Assessment of bacterioplankton community structure in relation to macrophyte cover in shallow lakes of the Pannonian Basin (Central Europe)

Study region: The Pannonian Basin, Hungary. Study focus: Macrophytes are essential components of freshwater ecosystems, influencing aquatic bacterial communities. This study investigated the impact of macrophytes on bacterioplankton abundance, metabolic activity and diversity across 12 shallow lakes in the Pannonian Basin. We assessed the interplay between macrophyte coverage and physicochemical parameters in structuring microbial communities using 16S rRNA gene metabarcoding and the most probable number (MPN) method. New hydrological insights for the region: While macrophyte presence appeared to influence the distribution of certain bacterial groups, most operational taxonomic units (OTUs) exhibited sitespecific patterns. Notably, Rhodocyclaceae and the genus Limnohabitans were more abundant in macrophyte- and reed-covered areas. 'Candidatus Patescibacteria' and Epsilonbacteraeota were found almost exclusively in certain samples, reaching 41.6% and 32.3% relative abundance, respectively. In contrast, freshwater SAR11 bacteria, particularly the LD12 clade, were present in nearly all sampled waters, with the highest observed relative abundance of 14.2% of total sequences. MPN values of heterotrophic bacteria (103-106 MPN mL-1) were highest in open water and submerged macrophyte samples; however, the abundance of other bacterial functional groups was linked with environmental factors such as dissolved oxygen levels. These findings suggest that beyond macrophyte coverage, other environmental parameters, such as hydrological conditions and surrounding land use, may exert a more deterministic effect on bacterioplankton in shallow lakes, even within a relatively small geographical area.