Environmentally induced changes to the diatom cell wall and the implications of these changes on biogeochemical cycles

Abstract

Diatoms are responsible for 20% of global primary production, control the marine silicon cycle, and are a major constituent of the organic matter that sinks into the deep ocean. Diatom cell walls, or frustules, are composed of silica and additional organic components, and the formation of the frustule is genetically controlled. Changes in frustule formation in response to shifting nutrient conditions can affect biogeochemical cycles due to the physiological responses of individual species and changes in the composition of species with diverse frustules and physiologies. To disentangle these factors, species-specific molecular approaches and fluorescent stains were used to identify changes to the diatom frustule in response to changing environmental conditions. First, the unknown aspects of cell wall formation were explored and chitin was discovered in the cell wall of several diatom species. The transcript abundance of a chitin synthase gene was correlated with the timing of maximum labeling of cellular chitin using a fluorescent probe. Chitin synthesis appeared to change when cells experienced silicic acid or iron starvation. Next, multiple genes related to frustule formation were studied in a community at the iron-limited Station Papa in the NE Pacific Ocean before and after iron-enrichment. The species specific influence on silica precipitation was evaluated with a quantitative fluorescent silica stain and was also evident from the diverse genetic potential among species. Changes in the transcript abundance of silicon transporters indicate that iron-enrichment may induce a functional shift in how silicon is taken up by the dominant species present. This was the first study to identify genetic patterns of frustule formation in natural diatom communities. Finally, the influence of silicic acid availability on coastal biogeochemistry was explored in natural communities and laboratory isolates. Growth experiments indicated that cellular silicification is highly influenced by the concentration of silicic acid available. Transcription of silicon transporters did not correlate with the changes in silicification, suggesting that diffusion of silicic acid into the cell may be important. When more silicic acid is available, cells are more silicified and sink faster. These changes in diatom frustule formation affect in the potential export of carbon from the surface ocean.