UC Santa Barbara

Ornately detailed frustules of silica encase the cells of one of the most productive primary producers in the world. Diatoms are responsible for roughly 20% of global primary production. They are unique among phytoplankton in their requirement of silicon for growth. Low concentrations of silicon in the surface ocean limit diatom silica production and possibly growth across ocean regions. The immense morphological and physiological diversity among diatoms suggests that community composition could be influenced by silicon availability and likewise difference in community compositions could affect the biogeochemical role of diatoms. However, until recently the ability to resolve relationships between community composition and silicon concentration was absent because methods to quantify differences in silica production and growth rate among co-occurring diatom taxa did not exist.

The first part of this dissertation (Chapter 2) describes the development of a new method to quantify single-cell silica production rates in mixed diatom assemblages. The method leverages the co-deposition of the fluorescent tracer, PDMPO (2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole) with newly deposited biogenic silica to measure single cell silica production rates using confocal microscopy. This method was used to quantify taxon-specific silica production and growth rates in the California upwelling zone for the remaining dissertation chapters.

Chapter three explores the contribution of co-occurring taxa to community silica production and examines the physiological attributes of the most influential diatom taxa. Over short time scales (~1 d) the most productive taxa in the community were the ones with the largest combined frustule surface area. This lead to a disproportionate fraction of community production occurring in large diatoms. Over longer timescales (3-10 d) the influence of growth rate increases so that over the course of the bloom a taxon with substantial cell size and high growth rate will be the largest contributor to silica production. These results offer a novel perspective into the mechanics of the influence of community composition in silicon cycling.

Chapter four begins to tease apart the role of resource limitation in these communities by examining the hierarchical physiologic response of taxa to silicon limitation. Co-occurring taxa, experiencing the same environment, showed a large range of silicon limitation. However, the physiological response to limitation was generally conserved among taxa. Taxa that were silicon limited preferentially decreased the silica content of their frustule and to a lesser extent decreased the rate of production of new frustule with little alteration to growth rate. The short-term relative contribution of individual taxa to community silica production or the relative abundance of taxa was not altered by silicon limitation. The combined results from this dissertation highlight some of the mechanistic ways that diatoms influence biogeochemical cycling, with community composition dictating downstream nutrient availability and nutrient induced physiologic changes affecting the fate of diatom bound nutrients.