The trophic role of glycolic acid in coastal seawater. I. Heterotrophic metabolism in seawater and bacterial cultures

Glycolic acid, a known algal excretory product, represents a potentially important energy source for heterotrophic bacteria in marine waters. Measurements of heterotrophic uptake and mineralization by the natural microbes in the plankton indicate quantitative use of glycolic acid comparable to other common microbial substrates. This activity shows vertical and horizontal variations that correlate in a general way with primary productivity. Glycolic acid appears to be present at higher concentrations than other low molecular weight substrates, and it also shows a lower rate of turnover. It is mineralized (respired) to a greater extent than other recorded substrates, an average of 70% of total uptake. Two-thirds of a total of 141 colonies of bacteria cultured from seawater on marine agar proved capable oftaking up and respiring glycolic acid. These same bacteria, however, were unable to utilize glycolic acid for growth when it was the sole carbon source. Glycolate may therefore occur at higher concentrations and show erratic fluctuations in natural waters because microbial use is not tied to glycolate production but to the presence of other substrates. The fact that so many bacteria are able to metabolize it indicates a possibly important trophic role as an energy source. It is suggested that glycolic acid may be a major source of energy for active transport of other substrates by marine bacteria.