last update: Nov 5, 2009
Bacterial Symbionts in the Earthworm Nephridia
Over 5000 species of earthworms distributed worldwide alter the soils they inhabit through mixing and processing of organic matter. Soil processing involves activities mediated by both the earthworm and associated microbiota.
Most studies of the microbial contribution to soil processing have focused on the gut and casts (fecal pellets) employing both culture based and more recently, molecular methods (Furlong, et al. 2002; Toyota and Makoto, 2000; Schönholzer, et al. 2002). We are investigating a less familiar association of a dense culture of bacteria in the nephridia,excretory organs present in each segment of the worm (Fig. 1). Although discovered around 1926, the identity and the activity of these bacteria within the worm had not been extensively investigated.
The nephridia pass fluid from the coelom of the worm to the outside through a continuous winding tube that forms three major loops for osmoregulation and excretion. The second loop contains a narrow part that widens into the ampulla, which is packed with bacterial cells (Fig. 1B). Our recently published comparative 16S rRNA gene sequence analyses revealed that the bacterial symbionts form a monophyletic group within the Acidovorax genus of the Betaproteobacteria (Schramm, et al. 2003). Different species of worms seem to harbor distinct strains of these associated, Acidovorax-like bacteria, suggesting a certain degree of co-evolution.
Figure 1. Verminephrobacter eiseniae symbionts inside a nephridium of the earthworm Eiseniae foetida. a) E. foetida and a simplified diagram of a longitudinal cross-section. b) Diagram of a nephridium illustrating the main three loops and the ampulla in the second loop, which harbors the bacterial symbionts. c) Laser scanning confocal micrograph of the ampulla showing the bacterial cells (yellow) and earthworm tissue (green). Bacterial cells in the nephridium were labelled by fluorescent in situ hybridization (FISH).
We have isolated betaproteobacterial symbionts from the nephridia of four different species of earthworms, including several bacterial strains from the compost earthworm Eiseniae foetida. Based on physiological and genomic traits unique to these isolates, we proposed the new genus Verminephrobacter, and named the isolates from E. foetida as V. eiseniae, the first species described within this genus (Pinel, et al., 2008). Isolated from other species of earthworms, are being characterized in order to explore the phenotypic plasticity of nephridial bacteria accross species of hosts.
In collaboration with the Joint Genome Institute, we sequenced the genome of V. eiseniae, and those of two other closely related Acidovorax species. Visit our genome projects site to learn more.
This association offers opportunities to study interactions between a beneficial bacterium and its host. Furthermore, this interaction may have influence on the ability of earthworms to function in the soil and thus on soil ecology. Currently we have NSF support to investigate the physiology of the symbiosis and interactions between bacterial symbiont and the host worm.