Specific long-chain fatty acids promote optimal growth of Frankia: accumulation and intracellular distribution of palmitic and propionic acid

Frankia isolates from nodules of the genera Casuarina (BR, S21, Thr), Allocasuarina (Allo2), and Gymnostoma (G80) were found to grow exponentially with high biomass yield and minimal sporangia formation in stirred propionate mineral medium when supplemented either with 2.4 μM palmitic acid (C16:0), pentadecanoic (C15:0), heptadecanoic (C17:0), or linoleic (C18:2, cis 9, 12) fatty acids. Strains also grew with lauric (C12:0) or myristic (C14:0) acids, but gave lower biomass yield. Stearic acid (C18:0) produced a good biomass yield, but cultures slowly accumulated sporangia; oleic acid (C18:1, cis-9) was detrimental to growth. Caprylic (C8:0) or capric (C10:0) acids proved to be prejudicial for long-term storage of Frankia strains. In experiments using labeled 1,2-dipalmitoyl phosphatidylcholine and palmitic acid, radioactivity bound rapidly to the insoluble, but solvent-extractable fraction of Frankia cells. In contrast, label from propionic acid accumulated in the cytosolic fraction. Therefore, the beneficial effect of some specific phospatidylcholines or free fatty acids on Frankia growth appears to result from their utilization as building blocks for the membrane, suggesting that membrane biosynthesis may be the limiting step for Frankia growth in unamended propionate mineral medium.