Diversity and functional responses of nitrogen-fixing microbes to three wetland invasions

Impacts of invasive species on microbial components of wetland ecosystems can reveal insights regarding functional consequences of biological invasions. Nitrogen fixation (acetylene reduction) rates and diversity of nitrogen fixers, determined by genetic fingerprinting (T-RFLP) of the nifH gene, were compared between native and invaded sediments in three systems. Variable responses of nitrogen fixing microbes to invasion by a non-native mussel, Musculista senhousia, and mangrove, Avicennia marina, in Kendall Frost-Northern Wildlife Preserve (Mission Bay) and salt cedar, Tamarisk (Tamarix spp.) in Tijuana Estuary suggest microbes respond to both species- and site-specific influences. Structurally similar invaders (the mangrove and salt cedar) produced different effects on activity and diversity of nitrogen fixers, reflecting distinct environmental contexts. Despite relative robustness of microbial community composition, subtle differences in total diversity or activity of nitrogen fixers reveal that microbes are not immune to impacts of biological invasions, and that functional redundancy of microbial diversity is limited, with significant consequences for functional dynamics of wetlands.