Bacterioplankton Metacommunity Processes across Thermal Gradients: Weaker Species Sorting but Stronger Niche Segregation in Summer than in Winter in a Subtropical Bay

ABSTRACT Thermal effluents from nuclear power plants greatly change the environmental and ecological conditions of the receiving marine water body, but knowledge about their impact on microbial ecology is limited. Here we used high-throughput sequencing of the 16S rRNA gene to examine marine bacterioplankton metacommunity assembly across thermal gradients in two representative seasons (i.e., winter and summer) in a subtropical bay located on the northern coast of the South China Sea. We found high heterogeneity in bacterioplankton community compositions (BCCs) across thermal gradients and between seasons. The spatially structured temperature gradient created by thermal effluents was the key determinant of BCCs, but its influence differed by season. Using a metacommunity approach, we found that in the thermal discharge area, i.e., where water is frequently exchanged with surrounding seawater and thermal effluent water, the BCC spatial patterns were shaped by species sorting rather than by mass effects from surrounding seawater or by dilution of thermal effluent water by surrounding seawater. However, this effect of species sorting was weaker in summer than in winter seawater. In both seasons, the bacterioplankton community structure was predominately determined by niche sharing; however, the relative importance of niche segregation was enhanced in summer seawater. Our findings suggest that for the seasonal differences in metacommunity processes, the BCCs of subtropical summer seawater were more sensitive to temperature and were more difficult to predict than those of winter seawater in the face of different intensities of thermal impacts. IMPORTANCE Understanding the mechanisms of bacterial community assembly across environmental gradients is one of the major goals of marine microbial ecology. Thermal effluents from two nuclear power plants have been present in the subtropical Daya Bay for more than 20 years and have generated a comparatively stable and long thermal gradient (a temperature increase from 0 to 10°C). The environmental patches across thermal gradients are heterogeneous and very strongly interconnected on a microbial scale; thus, this is a useful model for the study of the metacommunity processes (i.e., patch dynamics, species sorting, mass effects, and neutral processes) that underlie marine bacterioplankton assembly. The significance of our research is to reveal how environmental conditions and dispersal-related processes interact to influence bacterioplankton metacommunity processes and their seasonal differences across thermal gradients. Our results may advance the understanding of marine microbial ecology under future conditions of global warming.

New records of Compsopogon aeruginosus (Rhodophyta) in rivers of central Europe

The representatives of the genus Compsopogon Montagne in Bory et Durieaux (Rhodophyta) are basically tropical or subtropical algae. In central Europe they are only found in aquaria and in water bodies affected by thermal effluents, like the species C. aeruginosus (J. Agardh) Kützing in the Pulkau River (Austria), a tributary of the Dyje /Thaya River (Austria, Czech Republic). The first observation of this species was made in 2007 in the Pulkau River. In autumn 2010 the species was also observed at lower densities in an approximately 20 km section of the Dyje/Thaya River between the mouth of the Pulkau River (A) and the Nové Mlýny Reservoir (CR). Recently, the species was found also in Skryjský Brook, a tributary of the Jihlava River, below the cooling water effluent from the Dukovany Nuclear Power Plant, South Moravia, Czech Republic. This is the second finding of representatives of the genus Compsopogon in rivers in central Europe.