Linking dissolved organic matter composition and bacterioplankton communities in an Amazon floodplain system

Abstract

Dissolved organic matter (DOM) is the main substrate for aquatic prokaryotes, fuelling their metabolism and controlling community composition. Amazonian rivers transport and process large fluxes of terrestrial DOM, but little is known about the link between DOM composition and heterotrophic bacteria in the Amazon basin. The aims of this study were to characterize DOM composition and investigate the coupling between DOM and bacterial community composition (BCC) during a complete hydrological cycle in an Amazon floodplain system (lake Janauacá). Our study revealed a clear seasonal pattern in DOM composition through the flood pulse, which affected the amounts of autochthonous and allochthonous inputs and consequently the extent of humification, molecular weight, and aromaticity of the DOM. BCC was tightly coupled to DOM fluorescence, which was also driven by differences over the hydrological cycle, with distinct components and operational taxonomic units being simultaneously more abundant and correlating with a specific season. This coupling was particularly well reflected for three of the four identified fluorescence components, two terrestrial humic‐like components (C1 and C3) and an autochthonous component (C4). Despite clear changes in DOM composition, dissolved organic carbon concentrations tended to be relatively stable throughout the year. Overall, our results suggest that BCC shifts were associated with DOM quality but not with its quantity (that remains relatively constant throughout the year), and that bacteria preferably use labile and freshly produced DOM in lake Janauacá.

Use of absorption optical indices to assess seasonal variability of dissolved organic matter in amazon floodplain lakes

“…weight than those surrounded by forest. These results agree with previous studies (Melo et al, 2019;Shen et al, 2012;Spencer et al, 2008) indicating the presence of HMW DOM during rising water. However, in the present study the authors do not have data to corroborate the optical analyses regarding the origin and molecular weight of DOM.…”

“…This oscillation represents a key aspect in the biogeochemistry, ecology and hydrology of floodplain lakes (Junk et al, 1989;Moreira-Turq et al, 2004). Among other effects, the flood pulse (sensu Junk et al, 1989) affects the proportion of autochthonous and allochthonous sources contributing to the dissolved organic matter (DOM) pool in floodplain lakes throughout the year (Melo et al, 2019). DOM represents the largest pool of organic carbon in the aquatic environment (Cole et al, 2007;Tranvik et al, 2009) and it has an important role in the ecosystem carbon budgets (Dalmagro et al, 2019;Pangala et al, 2017;Richey et al, 2002).…”

Local and Geographic Factors Shape the Occupancy-Frequency Distribution of Freshwater Bacteria

“…These results seem to indicate that pH is more strongly related to the composition of the most abundant bacteria, while the mild bacteria are related to pH, along with nutrients, and the satellite bacteria with stochasticity. The increased importance of T-fDOM, an indirect indicator of autochthonous carbon supply [23,24], highlights the importance of an autochthonous metabolism at these sites [67] and is a potential indicator of phytoplankton-bacteria interaction, which is presumably high in tropical regions due to high phytoplankton excretion rates at high temperatures and light [68].…”

Organic Molecular Signatures of the Congo River and Comparison to the Amazon

“…In contrast, the decoupling of the R 1 and R 2 formulae in the Congo suggests that material representing R 1 peaks could be mobilized from different sources, due to the bimodal nature of the Congo River hydrograph (Mann et al., 2014; Spencer et al., 2016). Processed DOM, representing R 2 formulae, are constantly supplied to the river from wetlands in the Cuvette Centrale at all stages of the hydrograph (Coynel et al., 2005; Lambert et al., 2016), in contrast to the seasonal pulses of biolabile organic matter that the Amazon River receives from floodplain lakes (Moreira‐Turcq et al., 2013), which would be rapidly consumed (de Melo et al., 2020) and not represented in the R 1 and R 2 peaks. This is also supported by the higher relative abundance of the Common Congo fraction than the Common Amazon fraction which consistently accounts for ∼90% of the total relative abundance in each sample (Figure 6; Table 1), suggesting a greater accumulation and export of more stable compounds that have undergone degradation in the Congo River (Mentges et al., 2017).…”

“…Concurrently, the abundance of fresh, protein‐like DOM, which is likely derived from phytoplankton biomass and aquatic macrophytes, increases in floodplains during low‐ and falling‐water periods and is transported into the mainstem as the water level recedes (de Melo et al., 2020; Moreira‐Turcq et al., 2013). Although this autochthonous DOM represents a small portion of the total DOM, it is often consumed by microorganisms on short time scales as a biologically labile (biolabile) carbon source (D’Andrilli et al., 2015; Patriarca et al., 2021; de Melo et al., 2020) and along with fresh terrestrial DOM, could contribute to Amazon River CO 2 outgassing (Richey et al., 2002; Ward et al., 2013). Additionally, DOM compounds sharing similar composition (i.e., aliphatic and heteroatom‐rich) are also characteristic of agricultural sources and riparian zones (Spencer et al., 2019; Wagner et al., 2015), found in Amazon tributaries (Gonsior et al., 2016); and have also been linked to biolability and microbial degradation across several major world rivers (Riedel et al., 2016).…”