Diminished efficiency in the oceanic silica pump caused by bacteria-mediated silica dissolution

Previous laboratory findings indicated that marine bacteria accelerate biogenic silica (bSiO2) 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 bSiO2 dissolution during a diatom bloom in Monterey Bay, California, following moderate upwelling. Inhibition of bacterial activity with antibiotics and protease inhibitors reduced specific bSiO2 dissolution rates (Vdis) at five of six stations, with a reduction of 44 6 27% (mean 6 SD, n 5 6, range 22–91%) over 24 h. Reduced Vdis 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 Vdis 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 bSiO2 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; a and g subclasses of Proteobacteria) mediate bSiO2 dissolution. The importance of diatoms to marine primary productivity has lead to an examination of factors that control their growth (Thompson 1999; Martin-Jézéquel et al. 2000). Silicic acid supply mechanisms have been emphasized because diatoms have an absolute silicon requirement for growth 1 To whom correspondence should be addressed. Present address: Institute of Marine and Coastal Science, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901. 2 Present address: Department of Oceanography, Texas A&M University, College Station, Texas 77843-3146.