Decreasing amounts of extractable phospholipid-linked fatty acids in a soil during decline in numbers of pseudomonads

Abstract

The hypothesis that phospholipid-linked fatty acid methyl ester (PL-FAME) analysis of a soil characterizes lipids from within microbial cells (cellular) as distinct from lipids stabilized in soil organic matter (extracellular) was tested by measuring the sensitivity of PL-FAME analysis to numbers of bacteria in samples of a soil. Autoclaved samples from the Ap horizon of a soil having a sandy loam texture were inoculated with suspensions of a Pseudomonas sp. (day 0) to form bacterial microcosms. On day 2, half of the microcosms were inoculated with an Acanthamoeba sp., a protozoan that grazes on pseudomonads.The numbers of bacteria in the soil samples without or with Acanthamoeba sp. increased about five-fold between days 2 and 4. Between days 4 and 7, the numbers of pseudomonads in grazed microcosms decreased 10-fold and the numbers of Acanthamoeba sp. increased 10-fold, relative to those in nongrazed microcosms. The 10-fold relative difference in numbers of pseudomonads between nongrazed and grazed samples persisted through day 12. Cluster analysis grouped PL-FAME profiles from grazed with those from nongrazed microcosms on day 4. On days 7 and 12, PL-FAME profiles from grazed or nongrazed microcosms clustered separately. The PL-FAME profiles from sterile samples of soil on days 2, 4, 7, and 12 clustered with profiles on day 2 from microcosms inoculated with Pseudomonas sp.We infer that PL-FAME analysis is selective for cellular lipids in the sandy loam soil with the limited range of active organisms used here. Further extension of PL-FAME analysis to a wider range of soils and soil conditions is warranted. Key words: Phospholipid-linked fatty acid, PL-FAME, soil lipids, Pseudomonas, Acanthamoeba