SEAWATER MICROORGANISMS HAVE A HIGH-AFFINITY GLYCINE BETAINE UPTAKE SYSTEM WHICH ALSO RECOGNIZES DIMETHYLSULFONIOPROPIONATE

In 24 h old seawater filtrate cultures, comprised mostly of free-livin g bacteria, the uptake of methyl-C-14-glycine betaine (GBT) displayed Michaelis-Menten-type saturation kinetics with half saturation constan ts (K-t) of <5 nM. The uptake of 5 nM C-14-GBT was strongly inhibited by additions of 1 to 30 nM unlabeled beta-dimethylsulfonicpropionate ( DMSP), a naturally occurring analog of GBT. The dose response to DMSP was consistent with simple dilution of labeled GBT. Conversely, the up take of S-35-DMSP was inhibited by the presence of GBT over a similar concentration range. Comparison of C-14-GBT and S-35-DMSP uptake kinet ics in the presence and absence of unlabeled analogs suggested that GB T and DMSP act as competitive inhibitors of their respective uptake by seawater microbes. Direct comparisons of C-14-GBT and S-35-DMSP uptak e in filtrate cultures yielded nearly identical kinetic patterns. It w as also found that a portion of the C-14-GBT taken up into particulate material could be chased from the particles by a 200-fold excess of u nlabeled GBT or DMSP, whereas it was retained as untransformed C-14-GB T in the particles for several hours in unchased samples. A screening of organic compounds revealed that compounds with close structural sim ilarity to GBT (DMSP, dimethylsulfonioacetate, proline betaine and dim ethylglycine) strongly inhibited (<36% of controls) C-14-GBT uptake. C ompounds showing moderate inhibition (48 to 89% of controls) included trigonelline, proline, glutamic acid, carnitine and choline, while com pounds bearing no structural or chemical similarity to GBT (glycine, g lucose, acrylic acid, and 3-methiolpropionate) had no effects on C-14- GBT uptake. Our results indicate that the GBT uptake system expressed by natural populations of microorganisms is multifunctional, displayin g high affinity for both GBT and DMSP and possibly other naturally occ urring betaine-like compounds. This microbial uptake system is likely to play an important role in the biogeochemical dynamics of GBT and DM SP.