Fitzpatrick, R. M.; Hungate, B. A.; Power, M. E.; Schwartz, E.; Hayer, M.; Samo, T.; Weber, P.; Pett-Ridge, J.; Foley, M.; Marks, J. C.

The branching green macroalga Cladophora glomerata and its epiphytic microbiome dominate summer biomass in the Eel River, a Northern California river under Mediterranean (summer drought, winter rain) seasonality. Green streamers of Cladophora proliferate in early summer, then change from green to yellow and then red-brown as epiphyte loads increase and their taxonomic composition shifts. We characterized successional changes in epiphytic bacteria and archaea on Cladophora, examining both abundance and growth rates, using quantitative Stable Isotope Probing (qSIP) and16S rRNA gene amplicon sequencing to assess taxonomy and potential function. The number of bacterial taxa increased with succession while growth rates peaked in the middle successional stage. NanoSIMS imaging confirmed high sulfur (S) concentrations in Cladophora cell walls relative to surrounding biomass, which may have contributed to the bloom of sulfur bacteria (bacteria that reduce or oxidize sulfur/sulfates) as cell walls decompose. In general, relative abundances and growth rates of epiphytic cells were independent, indicating that either metric alone is insufficient for understanding how species and functional groups affect ecosystem processes. For instance, the relative abundance of N-fixers was highest on red Cladophora when their relative growth rates were lowest. Similarly, denitrifying bacteria were most abundant on green Cladophora when their growth rates were lowest. Such patterns may be driven by growth limitation due to space competition. Together, changes in abundance and relative growth rates suggest different limiting factors for different functional groups in the Cladophora microbiome at multiple successional stages.