Bacterial and fungal endophyte response to crude oil exposure

Background/Question/Methods

Endophytes are symbiotic organisms that live cryptically within all plant tissue. These symbionts can positively affect the physiology and immunity of terrestrial plants, but little is known about their role in coastal plants, or how environmental conditions can affect these symbionts.  Some organisms can be resistant or resilient to stress or disturbance, and even may become usurpers in some cases, while others are susceptible and may not be able to recover after extreme changes in the environment.  To begin to understand endophyte response to their environment, we studied leaf and root endophyte communities of Spartina alterniflora in two coastal marshes in Louisiana that were affected by the Deepwater Horizon Oil Spill event in 2010. Within each site, we selected oiled and unoiled areas. We collected S. alterniflora tissues and used culture-dependent, culture-independent, and staining methods to isolate and quantify endophyte abundance, diversity, and community composition. We tested the hypotheses that endophyte diversity decreases with oil exposure, and that endophyte composition and diversity differs between plant parts (roots vs. leaves). We used nonmetric multidimensional scaling and ANOVA to detect differences in endophyte community composition and diversity respectively between oiled and non-oiled plants, as well as between roots and leaves.

Results/Conclusions

Three years after the Deepwater Horizon oil spill, we detected significant effects of oil on the S. alterniflora symbiont community composition and their relative diversity.  Based on the culture-dependent approach, we observed the following patterns: (1) In leaves, both fungal abundance and diversity decreased significantly with oil, whereas in roots, only diversity decreased;  (2) Neither bacterial diversity nor abundance was affected by oil in leaves, whereas both factors decreased in roots,  (3) Bacterial community composition did not differ between oiled and unoiled areas in either roots or leaves, (4) Fungal communities in leaves were different between the oiled and unoiled areas. Root fungal endophytes were not different between or within sites.  These results indicate that environmental stress and disturbance elicit different responses from different organisms, but that this, in part, depends on the niche they occupy.  In general, foliar fungal endophytes are susceptible to oil exposure, whereas some root fungal endophytes can become usurpers, growing aggressively and displacing other fungi.  Conversely, foliar bacterial endophytes seem to be resistant or resilient to oil exposure.  While root bacteria decreased overall in roots, community similarity between oiled and unoiled areas suggests that these symbionts are resilient and ultimately will recover.