Award: OCE-1334387

The vast majority of photosynthesis in the oceans is conducted by microscopic organisms called phytoplankton. The phytoplankton contain thousands of individual species. Within the phytoplankton a group known as the diatoms stands out for literally living in a house of glass that they construct using dissolved silicon in seawater. These opal shells are exquisitely detailed and the beauty of diatoms has been appreciated since the first microscopes were invented. The need for diatoms to build these structures extends beyond mere curiosity. Unlike other phytoplankton diatoms absolutely must have adequate silicon or their growth ceases. The importance of this constraint becomes more obvious when placed in context of the amount of photosynthesis done by diatoms. Twenty percent of photosynthesis on planet Earth is done by diatoms and that includes comparison with photosynthesis on land and sea. For every fifth breadth of oxygen to breath you can thank a diatom. This project sought to understand how the availability of the silicon dissolved in seawater controls the distribution and abundance of diatoms. For decades scientists have been making measurements of how fast diatoms take up silicon from seawater and how those rates relate to factors such as the diatom species present, their biomass, the amount of silicon available and other environmental variables. At the same time the relatively new science of molecular biology has advanced to where molecular methods are being applied to environmental problems. Molecular methods revealed the identity of organisms through DNA sequencing and how gene expression varies with changes in the environment. Biogeochemistry and molecular biology each have their strengths and weaknesses. Biogeochemistry can precisely quantify how fast processes occur in nature but those methods cannot always reveal the underlying biological mechanisms. Molecular techniques can identify what biochemical pathways are activate but struggle to quantify what patterns of gene expression mean for the rate of physiological processes. In this project we sought to combine biochemistry and molecular biology to play to their complementary strengths to better understand how silicon metabolism controls the abundance, distribution and activity of diatoms in the sea. One challenge for the project is that the biochemical pathways associated with silica biomineralization in diatoms, or any other organisms, is unknown. Thus our project also sought to discover additional biomolecules that are part of the silicification pathway. We conducted our work during three oceanographic expeditions. On the first we related measures of silica production rates and Si limitation to the gene expression profile of diatoms. By examining changes in gene expression across gradients in Si stress we identified sets of genes that showed a strong response, two order of magnitude change in expression, with severe Si stress. Examination of these genes will reveal a new set of genes that are candidates for being key to diatom silicificiation. On the second expedition we studied interactions between diatom silicification and the trace nutrient iron. The reason is prior findings that diatoms that have low iron dramatically increase their demand for silicon relative to nitrogen. This physiological response has implications for diatoms running out of silicon earlier than when iron is plentiful and can drive changes in diatom productivity which feeds back into marine food-web structure. What we discovered for iron-stressed diatoms off California was that despite silicon transporters being upregulated the silicon content of diatoms was unaltered by low iron. Nitrogen use was compromised as indicated by reduced uptake of nitrate and the downregulation of the nitrate assimilation pathway. So this was a case where the increased Si uptake implied by the molecular expression data was not manifest opening a new avenue for exploring the relationship between transporter expression and silicon use. Finally, on our third expedition we examined how diatoms adjust to abundant silicon by shutting off silicon transporters relying on diffusion to acquire silicon and we are examining interactions between light availability and silicon metabolism. Beyond our discoveries this project is training a new graduate student who will use the data that we collected for their Ph.D. dissertation. The student has a unique opportunity to be cross-trained in both biogeochemistry and molecular biology making them one of the new generation of marine scientists that view the ocean from a multidisciplinary perspective allowing ever more sophisticated insights into how phytoplankton, and in our case diatoms, contribute to processes that sustain the ocean ecosystem to the benefit of humankind. Last Modified: 11/20/2018 Submitted by: Mark A Brzezinski