Bacteria Contribute to Sediment Nutrient Release and Reflect Progressed Eutrophication-Driven Hypoxia in an Organic-Rich Continental Sea

Abstract

In the sedimental organic matter of eutrophic continental seas, such as the largest dead zone in the world, the Baltic Sea, bacteria may directly participate in nutrient release by mineralizing organic matter or indirectly by altering the sediment’s ability to retain nutrients. Here, we present a case study of a hypoxic sea, which receives riverine nutrient loading and in which microbe-mediated vicious cycles of nutrients prevail. We showed that bacterial communities changed along the horizontal loading and vertical mineralization gradients in the Gulf of Finland of the Baltic Sea, using multivariate statistics of terminal restriction fragments and sediment chemical, spatial and other properties of the sampling sites. The change was mainly explained by concentrations of organic carbon, nitrogen and phosphorus, which showed strong positive correlation with Flavobacteria, Sphingobacteria, Alphaproteobacteria and Gammaproteobacteria. These bacteria predominated in the most organic-rich coastal surface sediments overlain by oxic bottom water, whereas sulphate-reducing bacteria, particularly the genus Desulfobacula, prevailed in the reduced organic-rich surface sediments in the open sea. They correlated positively with organic nitrogen and phosphorus, as well as manganese oxides. These relationships suggest that the bacterial groups participated in the aerobic and anaerobic degradation of organic matter and contributed to nutrient cycling. The high abundance of sulphate reducers in the surficial sediment layers reflects the persistence of eutrophication-induced hypoxia causing ecosystem-level changes in the Baltic Sea. The sulphate reducers began to decrease below depths of 20 cm, where members of the family Anaerolineaceae (phylum Chloroflexi) increased, possibly taking part in terminal mineralization processes. Our study provides valuable information on how organic loading affects sediment bacterial community compositions, which consequently may maintain active nutrient recycling. This information is needed to improve our understanding on nutrient cycling in shallow seas where the dead zones are continuously spreading worldwide.

Sediment prokaryote communities in different sites of eutrophic Lake Taihu and their interactions with environmental factors

“…During the November, February, May and August, the relative abundance of most of the predominant groups showed different patterns of variation in the sites 16 (heavy bloom area), 24 (light bloom area or macrophylic area) and 7 (mediate bloom area), implying they may play different ecologic functions in the sediments. These changes might be related to the responds of these bacteria to the nutrient conditions of the sediments during different bloom stages, since they have been described as the degraders of organic nutrient in marine sediment (Sinkko et al 2013). Our results demonstrated that the Alphaproteobacteria were more abundant during the algal bloom, which is similar to the findings elsewhere (Eiler et al 2006;Shao et al 2011;Ye et al 2009).…”

“…The sediments are the reservoir of nutrients for the water body and the sediment microorganisms drive the nutrient interchange between sediment and water which affects the trophic level of the water body (Sinkko et al 2013). The important roles of sediment microbes in energy and matter transformation in aquatic environments have been reported previously (Boyle-Yarwood et al 2008;Glissman et al 2004;Schmalenberger et al 2008;Tamaki et al 2005).…”

Discordant Temporal Turnovers of Sediment Bacterial and Eukaryotic Communities in Response to Dredging: Nonresilience and Functional Changes

“…The pH, DO, and E h were recorded by a calibrated Myron L Ultrameter (Myron L Company) in situ. Fresh sediment samples were cultured aerobically in 250-ml flasks for 2 weeks at 28°C with the addition of 200 ppm NH 4 ϩ -N; concentrations of NO 3 Ϫ -N were measured before and after the incubation to determine the sediment nitrification potential (56). The sediment denitrification potential was estimated through anaerobic incubation at 28°C for 3 days in sealed 250-ml flasks filled with N 2 ; the consumption of NO 3 Ϫ was calculated with an initial addition of 50 ppm NO 3 Ϫ -N (57).…”

“…Rapid and significant changes in the structure and functions of freshwater and marine ecosystems caused by humaninduced disturbances are of major concern (3). Nutrient cycles and pollutant clearing up in freshwater ecosystems mainly occur in the sediment surface layer through biological metabolic processes (4,5). The microbial communities can be responsible for 76% to 96% of the total sediment respiration, and the communities are generally known as a dominant driving force in the transformation of complex organic compounds (6,7).…”

“…Canonical correspondence analysis (CCA) was performed to link the relationship between geochemical attributes (i.e., pH, OM, NH 4 ϩ , NO 3 Ϫ , P, SO 4 2Ϫ , DO, Eh, and temperature) and sediment community structure (Fig. 4).…”