Afternoon Session: Oceanography (1430-1630)
Chair: Chris Bowler

Kimberlee Thamatrakoln1, Kay D. Bidle1, Heather McNair2, Michael Maniscalco2, Mark A. Brzezinski2, Bethanie R. Edwards3, Benjamin A.S. Van Mooy3, Matthew D. Johnson4, Andrew E. Allen5, Lisa Z. Allen5, and Jeffrey W. Krause6
1 Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ, USA
2 Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA, USA
3 Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
4 Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
5 J. Craig Venter Institute, San Diego, CA, USA
6 Dauphin Island Sea Lab, Dauphin Island, AL, USA

In June/July 2013, we embarked on the “DYE labeling of diatom silica” (DYEatom) cruise to perform a multi-faceted, comprehensive study on diatom bloom dynamics and silicon cycling. We examined multiple water masses between Monterey Bay and Bodega Canyon which were characterized by diatom assemblages under varying degrees of silicon limitation. Biogenic silica production rates were measured at the assemblage level using the radioisotope, 32Si and the fluorescent dye, PDMPO (2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl)methoxy)phenyl)oxazole). Taxon-specific silica production was quantified using a newly developed and optimized quantitative PDMPO-based method. To capture the underlying biological mechanisms that regulate the observed rates, we performed metatranscriptomic analysis of silicification-related gene expression and targeted measurements of silicon transporter protein expression. Phytoplankton mortality by microzooplankton grazing was assessed by dilution experiments, while diatom-specific viral infection was assessed through analysis of viral gene signatures in the metatranscriptomes. Mass spectrometry paired with novel lipidomic-based approaches was used to determine the potential role of allelopathic compounds in mediating biogenic silica dissolution and regeneration through either enhanced bacterial proteolysis or microzooplankton grazing. In this talk, I will provide an overview of the DYEatom cruise and initial results on the role and impact of diatom growth and mortality on silicon biogeochemistry.

Michael Maniscalco1, Mark A. Brzezinski1, Heather McNair1, Jeffrey W. Krause2 and Kimberlee Thamatrakoln3
1University of California, Santa Barbara, CA 93106, USA
3Dauphin Island Sea Lab, University of South Alabama, Dauphin Island, AL 36528, USA
3Department of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, USA

Diatoms serve as the major link between the marine carbon and silicon biogeochemical cycles through their requirement for silicic acid for the production of their siliceous frustules.  The ecological importance of diatoms has prompted extensive research, providing a wealth of information about the dynamics of biogenic silica production in global oceans.  These studies provide insight into the degree that low silicic acid concentrations limit biogenic silica production, and ultimately diatom carbon fixation.  However, these community-level measurements do not address the physiological mechanisms driving the observed patterns.  To bridge this gap we are linking measurements of silicic acid uptake using the radioisotope 32Si with molecular analysis of the silicon transporters (SITs), diatom-specific proteins responsible for the uptake of silicic acid into the cell.  Preliminary results demonstrate elevated SIT expression at low external silicic acid concentrations that decrease upon Si addition. These data support the hypothesis that natural diatom assemblages, similar to laboratory cultures, switch from SIT-mediated silicon transport at low Si concentrations to diffusion-mediated uptake at high concentrations.

Klaus Valentin1, Bank Beszteri1, Madlen Franze1, Anique Stecher1, Michael Ginzburg2, Mar Fernandez3, , Antje Boetius3, and Gernot Glöckner4
1 Alfred Wegener Institute Bremerhaven, Germany
2 University Kiel, Germany
3 Max Planck Institute Bremen, German
4 University Cologne, Germany

Melosira arctica has been identified as a key species of the ice-covered Arctic Ocean. This diatom forms large standing stocks as several meters long mats underneath the sea ice and sinking of M. arctica mats can contribute up to 80% of carbon export. Our results indicate that M. arctica maybe enriched at multi-year ice which will decrease in the future (see contribution by Stecher et al.). We therefore launched a project to study the molecular ecology of the species using genomic, transcriptomic, and physiological tools. In the past, studies on M. arctica were hindered by the absence of a clean laboratory culture. We therefore established several unialgal and well-growing lab cultures which were subjected to a temperature range between 0 °C and 7 °C; in contrast to previous reports the diatom grows above 4°C. For comparative studies, a temperate Melosira from River Weser at Bremerhaven was also isolated. A genome project for M. arctica was started and a draft genome with a size range of about 350 mb was assembled. A reference transcriptome was also produced but not yet fully analysed. Future studies will include an experimental evolution experiment under elevated temperatures.

Bethany D. Jenkins1,2, Joselynn Wallace1, P. Dreux Chappell3, Kristofer Gomes1
1 Department of Cell and Molecular Biology, The University of Rhode Island, Kingston, RI, USA
2 Graduate School of Oceanography, The University of Rhode Island, Narragansett, RI, USA
3 Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, Norfolk, VA, USA

It has long been appreciated that the mosaic of nutrient availability in the surface ocean is a driver in shaping diatom populations.  To better understand the nutritional ecology of diatoms and why sometimes closely related species occupy different niches in the marine environment, we are comparing the transcriptional response of diatoms to a variety of nutrient stressors.  We have been focusing on the Thalassiosiroid genus because of their cosmopolitan distribution in the global ocean and because of the availability of genome data for a few species in this genus.  We have found that the compliment of genes responding to iron limitation varies amongst members of this genus, with oceanic isolates containing a wider complement of iron-responsive genes. We have also found that the oceanic diatom Thalassiosira oceanica lacks silicon transporters and genes encoding proteins involved in silicon deposition that are induced by silicon stress in Thalassiosira weissflogii.   These examples highlight comparative transcriptomics as a powerful means of helping us understand the repertoire of nutrient metabolism diatoms have the potential to employ in the environment and how this may relate to species distribution.