Submitted to: Journal of Protozoology Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2006
Publication Date: 9/26/2006
Citation: Cohen, M., Mazzola, M. 2006. Effects of brassica napus seed meal amendment on soil populati0ns of resident bacteria and naegleria americana and the unsuitability of arachidonic acid as a protozoan-specific marker. Journal of Protozoology Research. 6:16-25.
Interpretive Summary: Organic soil amendments often have been promoted as a means to control soilborne plant diseases. However, the effective use of such an environmentally sensitive method of disease control has been impeded by a lack of understanding of how such amendments modify soil microbial communities. Application of Brassica napus seed meal (RSM) altered bacterial population in soil resulting in a decline in fluorescent pseudomonad populations but increases in Streptomyces numbers. This finding is important as both of these bacterial groups have been cited as active plant disease suppressing soil microorganisms. It was shown that resident protozoan, Naegleria americana, could be responsible for the differential trend in populations of these two bacterial groups. Numbers of N. americana increase in RSM amended soils and while they feed on fluorescent pseudomonads, they do not feed at all on Streptomyces. Many scientists have tracked the habit of protozoa in soils by monitoring a key fatty acid, arachononic acid (AA). Although reported to be a specific indicator of protozoan densities, AA was not detected in either straminipile Pythium spp. AA levels increased dramatically in response to RSM amendment of soils, and this corresponded with dramatic increases in Pythium spp. populations. When Pythium spp. were suppressed through application of mefenoxam to soil the accumulation of AA in RSM amended soils was abolished. These findings demonstrate that AA is not a reliable fatty acid marker for describing protozoan densities in soil systems.
Technical Abstract: Application of organic amendments to soil systems has the potential to alter both the structure and function of resident microbial communities. Addition of Brassica napus seed meal (RSM) to orchard soils consistently led to a decline in populations of fluorescent Pseudomonas spp. and a significant and sustained increase in numbers of Streptomyces spp. Numbers of protozoa also increase significantly in response to RSM amendment, with the population in orchard soils dominated by Naegleria americana. This amoeba-lagellaete showed a marked feeding preference in culture for fluorescent pseudomonads relative to Streptomyces spp., with virtually no feeding by the protozoan on the latter. The preferential survival of Streptomyces spp. in RSM amended soils may bear some relation to the differential sensitivity of these organisms to feeding by protozoa. Analysis of fatty acid composition from environmental samples is a common approach to the study of protozoa and other soil microbial populations. This approach relies upon the existence of group-definitive fatty acid markers to monitor microbial populations of interest. Arachidonic acid (AA) is commonly cited as a marker of protozoan density. However, in our studies AA was not detected in cultures of either N. americana nor a Bodo sp, both common culturable protozoa found in RSM amended soil. Based on our data, the abrupt and transient increase in AA levels recorded in soils in response to RSM amendment was likely due to the post-amendment amplification of Pythium spp. Application of the oomycete-specific pesticide mefenoxam abolished AA accumulation in RSM amended soils. It is clear from this study that AA is not a specific indicator of protozoan densities in soil, a finding that will carry significant implication in the interpretation of previous and future reports concerning protozoa in soil systems.