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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 Si-metabolism 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 Transitions in phytoplankton community composition are typically attributed to ecological succession even in physically dynamic upwelling systems like the California Current Ecosystem (CCE). An expected succession from a high‐chlorophyll (~ 10μg L⁻¹) diatom‐dominated assemblage to a low‐chlorophyll (< 1.0μg L⁻¹) non‐diatom dominated assemblage was observed during a 2013 summer upwelling event in the CCE. Using an interdisciplinary field‐based space‐for‐time approach leveraging both biogeochemical rate measurements and metatranscriptomics, we suggest that this successional pattern was driven primarily by physical processes. An annually recurring mesoscale eddy‐like feature transported significant quantities of high‐phytoplankton‐biomass coastal water offshore. Chlorophyll was diluted during transport, but diatom contributions to phytoplankton biomass and activity (49–62% observed) did not decline to the extent predicted by dilution (18–24% predicted). Under the space‐for‐time assumption, these trends infer diatom biomass and activity and were stimulated during transport. This is hypothesized to result from decreased contact rates with mortality agents (e.g., viruses) and release from nutrient limitation (confirmed by rate data nearshore), as predicted by the Disturbance‐Recovery hypothesis of phytoplankton bloom formation. Thus, the end point taxonomic composition and activity of the phytoplankton assemblage being transported by the eddy‐like feature were driven by physical processes (mixing) affecting physiological (release from nutrient limitation, increased growth) and ecological (reduced mortality) factors that favored the persistence of the nearshore diatoms during transit. The observed connection between high‐diatom‐biomass coastal waters and non‐diatom‐dominated offshore waters supports the proposed mechanisms for this recurring eddy‐like feature moving seed populations of coastal phytoplankton offshore and thereby sustaining their activity.