Sea level rise and the microbial communities of baldcypress

Sunshine A. Van Bael, Elizabeth R. Kimbrough and Stephen K. Formel, Ecology and Evolutionary Biology, Tulane University, New Orleans, LA, Candice Y. Lumibao, Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN, Lorena Torres Martinez, Ecology and Evolutionary Biology, University of California Riverside, Riverside, CA, William H. Conner, Baruch Institute of Coastal Ecology and Forest Science, Clemson University, Georgetown, SC, Richard H. Day, U.S. Geological Survey, National Wetlands Research Center, Lafayette, LA, Ken W. Krauss, Wetland and Aquatic Research Center, U.S. Geological Survey, Lafayette, LA

Background/Question/Methods
As coastal ecosystems are subjected to more extreme climatic events and rising sea levels, there is a pressing need to examine the effects of saltwater intrusion on coastal communities. Little is known about the effects of salinity and flooding on plant symbionts, including baldcypress trees (Taxodium distichum), the dominant trees in many swamp ecosystems in the southeastern US. We focused our research on baldcypress endophytes and rhizosphere communities, including both bacteria and fungi. Our research team asked: (1) What are the abiotic and biotic factors that structure microbial communities associated with baldcypress? (2) What are the differences between rhizosphere and endosphere microbial communities? (3) How do extreme flooding events influence the fungal community of baldcypress? Our research was based on field-collected samples and we described microbial communities using cultures and Illumina sequencing. We also ran experiments on extreme flooding/drought events for baldcypress seedlings in a growth chamber.
Results/Conclusions
The abiotic factors that correlated most with microbial community structure were water level and salinity – this was true for both bacteria and fungi in leaves and roots. Salinity strongly influenced both rhizosphere and endosphere fungal diversity in opposite patterns, e.g., at intermediate salinity levels we observed the highest endosphere diversity and lowest rhizosphere diversity. These results indicate that the assembly and structure of the root endosphere and rhizosphere within a host can be shaped by different processes. For biotic factors, key variables associated with microbial community structure were woody debris (rhizosphere and endosphere fungi) and the density of host trees (endosphere fungi only). We found that flooding reduced the colonization rates of dark septate endophytes (potential mutualists) and oomycetes (pathogens), but arbuscular mycorrhizal fungi colonization was not influenced by flooding. We did not observe strong correlations of baldcypress growth with fungal mutualists/pathogens. Together, our results show how the baldcypress rhizosphere and endosphere communities are shaped by high water levels and salinity; however future work on the implications for tree growth and survival is necessary. Such experiments will bridge the knowledge gap between microbial community patterns and their functional significance.