Skip to main content

Evolution of symbiosis between ciliates and nitrogen-fixing prokaryotes

Final Report Summary - SYMNIF (Evolution of symbiosis between ciliates and nitrogen-fixing prokaryotes)

Nitrogen-fixing bacteria affect the local and global nitrogen cycle by channeling gaseous nitrogen into the food web. For a long time only plant-associated diazotrophs and cyanobacteria in aquatic environments have been in scientific focus but more and more evidence show that also in the marine and freshwater systems non-cyanobacterial nitrogen-fixers can be important for actual nitrogen fixation. In this project, we studied if heterotrophic nitrogen fixers find a refuge in aquatic ciliates and if these symbionts may have an effect on the local nitrogen cycle.
More than 100 ciliate strains from Danish coastal and North Atlantic open waters were studied with nifH specific PCR and acetylene reduction assay if they harbor active diazotrophic symbionts. Almost half of all tested ciliates harbored bacteria with the nifH gene. Interestingly, all ciliates from one culture showed the same results with specific PCR even with old cultures established more than 5 years ago. In addition to this observation, we found that ciliates sampled in spring and summer had a higher probability to contain diazotrophs than ciliates in winter time when the ambient water was not depleted in bioavailable nitrogen compounds. However, addition of nitrogen to the cultures showed no effect on nitrogen fixation rates highlighting the enclosed position of the symbionts. Activity measurements showed low but present nitrogen fixation and thus support the hypothesis that diazotrophic symbionts in aquatic ciliates can have high impact on local and even global matter cycling. This notion is also underlined by the fact that ciliates obtained from open water in the North Atlantic harbored diazotrophs so that nitrogen-fixing symbionts in aquatic ciliates seems to be a global phenomenon. Incubation experiments with 15N showed also nitrogen fixation in the respective size class.
To study if diazotrophic symbionts can be in general important for the marine system we also investigated marine sponges and copepods. The investigated ciliates contained one to three different nitrogen fixind bacteria indicating a close and relative stable symbiosis. However, sponges with high microbial abundance (HMA) harboured only seldom diazotrophs and sponges with low microbial abundance (LMA) contained a highly diverse diazotrophic community. Copepods on the other hand harboured diverse nitrogen-fixing bacteria – even starving the copepods did not change this observation.
The project highlights the importance of associated and symbiotic bacteria to global matter cycling and indicates the importance to investigate these groups of bacteria more closely than done before