Potential role of oxygen and inorganic nutrients on microbial carbon turnover in the Baltic Sea

Abstract

Oxygen (O2) deficiency and nutrient concentrations in marine systems are impacting organisms from microbes to higher trophic levels. In coastal and enclosed seas, O2 deficiency is often related to eutrophication and high degradation rates of organic matter. To investigate the impact of O2 concentration on bacterial growth and the turnover of organic matter, we conducted multifactorial batch experiments with natural microbial communities of the central Baltic Sea. Water was collected from suboxic (<5 µmol l−1) depths in the Gotland Basin during June 2015. Samples were kept for 4 d under fully oxygenated and low O2 conditions (mean: 34 µmol l−1 O2), with or without nutrient (ammonium, phosphate and nitrate) and labile carbon (glucose) amendments. We measured bacterial abundance, bacterial heterotrophic production, extracellular enzyme rates (leucine-aminopeptidase) and changes in dissolved and particulate organic carbon concentrations. Our results show that the bacterial turnover of organic matter was limited by nutrients under both oxic and low O2 conditions. In nutrient- and glucose-replete treatments, low O2 concentrations significantly reduced the net uptake of dissolved organic carbon and led to greater accumulation of more labile dissolved organic matter. Our results therewith suggest that the combined effects of eutrophication and deoxygenation on heterotrophic bacterial activity might favor the accumulation of dissolved organic carbon in the Baltic Sea.