Studies on glycolic acid metabolism by freshwater bacteria1

Measurements of uptake and mineralization of 14C-labeled glycolic acid by freshwater heterotrophs showed that respiration (mineralization) accounts for 69% of total uptake, and that the heterotrophic potential ( V,,,,, ) for glycolate is comparable with that for other commonly used substrates. Bacteria cultured from lake water showed uptake and mineralization patterns similar to those from the natural plankton. Enrichment of lake water with glycolic acid resulted in measured disappearance of the glycolic acid, in agreement with rates predicted by uptake kinetics, but bacterial response was slow and independent of added glycolate concentration. Under natural conditions the heterotrophic bacteria prevent glycolatc from accumulating by mineralizing most of it to CO, soon after it is released by algae. The interaction of bacteria and organic solutes represents an area of considerable importance in tracing the flow of energy and the cycling of elements in aquatic ecosystems. Recent methods using 14C have made possible the investigation of the relationships of heterotrophic microorganisms and specific organic solutes. The heterotrophic bacteria of natural waters can utilize organic solutes at concentrations of a few micrograms per liter (e.g. Wright and IIobbie 1966; Vaccaro and Jannasch 1966; Williams 1970; Robinson et al. 1973), and it seems clear that the bacteria are the major agents of mineralization of low molecular weight organic solutes in fresh and marinc waters (Wright 1974). Enrichment studies have shown that bacterial response to sudden increases in substrate is rapid, a few days being sufficient for complete removal and oxidation of glucose and amino acids artificially added at levels one to two orders of magnitude above ambient (Vaccaro 1969; Williams and Gray 1970). Lake water contains a great variety of organic solutes, some no doubt more important than others to the heterotrophic bacteria because of their concentrations and rates of production. Among the substratcs of primary importance, glycolic acid must be given serious consideration. 1 The work was supported by National Science Foundation grant GB 7741 and Science Faculty Fellowship 69210, also F.W.Q.A. program 16010 to R. Y. Morita. It occurs in lake waters (Fogg et al. 1969)) although ambient concentrations are often so low as to be undetectable by some techniques (Spear and Lee 1968). Glycolic 8Cl ‘d is readily traced to its source as a major algal excretory product (Tolbert and Zill 1956; Fogg 1966; Watt 1966,1969). Excretion of glycolic acid by algae, a puzzling loss of energy, is related to the dark reactions of photosynthesis as well as to environmental conditions. In a thorough review of glycolate metabolism in algae, Tolbert ( 1974) indicated that the principal path of glycolate formation is by oxidation of ribulosc diphosphate, in a reaction that involves competition between oxygen and carbon dioxide for this substrate. After its formation glycolate is either released ( excreted) or metabolically changed to glycine and serine via glyoxylic acid. It is not readily reassimilated by most algae. Wright (1970) found that plankton in the upper layers of a lake could take up glycolic acid at low concentrations. Circumstantial evidence indicated that this uptake was bacterial. Because of the magnitude of the uptake and the apparent source in algal excretion, I suggested that glycolic acid might be the most important energy source for the epilimnetic heterotrophic bacteria. However, at the time I did not know the rate at which glycolic acid was being respired, nor did I know how the bacteria would respond to enrichment by glycolatc. IIere I would like to LIMNOLOGY AND OCEANOGRAPHY 626 JULY 1975, V. 20(4) Glycolic acid and bacteria 627 address these problems by presenting more recent work on glycolic acid in freshwater. I will deal with three basic approaches before attempting to give an evaluation of the role of glycolic acid. These are the uptake and mineralization of 14C-labeled glycolic acid by natural plankton, the uptake and mineralization of glycolic acid by bacteria cultured from lake water, and results of artificially enriching lake water with glycolic acid.