The role of oxygen conditions in the microbial dissolution of biogenic silica under brackish conditions

Abstract

Regeneration of biogenic silica (BSi) is essential for the constant supply of dissolved silica (DSi) to diatoms in aquatic ecosystems, where these primary producers play an important role in carbon (C) drawdown. Increased DSi concentrations have been detected in the oxygen (O2) depleted bottom waters of e.g. the brackish Baltic Sea, which is suffering from one of the largest anoxic or hypoxic marine areas in the world. As hypoxia is likely to become more frequent also in coastal areas, the effects of suboxic conditions on the silicon (Si) cycle should be explored. To examine the effects of hypoxia on microbially mediated DSi release from diatom detritus and sediment to brackish seawater, a batch laboratory experiment was conducted. Hypoxic conditions facilitated more complete dissolution of diatoms than oxic conditions. However, the age, origin and physiological condition of the BSi significantly affected the dissolution rate, and the presence of sedimentary material decreased BSi dissolution. Inhibition of bacterial activity reduced the dissolution of diatoms under both oxic and hypoxic conditions, and T-RFLP of 16S rRNA genes confirmed that O2 conditions affected the composition of the bacterial community. Taken together, these results demonstrate that oxygen conditions and microbes play a fundamental role in regeneration of diatom BSi. Moreover, the results may explain the elevated DSi concentrations associated with O2 depletion in the bottom waters of the Baltic Sea.