Glycine betaine uptake, retention, and degradation by microorganisms in seawater
Limnol. Oceanogr., 43(7), 1998, 1592-1603 | DOI: 10.4319/lo.19126.96.36.1992
ABSTRACT: The uptake of nanomolar levels of methyl-14C-glycine betaine (GBT) was studied in surface water samples from the Gulf of Maine, Mobile Bay, and the Gulf of Mexico. At all locations, 14C-GBT was rapidly taken up into particulate material (>0.2 µm). Formalin treatment, autoclaving, or prefiltration of water through 0.2-µm membranes completely inhibited 14C-GBT uptake, indicating the process was biologically mediated, probably by bacteria. During the first several hours of uptake, >80% of the label found in particulates was recovered as untransformed 14C-GBT This fraction diminished over longer incubations but never fell below 16%, indicating some longer term storage of GBT in microorganisms. Respiration of the added label to 14CO2 was typically <5% of the total uptake during 5-10-min incubations but increased to approximately 40-60% after 24-48 h. Increasing the salinity of an estuarine water filtrate culture from 14 to 25 or 35 resulted in greater retention of 14C-GBT in particulates and less production of 14CO2, suggesting that GBT may be used as an osmolyte by microorganisms. Uptake, retention, and degradation patterns were similar in <0.8-µm filtrates and whole seawater, and size fractionation experiments showed that most GBT uptake was by organisms of <l µm, suggesting bacteria as the microbes most likely responsible for GBT uptake. Uptake rates followed Michaelis-Menten kinetics with saturation at <20 nM added GBT in most cases and Kt + Sn values ranging from 1.2 to 49 nM. Turnover times for Sn were short, ranging from 0.52 to 11 h. Vmax values ranged from 0.39 to 44 nmol liter-1 h-1, and Vmax could be increased by enriching <1.0-µm filtrates for 48 h with 100 µM glucose or acrylate. GBT uptake displayed temperature optima from 20°C to 30°C, with optima close to the in situ temperature on the day of sample collection. Together, the results indicate that nanomolar levels of GBT are rapidly scavenged by seawater microorganisms for use as an osmolyte or carbon and energy source. The relative proportion of retention vs. degradation is likely to depend on the composition of the microbial community and environmental factors such as osmotic stress and the availability of alternative osmotic or carbon and energy substrates.