Degradation of potassium rock by earthworms and responses of bacterial communities in its gut and surrounding substrates after being fed with mineral

Abstract

Background: Earthworms are an ecosystem's engineers, contributing to a wide range of nutrient cycling and geochemical processes in the ecosystem. Their activities can increase rates of silicate mineral weathering. Their intestinal microbes usually are thought to be one of the key drivers of mineral degradation mediated by earthworms,but the diversities of the intestinal microorganisms which were relevant with mineral weathering are unclear.

Methodology/principal findings: In this report, we show earthworms' effect on silicate mineral weathering and the responses of bacterial communities in their gut and surrounding substrates after being fed with potassium-bearing rock powder (PBRP). Determination of water-soluble and HNO(3)-extractable elements indicated some elements such as Al, Fe and Ca were significantly released from mineral upon the digestion of earthworms. The microbial communities in earthworms' gut and the surrounding substrates were investigated by amplified ribosomal DNA restriction analysis (ARDRA) and the results showed a higher bacterial diversity in the guts of the earthworms fed with PBRP and the PBRP after being fed to earthworms. UPGMA dendrogram with unweighted UniFrac analysis, considering only taxa that are present, revealed that earthworms' gut and their surrounding substrate shared similar microbiota. UPGMA dendrogram with weighted UniFrac, considering the relative abundance of microbial lineages, showed the two samples from surrounding substrate and the two samples from earthworms' gut had similarity in microbial community, respectively.

Conclusions/significance: Our results indicated earthworms can accelerate degradation of silicate mineral. Earthworms play an important role in ecosystem processe since they not only have some positive effects on soil structure, but also promote nutrient cycling of ecosystem by enhancing the weathering of minerals.

Figure 1. The amounts of water-soluble and HNO₃-extractable K Al, Fe and Ca released from PBRP.

The numbers 0K, 10K and 10EK mean the original prepared PBRP, the PBRP incubated at proper humidity level for ten days without earthworms and the PBRP after being fed to earthworms for ten days, respectively. Error bars are ± standard deviation (n = 2). The asterisks (*) above 10K and 10EK denote the value significantly greater than the values of 0K and 10K (p<0.05), respectively.

Figure 2. TEM images are of the soil, the PBRP and the gut's contents of earthworms feeding on PBRP and soil.

Among these images, a and b showed the TEM images of the soil and the gut's content after feeding earthworms with soil for ten days, respectively; c and d showed that of the mixture of PBRP and soil and the gut's content after being fed with the mixture respectively; e showed the images of fresh PBRP; f is the images of the surrounding PBRP after being fed; g and h showed the images of the gut's content after being fed with PBRP, respectively.

Figure 3. Neighbour-joining tree of 16SRNA gene sequences depicting the phylogenetic relationships of clones from the soil samples after having been processed by the earthworms in ten days.

The scale bars represent a 5% sequence divergence and percentages of 1000 bootstrap resamplings are shown at the nodes. Species names and OTUs' numbers are followed by their GenBank accession numbers their proportion in all clones of the library, respectively.

Figure 4. Neighbour-joining tree of 16SRNA gene sequences depicting the phylogenetic relationships of clones from the gut contents of the earthworms fed with soil for ten days.

The scale bars represent a 5% sequence divergence and percentages of 1000 bootstrap resamplings are shown at the nodes. Species names and OTUs' numbers are followed by their GenBank accession numbers their proportion in all clones of the library, respectively.

Figure 5. Neighbour-joining tree of 16SRNA gene sequences depicting the phylogenetic relationships of clones from the natural PBRP sample.

The scale bars represent a 2% sequence divergence and percentages of 1000 bootstrap resamplings are shown at the nodes. Species names and OTUs' numbers are followed by their GenBank accession numbers their proportion in all clones of the library, respectively.

Figure 6. Neighbour-joining tree of 16SRNA gene sequences depicting the phylogenetic relationships of clones from the PBRP samples after having been processed by the earthworms in ten days.

The scale bars represent a 5% sequence divergence and percentages of 1000 bootstrap resamplings are shown at the nodes. Species names and OTUs' numbers are followed by their GenBank accession numbers their proportion in all clones of the library, respectively.

Figure 7. Neighbour-joining tree of 16SRNA gene sequences depicting the phylogenetic relationships of clones from the gut contents of the earthworms fed with PBRP for ten days.

The scale bars represent a 5% sequence divergence and percentages of 1000 bootstrap resamplings are shown at the nodes. Species names and OTUs' numbers are followed by their GenBank accession numbers their proportion in all clones of the library, respectively.

Figure 8. Distribution of clone numbers of each phylogenetic affiliations in each 16S rRNA gene clone library.

Figure 9. Hierarchical clustering of the five samples with weighted and unweighted UniFrac.

The percentage support for nodes supported at least 70% of the time with sequence jackknifing is indicated.

Figure 10. UniFrac unweighted principal coordinate analyses (PCA) (a) and weighted PCA (b) for the five samples.