cultivation.…”