Diminished Efficiency in the Oceanic Silica Pump Caused by Bacteria-Mediated Silica Dissolution

Previous laboratory findings indicated that marine bacteria accelerate biogenic silica $(\text{bSiO}_{2})$ dissolution rates in the sea by degrading the organic coating surrounding diatom frustules and exposing the underlying silica to chemical attack by undersaturated seawater. We examined the effectiveness of bacterial activity in facilitating in situ $\text{bSiO}_{2}$ dissolution during a diatom bloom in Monterey Bay, California, following moderate upwelling. Inhibition of bacterial activity with antibiotics and protease inhibitors reduced specific $(\text{bSiO}_{2})$ dissolution rates $(\text{V}_{\text{dis}})$ at five of six stations, with a reduction of 44 ± 27% (mean ± SD, n = 6, range 22-91%) over 24 h. Reduced $\text{V}_{\text{dis}}$ in inhibitor treatments corresponded with reductions in abundance, production, and proteolytic activity of attached bacteria. Dissolution rates were highly correlated with protease activity integrated from the surface down to the depth where each dissolution was measured, suggesting that increased $\text{V}_{\text{dis}}$ with depth in the upper 20-80 m of the ocean is caused by the progressive removal of organic matter from frustules during sinking. Facilitation of $\text{bSiO}_{2}$ dissolution by in situ bacterial assemblages varied between stations and was likely influenced by the physiological condition of resident diatom assemblages. Denaturing gradient gel electrophoresis and 16S rRNA gene sequencing of bacteria colonizing in situ diatom assemblages confirmed previous findings that specific bacterial phylotypes (Cytophaga/Flavobacteria/Bacteriodes; α and γ subclasses of Proteobacteria) mediate $\text{bSiO}_{2}$ dissolution.