Comparable light stimulation of organic nutrient uptake by SAR11 and Prochlorococcus in the North Atlantic subtropical gyre

Abstract

Subtropical oceanic gyres are the most extensive biomes on Earth where SAR11 and Prochlorococcus bacterioplankton numerically dominate the surface waters depleted in inorganic macronutrients as well as in dissolved organic matter. In such nutrient poor conditions bacterioplankton could become photoheterotrophic, that is, potentially enhance uptake of scarce organic molecules using the available solar radiation to energise appropriate transport systems. Here, we assessed the photoheterotrophy of the key microbial taxa in the North Atlantic oligotrophic gyre and adjacent regions using 33 P-ATP, 3 H-ATP and 35 S-methionine tracers. Light-stimulated uptake of these substrates was assessed in two dominant bacterioplankton groups discriminated by flow cytometric sorting of tracer-labelled cells and identified using catalysed reporter deposition fluorescence in situ hybridisation. One group of cells, encompassing 48% of all bacterioplankton, were identified as members of the SAR11 clade, whereas the other group (24% of all bacterioplankton) was Prochlorococcus. When exposed to light, SAR11 cells took 31% more ATP and 32% more methionine, whereas the Prochlorococcus cells took 33% more ATP and 34% more methionine. Other bacterioplankton did not demonstrate light stimulation. Thus, the SAR11 and Prochlorococcus groups, with distinctly different light-harvesting mechanisms, used light equally to enhance, by approximately one-third, the uptake of different types of organic molecules. Our findings indicate the significance of light-driven uptake of essential organic nutrients by the dominant bacterioplankton groups in the surface waters of one of the less productive, vast regions of the world's oceans-the oligotrophic North Atlantic subtropical gyre.

Leucine incorporation by aerobic anoxygenic phototrophic bacteria in the Delaware estuary

“…The lack of a light effect in our experiments is consistent with previous work in Delaware and elsewhere in which light effects were complex and often contradictory Straza and Kirchman, 2011;Ruiz-González et al, 2012a;Ruiz-González et al, 2012b). Light did stimulate the activity of the NOR5 gammaproteobacterial clade known to contain the AAP bacterial taxa (Ruiz-González et al, 2012a) and of another likely photoheterotroph (SAR11 bacteria) in short term experiments (Gomez-Pereira et al, 2013). In any case, light seems to be the most likely factor, along with top-down factors, affecting AAP bacterial abundance and activity relative to the rest of the bacterial community.…”

Diurnal expression of SAR11 proteorhodopsin and 16S rRNA genes in coastal North Atlantic waters

“…It would be difficult to detect a small contribution, especially with natural microbial communities. Light appears to enhance uptake of methionine and ATP by SAR11 cells in the North Atlantic Ocean by about 30% on average (Gómez-Pereira et al 2013), which is more than predicted by theoretical calculations (Kirchman & Hanson 2013), but still near the detection limits of many methods. This level of enhancement may be too small to detect with 16S rRNA:rRNA ratios, especially if there is uncoupling between 16S rRNA synthesis and growth.…”

Faster growth of the major prokaryotic versus eukaryotic CO2 fixers in the oligotrophic ocean

“…The amount of tracer in flow-sorted LNA cells was statistically indistinguishable from the background (Supplementary Fig. 3), indicating that the SAR11 alphaproteobacteria, which comprised the LNA cell population13, did not fix CO 2 . The increased 14 C-radioactivity of the A-Pl cells in the dark has a ready explanation—the A-Pl cells continued feeding on 14 C-labelled CO 2 fixers in the dark.…”