In 24 h old seawater filtrate cultures, comprised mostly of free-living bacteria, the uptake of methyl-14C-glycine betaine (GBT) displayed Michaelis-Menten-type saturation kinetics with halfsaturation constants (K,) of <5 nM. The uptake of 5 nM I4C-GBT was strongly inhibited by additions of 1 to 30 nM unlabeled P-dimethylsulfoniopropionate (DMSP), a naturally occurring analog of GBT The dose response to DMSP was consistent with simple dilut~on of labeled GBT Conversely, the uptake of %-DMSP was

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... inhibited by the presence of GBT over a similar concentration range. Comparison of 'T-GBT and 35S-DMS~ uptake kinetics in the presence and absence of unlabeled analogs suggested that GBT and DMSP act as colnpetitive inhibitors of their respective uptake by seawater microbes. D~rect comparisons of 14C-GBT and 3 5 S -D M~~ uptake in filtrate cultures y~elded nearly identical kinetic patterns. It was also found that a portion of the "C-GBT taken up into particulate material could be chased from the particles by a 200-fold excess of unlabeled GBT or DMSP, whereas it was retained as untransformed I4C-GBT in the particles for several hours in unchased samples. A screening of organic compounds revealed that compounds with close structural similarity to GBT (DMSP, dimethylsulfonioacetate, proline betaine and dimethylglycine) strongly inhibited (<36% of controls) I4C-GBT uptake. Compounds showing moderate inhibition (48 to 89 % of controls) included trigonelline, proline, glutamic acid, carnitine and choline, while compounds bearing no structural or chemical similarity to GBT (glycine, glucose, acrylic acid, and 3-methiolpropionate) had no effects on "C-GBT uptake. Our results indicate that the GBT uptake system expressed by natural populations of rmcroorganisms is multifunctional, displaying high affinity for both GBT and DMSP and possibly other naturally occurring betaine-like compounds. This microbial uptake system is likely to play an important role in the biogeochemical dynamics of GBT and DMSP.