Ecology Microbial Interactions

Morning Session 1: Ecology/Microbial Interactions (0900-1030)
Chair: Mariella Ferrante

0900-0930 (Invited Speaker)
Yuji Tomaru1, Kei Kimura2
1 National Research Institute of Fisheries and Environment of the Inland Sea, Fisheries Research Agency, Japan
2 Institute of Lowland and Marine Research, Saga University, Japan.

Viruses infectious to microorganisms are currently considered to play a significant role for biogeochemical cycles in marine environments. The viruses infectious to diatoms also have predicted to be primary agent to shape their dynamics, especially for the population size decreases. Isolations of diatom viruses, however, were not reported in 20th century. Diatom viruses were discovered as recently as a decade ago. Since then, data on diatom viruses have rapidly accumulated. At least 18 diatom viruses have been isolated and characterized. They are roughly divided into 2 groups based on genome conformation: ssDNA (Bacilladnavirus) or ssRNA (Bacillarnavirus) viruses. The former viruses are generally harboring 5–6kb closed circular ssDNA and the latter are ca. 9kb linear ssRNA genome. They are icosahedral, <40 nm in diameter, and lack a tail. Viral infection causes a complete lysis of the host in axenic culture. However, in nature, diatom host populations maintain their sizes even with the presence of the infectious viruses, implying diverse virus resistance strategies. We summarise basic features of isolated diatom viruses and their ecology in nature.

Michael Carlson1, Nicolette McCary1, Kyle Frischkorn1,2, Terence Leach1, Gabrielle Rocap1
1University of Washington, School of Oceanography, Seattle, WA, USA
2Columbia University, Department of Earth and Environmental Sciences

Viruses are major catalysts of biogeochemical cycling, architects of microbial community structure, and terminators of phytoplankton blooms, but the impact of viruses on diatom communities is unknown. Diatom-virus dynamics were explored by sampling every month at 2 locations in the Pacific Northwest resulting in 41 new Pseudo-nitzschia isolates and 20 environmental virus samples. Diatom hosts ranged in permissivity to infection. Titers of viruses were variable and dependent on host strains; isolates that were most infected yielded the highest viral titers. Genotyping of the ITS1 region revealed subgroups of genetically identical hosts that were differentially infected. Molecular fingerprints of Pseudo-nitzschia communities indicated that monospecific blooms of Pseudo-nitzschia are composed of multiple viral infection phenotypes. Additionally, the characterization of the Pseudo-nitzschia multiseries DNA virus (PmDNAV), which infects both centric and raphid diatoms, highlighted the complexity of host-virus dynamics. Strains of Pseudo-nitzschia and T. pseudonana showed variability in burst size, latent period, and susceptibility. Genome sequencing of the viruses from these various cultures yielded a novel linear single-stranded DNA virus. The characterization of the PmDNAV links viral host range and host permissivity to the host dependent patterns observed in field data, revealing how viruses control Pseudo-nitzschia communities in the environment.

Lander Blommaert1,4, Marie J. J. Huysman2,3 Wim Vyverman1, Johann Lavaud4 & Koen Sabbe1
1Ghent University, Lab. Protistology & Aquatic Ecology, B-9000 Ghent, Belgium
2 VIB, Department of Plant Systems Biology, B-9052 Ghent, Belgium
3Ghent University, Department of Plant Biotechnology and Bioinformatics, B-9052 Ghent, Belgium
4CNRS/Université de La Rochelle, UMR7266 LIENSs, Institut du Littoral et de l’Environnement, 17000 La Rochelle, France

Intertidal marine sediments belong to the most productive ecosystems on earth, despite being characterized by rapidly fluctuating and often extreme light conditions. Its main primary producers are benthic diatoms which possess physiological protection mechanisms against oversaturating light conditions (i.e. ‘high light’-HL). Among these mechanisms Non Photochemical Quenching (NPQ) is thought to be the most important one. Its main component (QE) is dependent on (1) thylakoid lumen acidification (2) the xanthophyll cycle pigment diatoxanthin, synthetized during thylakoid lumen acidification, and (3) LHCX (Light harvesting complex X) proteins which function as NPQ modulators. Intertidal benthic diatoms consist of two main growth forms. The epipelon comprises larger motile diatoms, which can position themselves along the vertical sediment light gradient, whereas epipsammic diatoms are largely immotile and have to undergo changes in light conditions. We recently showed that epipelic and epipsammic diatoms show fundamentally different photoprotective responses (Barnett et al. 2015): epipsammic diatoms have higher NPQ and associated xanthophyll cycle capacities compared to epipelic diatoms. In the latter group, the behavioural response (motility) is more important. The regulation and performance of NPQ was studied using model representatives of each functional group during and after exposure to HL (2000 µmol quanta m-2s-1 for 1 h). We observed clear differences in xanthophyll cycle pigment (HPLC) and LHCX protein dynamics (Western blotting) between both representatives. LHCX regulation at the transcript level was studied in the epipelic representative S. robusta only. Seven members of the LHCX family were detected, all of which but one showed distinct upregulation during HL exposure. Our results indicate that benthic diatoms have an elaborate regulatory network to cope with HL stress, likely due to the harsh light environment of intertidal sediments.

Jacob Valenzuela1, Allison Lee1, Mónica V. Orellana1,2,3, Nitin S. Baliga1,4
1 Institute for Systems Biology, Seattle, WA USA
2 Polar Science Center, Applied Physics Lab, Seattle, WA, USA
3 University of Washington, Seattle, WA, USA
4 Department of Microbiology, University of Washington, Seattle, WA, USA

Climate change models continue to project high atmospheric concentrations of carbon dioxide (CO2) at the end of the century. The oceans act as carbon sinks for CO2 which affects the pH, higher CO2 concentrations cause a lower pH, termed ocean acidification. Each year diatoms account for up to 40% of the total marine primary productivity. It is imperative we understand how they respond to ocean acidification in order to accurately model the consequences of climate change. Historically environmental tipping points have been identified post hoc, we aim to project bifurcation points in diatom ecosystems in an effort to be proactive about niche breadth implications. We employ the model diatom, Thalassiosira pseudonana as a “canary” for possible tipping point events. Results have shown different population trajectories and phenotypic plasticity in response to high carbon environments. In particularly, long term exposure of short term daily doses of ultraviolet radiation resulted in slower recovery of T. pseudonana to cycles of nitrogen depletion at low carbon (high pH) compared to high carbon (lower pH) environments. Results show implications on tipping points, phenotypic plasticity and niche breadth of diatoms as ocean acidification shifts marine primary production diversity and abundance.

BREAK: 1030-1100

Morning Session 2: Ecology/Microbial Interactions (1100-1230)
Chair: Assaf Vardi

1100-1130 (Invited Speaker)
Mary Ann Moran
Department of Marine Sciences, University of Georgia, Athens, Georgia, USA

Each year in the surface ocean, ~ 20 Gt of biologically labile carbon liberated from phytoplankton cells as dissolved organic matter (DOM) is rapidly taken up by heterotrophic bacteria. Early studies of this highly labile DOM focused intuitively on metabolic intermediates common to microbial cells, such as free amino acids and sugars. Yet other important conduits of rapid C flux into the microbial food web may have escaped our attention, both because they are assimilated too rapidly to accumulate in seawater and because they are difficult to identify against the complex chemical background of marine DOM. One emerging strategy with the promise of expanding our knowledge of phytoplankton-derived DOM leverages a genomics approach: using gene expression patterns invoked by bacteria-phytoplankton interactions to identify metabolites that are exchanged between cells. We performed RNA-Seq analyses of mutualistic model microbial system in which a bacterium (marine Roseobacter Ruegeria pomeroyi) relied on a co-cultured diatom (Thalassiosira pseudonana) as it sole source of organic matter and the diatom relied on the bacterium for vitamin B12. Two little-known sulfonates emerged as major biolabile compounds supporting the growth of the bacterium, and evidence was found that these compounds actively cycle in a diatom-dominated marine system. Such exchanges of previously unrecognized metabolites between phytoplankton and associated bacterial cells may sum to sizable fluxes in global elemental cycles.

Katherine E. Helliwell1, Andrew Lawrence2, Andre Holzer1, Martin Warren2, Alison G. Smith1
1Department of Plant Sciences, University of Cambridge, Cambridge, CB2 3EA, UK.
2 Department of Biosciences, University of Kent, Canterbury, Kent, CT2 7NJ, UK

Photosynthesis by marine phytoplankton is a major component of the global carbon cycle. Understanding the factors that influence their growth is therefore crucial. Over half of all eukaryotic microalgae are auxotrophic for vitamin B12. Since vitamin B12 biosynthesis is confined to prokaryotes, a key question is what is the source of the vitamin for eukaryotic algae? In many parts of the world’s oceans the predominant prokaryotes are cyanobacteria. Evidence gathered from whole-genome searches of diverse bacterioplankton species has revealed that genes for vitamin B12 biosynthesis are present in the majority of cyanobacterial taxa surveyed. In this study, we investigate vitamin B12 biosynthesis in strains of the abundant and ecologically significant marine Synechococcus genus. The physiological activity of cyanobacterially derived vitamin B12 to eukaryotic algae is studied in a survey of diverse algal species including the marine diatoms Thalassiosira pseudonana and Phaeodactylum tricornutum. Our results represent an important shift in our understanding of the dynamics, and complexity of vitamin B12 cycling in aquatic microbial communities.

Bryndan P. Durham1, Stephen P. Dearth2, Shady A. Amin3, Laura Truxal1, Shalabh Sharma4, Christa B. Smith4, Shawn R. Campagna2, Mary Ann Moran4, Anitra E. Ingalls1, E. Virginia Armbrust1
1 School of Oceanography, University of Washington, Seattle, Washington, USA
2 Department of Chemistry, University of Tennessee, Knoxville, TN, USA
3 Department of Chemistry. New York University Abu Dhabi, Abu Dhabi, UAE
4 Department of Marine Sciences, University of Georgia, Athens, Georgia, USA

The aggregation of heterotrophic bacteria in phycospheres of marine phytoplankton fosters conditions for signaling and metabolite exchange over micron-scale distances. Defining the chemical currencies that form the basis of these trophic interactions is a crucial step in understanding the complex microbial networks that drive ocean elemental cycles. Recently, two model microbial systems between marine Roseobacter clade bacteria and diatoms were developed to explore bacterial-phytoplankton exchanges via gene expression and metabolite analyses. Although these two systems differ in the basis of their relationship (one is based on B12 auxotrophy, the other on auxin signaling), sulfonate exchange is a common thread across these mutualisms. Further, exchange of two novel sulfonates 2,3-dihydroxypropane-1-sulfonate (DHPS) and N-acetyltaurine was detected, neither of which had been recognized in the marine microbial food web. Genes for both sulfonate catabolism and auxin synthesis have limited distribution among bacterioplankton, suggesting that sulfonate and auxin cycling underlie targeted interactions between mutualistic bacteria and diatoms. Bacterial gene expression and plankton metabolite analyses in seawater communities confirm prevalence of both auxin signaling and sulfonate feeding in surface ocean communities. Such exchange of signaling molecules and metabolic currencies between phytoplankton and associated bacteria likely represents new links in oceanic ecosystem interdependencies and biogeochemical cycles.