Seasonal Dynamics of a Photosynthetic Mat in an Intertidal Marine Environment

Photosynthetic microbial mats are complex, stratified microbial ecosystems found in a variety of environments including intertidal regions of salt marshes. Generally, phototrophic organisms dominate the top layers, while diverse heterotrophic organisms can be found throughout a well-developed mat. This study focuses on how seasonally fluctuating environmental factors (salinity, temperature, and pH) influence the diversity and composition of an intertidal microbial mat located in Southern California, USA. Using high-throughput sequencing of the 16S rRNA gene and the ITS1 region of fungi, I characterized the microbial communities of the top (0 – 0.5cm) and bottom layers (0.5 – 1.0cm), and the underlying sediments of a mat system seasonally for two years beginning in the summer of 2014. Over the observation period, salinity (3.7% – 10%), temperature (11.0°C – 30.4°C) and pH (5.6 – 8.4) were variable, revealing a dynamic environment influenced by seasonal changes in tide height, solar radiation, and precipitation. Microbial communities were compared for changes in diversity and community composition. The top layer of the mat was composed primarily of Cyanobacteria, algae, purple sulfur bacteria, and aerobic heterotrophs, but the relative abundance of each varied seasonally. In the bottom layers of the mat and underlying sediment, the depletion of oxygen and filtering of light selected for anaerobic heterotrophs, primarily sulfate reducing bacteria. In contrast, fungal communities throughout the layers of the mat were less variable, but some fungal lineages did change seasonally. Ongoing seasonal characterization of this mat system will provide foundational data that will allow for the identification of factors structuring this mat system and will ultimately lead to a better understanding of how photosynthetic microbial mats contribute to carbon cycling within salt marsh environments.