Bacteria, are a major component of the microbial loop, and play an important role in biogeochemical processes. They mineralize dissolved organic matter, such as algal exudate and transfer carbon and energy to higher trophic levels, and play an essential role in the microbial food web (Azam et al., 1983). Furthermore, bacterioplankton can play different roles in the initiation, development and termination of a broad range of algal blooms (Paver et al., 2013; Buchan et al., 2014). Thereby, the bacterioplankton community can, to some extent, indicate trophic status and certain organic or heavy metal pollution, mirroring the state of water quality (Pinto et al., 2021). Therefore, it was proposed that bacteria act as sentinels of environmental change due to their sensitivity to changing environmental conditions including trophic status and anthropogenic pollution (Harnisz, 2013; Savio et al., 2015). Based on their life style, there are two bacterial fractions, i.e. free-living (FL), with a size range between 0.22 to 3.0 µm or 5 µm (Crespo et al., 2013; Grossart, 2010), and particle-attached (PA), which can be separated from FL by filtration on 3.0 µm/ 5 µm filters. FL refers to bacteria floating freely in the water column, while the PA fraction comprises bacteria attached to organic particles and living organisms. Several studies revealed that there are significant phylogenetic differences between FL and PA fractions, whereby some bacteria can rapidly exchange between the size fractions (Tang et al., 2015; Bizic-Ionescu et al., 2014). The first step to study FL and PA assemblages is to collect the most representative PA samples. Generally, the prevalent method is filtration and centrifugation, earlier study has proposed that centrifugation was proper in eutrophic and turbulent lakes, but it could enrich some taxa disproportionately due to various sinking velocity (Xie et al., 2020). Filtration with 3.0 µm (ImLiu et al., 2019; Wang et al., 2020) or 5.0 µm (Allgaier and Grossart, 2006; Parveen et al., 2011; Zhao et al., 2017; Hu et al., 2020) pore size filter is the most frequently used. Different pore sizes may affect the structure of community. Moreover, the methods applied for collection may be trophic-dependent because nutrients are associated with the composition and size of particles in the water column (Tang et al., 2017).
Research about bacterioplankton in river reservoirs on Yungui-Plateau are scarce. Zhang et al. (2020) investigated the relationship between phytoplankton and bacterioplankton in Dianchi Lake. Wu et al. (2019) revealed the vertical bacterial profile in Lake Lugu. Both studies were conducted in Yunnan Province. Wu River is the largest southern tributary of Yangtze River and also the largest river in Guizhou Province. Wujiangdu reservoir was the first major dam established in the middle and upper reaches of the Wu River. There are many streams supplying water to the impoundment, such as Xifeng River and Pianyan River. Due to the karstification characteristics in this region, Wu River water is abundant with carbonated weathering (Han and Liu, 2004). Yue et al. (2021) investigated the vertical distribution of bacterial community in Wujiangdu reservoir and emphasized the role of stratification on the vertical pattern of bacterial community structure. Cascade dams along Wu River could affect overall microbial community via altering hydrological features. However, their community structure and function, classified as FL and PA fraction, in such plateau reservoirs are still poorly understood. Information about the bacterial distribution, assembly, diversity and functioning in this reservoir could help us understand the mechanisms underlying microbial food web and phytoplankton succession in such plateau freshwater ecosystems. Therefore, it is necessary to unveil the bacterial community structure in reservoirs of Yungui Plateau.
We collect samples from 14 sites along Wujiangdu reservoir, a typical canyon river reservoir. Based on 16S rRNA marker gene, we firstly compared the results of PA bacterioplankton by using 3.0 µm and 5.0 µm filter, and simultaneously investigate both FL and PA bacterial community structure, diversity and gene function. We hypothesized that (1) different pore size filters would result in different PA bacterioplankton community; (2) FL and PA bacterioplankton differ in composition, diversity and functions in Wujiangdu reservoir.