Online first articles

Functional Group V (FG V), which comprises anoxygenic phototrophic purple and green sulfur bacteria, has traditionally been associated with stratified lakes, where pronounced vertical redox gradients are typically described. Here, we report the repeated occurrence of anoxygenic phototrophic sulfur bacteria in two shallow riparian wetlands, suggesting that the ecological context of this functional group may extend beyond classical stratified systems.

Environmental variables and phytoplankton functional structure were analysed in a lowland and a semi-mountain wetland characterized by dense macrophyte cover, high summer temperatures, elevated organic matter content, and low dissolved oxygen conditions. FG V, represented by Chromatium okenii and Thiospirillum jenense, was detected in all samples and episodically reached high relative abundance. It co-occurred within a functionally coherent assemblage of motile flagellates (W1, X2, Y), filamentous cyanobacteria (TC), and metaphytic diatoms (MP), typical of shallow, weakly mixed systems.

Although direct redox measurements were not performed, the combination of low dissolved oxygen, high organic loading, and restricted mixing indicates the potential for reduced conditions and localized redox discontinuities. Our results are consistent with the interpretation that macrophyte-dominated shallow wetlands may locally provide environmental conditions functionally analogous to those of deeper stratified systems, allowing FG V to persist within phytoplankton communities. These findings support a process-based view of FG V ecology, emphasizing the role of redox-related conditions over water depth, and highlight shallow wetlands as potentially important but underexplored habitats for redox-dependent phototrophic communities.