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Diatoms serve as the major link between the marine carbon (C) and silicon (Si) biogeochemical cycles through their contributions to primary productivity and requirement for Si during cell wall formation. Although several culture-based studies have investigated the molecular response of diatoms to Si and nitrogen (N) starvation and replenishment, diatom silicon metabolism has been understudied in natural populations. A series of deckboard Si-amendment incubations were conducted using surface water collected in the California Upwelling Zone near Monterey Bay. Steep concentration gradients in macronutrients in the surface ocean coupled with substantial N and Si utilization led to communities with distinctly different macronutrient states: replete ('healthy'), low N ('N-stressed'), and low N and Si ('N- and Si-stressed'). Biogeochemical measurements of Si uptake combined with metatranscriptomic analysis of communities incubated with and without added Si were used to explore the underlying molecular response of diatom communities to different macronutrient availability. Metatranscriptomic analysis revealed that N-stressed communities exhibited dynamic shifts in N and C transcriptional patterns suggestive of compromised metabolism. Expression patterns in communities experiencing both N and Si stress imply that the presence of Si stress may partially ameliorate N stress and dampen the impact on organic matter metabolism. This response builds upon previous observations that the regulation of C and N metabolism is decoupled from Si limitation status, where Si stress allows the cell to optimize the metabolic machinery necessary to respond to episodic pulses of nutrients. Several well-characterized Simetabolism associated genes were found to be poor molecular markers of Si physiological status; however, several uncharacterized Si-responsive genes were revealed to be potential indicators of Si stress or silica production. [ABSTRACT FROM AUTHOR]

Previous laboratory findings indicated that marine bacteria accelerate biogenic silica (bSiO[sub2]) 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 bSiO[sub2] dissolution during a diatom bloom in Monterey Bay, California, following moderate upwelling. Inhibition of bacterial activity with antibiotics and protease inhibitors reduced specific bSiO[sub2] dissolution rates (V[sub dis]) at five of six stations, with a reduction of 44 ± 27% (mean ± SD, n = 6, range 22-91%) over 24 h. Reduced V[sub 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 V[sub 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 bSiO[sub2] 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; &alpha and γ subclasses of Proteobacteria) mediate bSiO[sub2] dissolution. [ABSTRACT FROM AUTHOR]

Silicon isotope tracers were used to examine the relative magnitude of silica dissolution and silica production in the Monterey Bay, California, upwelling system. A diatom bloom dominated by Skeletonema costatum and Chaetoceros spp. was encountered under conditions of moderate upwelling. Profiles of silica production and dissolution rates were obtained at seven stations that sampled both inside and outside the bloom. Integrated silica production rates ranged from 5.4 to 108 mmol Si m[sup-2] d[sup-1], averaging 42.8 mmol Si m[sup-2] d[sup-1]. Integrated silica dissolution rates were considerably lower than production rates with values between 0.63 and 6.5 mmol Si m[sup-2] d[sup-1] (mean = 2.90 mmol Si m[sup-2] d[sup-1]). The mean ratio of integrated silica dissolution to integrated silica production (∫ D: ∫ P) between the surface and the 0.1% light depth was 0.075, omitting one station with an unusually high ∫ D: ∫ P of 0.61, indicating that, on average, 93% of silica production was supported by new silicic acid. The f-ratio for diatom nitrogen use estimated from silicic acid and nitrate depletion curves and the mean ∫ D: ∫ P ratio was found to be 0.83, indicating that silica was being regenerated at a rate that was only slightly slower than that for particulate organic nitrogen. These data provide direct evidence confirming earlier hypotheses that the silica pump is weak in Monterey Bay. Analysis of the global data set on ∫ D: ∫ P in the surface ocean leads to the hypothesis that low ∫ D: ∫ P (∼ 0.10 or less) are typical of diatom bloom events, with ∫ D: ∫ P rising to values in excess of... [ABSTRACT FROM AUTHOR]