I am a Research Associate in the UTS Climate Change Cluster. My role involves integrating research across several teams investigating microbial processes in marine systems. It is focussed on the role of algal-bacterial interactions in the context of oceanography, biotechnology, symbiosis and algal blooms. I am using my integrated expertise in biology and bioinformatics to assist in laboratory-based experiments and analyse metagenomic/metabarcoding and transcriptomic data.
Prior to joining UTS I held a Postdoctoral Fellowship at Macquarie University in the Department of Chemistry and Biomolecular Sciences.
Hughes, DJ, Varkey, D, Doblin, MA, Ingleton, T, Mcinnes, A, Ralph, PJ, van Dongen-Vogels, V & Suggett, DJ 2018, 'Impact of nitrogen availability upon the electron requirement for carbon fixation in Australian coastal phytoplankton communities', Limnology and Oceanography, vol. 63, no. 5, pp. 1891-1910.View/Download from: UTS OPUS or Publisher's site
© 2018 Association for the Sciences of Limnology and Oceanography Nitrogen (N) availability affects phytoplankton photosynthetic performance and regulates marine primary production (MPP) across the global coast and oceans. Bio-optical tools including Fast Repetition Rate fluorometry (FRRf) are particularly well suited to examine MPP variability in coastal regions subjected to dynamic spatio-temporal fluctuations in nutrient availability. FRRf determines photosynthesis as an electron transport rate through Photosystem II (ETRPSII), requiring knowledge of an additional parameter, the electron requirement for carbon fixation (KC), to retrieve rates of CO2-fixation. KC strongly depends upon environmental conditions regulating photosynthesis, yet the importance of N-availability to this parameter has not been examined. Here, we use nutrient bioassays to isolate how N (relative to other macronutrients P, Si) regulates KC of phytoplankton communities from the Australian coast during summer, when N-availability is often highly variable. KC consistently responded to N-amendment, exhibiting up to a threefold reduction and hence an apparent increase in the efficiency with which electrons were used to drive C-fixation. However, the process driving this consistent reduction was dependent upon initial conditions. When diatoms dominated assemblages and N was undetectable (e.g., post bloom), KC decreased predominantly via a physiological adjustment of the existing community to N-amendment. Conversely, for mixed assemblages, N-addition achieved a similar reduction in KC through a change in community structure toward diatom domination. We generate new understanding and parameterization of KC that is particularly critical to advance how FRRf can be applied to examine C-uptake throughout the global ocean where nitrogen availability is highly variable and thus frequently limits primary productivity.
Varkey, D, Mazard, S, Jeffries, T, Hughes, D, Seymour, J, Paulsen, IT & Ostrowski, M 2018, 'Stormwater influences phytoplankton assemblages within the diverse, but impacted Sydney Harbour estuary', PLoS ONE, vol. 13, no. 12, pp. e0209857-e0209857.View/Download from: UTS OPUS or Publisher's site
Sydney Harbour is subjected to persistent stress associated with anthropogenic activity and global climate change, but is particularly subjected to pulse stress events associated with stormwater input during episodic periods of high rainfall. Photosynthetic microbes underpin metazoan diversity within estuarine systems and are therefore important bioindicators of ecosystem health; yet how stormwater input affects their occurrence and distribution in Sydney Harbour remains poorly understood. We utilised molecular tools (16S/18S rRNA and petB genes) to examine how the phytoplankton community structure (both prokaryotes and eukaryotes) within Sydney Harbour varies between high and low rainfall periods. The relative proportion of phytoplankton sequences was more abundant during the high rainfall period, comprising mainly of diatoms, an important functional group supporting increased productivity within estuarine systems, together with cyanobacteria. Increased spatial variability in the phytoplankton community composition was observed, potentially driven by the steepened physico-chemical gradients associated with stormwater inflow. Conversely, during a low rainfall period, the proportion of planktonic photosynthetic microbes was significantly lower and the persistent phytoplankton were predominantly represented by chlorophyte and dinoflagellate sequences, with lower overall diversity. Differences in phytoplankton composition between the high and low rainfall periods were correlated with temperature, salinity, total nitrogen and silicate. These results suggest that increased frequency of high-rainfall events may change the composition, productivity and health of the estuary. Our study begins to populate the knowledge gap in the phytoplankton community structure and substantial changes associated with transient environmental perturbations, an essential step towards unravelling the dynamics of primary production in a highly urbanised estuarine ecosystem in response to cli...
Lagos, L, Tandberg, JI, Repnik, U, Boysen, P, Ropstad, E, Varkey, D, Paulsen, IT & Winther-Larsen, HC 2017, 'Characterization and Vaccine Potential of Membrane Vesicles Produced by Francisella noatunensis subsp orientalis in an Adult Zebrafish Model', CLINICAL AND VACCINE IMMUNOLOGY, vol. 24, no. 5.View/Download from: UTS OPUS or Publisher's site
Tandberg, JI, Lagos, LX, Berger, E, Rishovd, A-L, Roos, N, Varkey, D, Paulson, I & Winther-Larsen, HC 2016, 'Zebrafish as a model host for studies of Piscirickettsia salmonis and its outer membrane vesicles', FISH & SHELLFISH IMMUNOLOGY, vol. 53, pp. 86-86.
Tandberg, JI, Lagos, LX, Langlete, P, Berger, E, Rishovd, A-L, Roos, N, Varkey, D, Paulsen, IT & Winther-Larsen, HC 2016, 'Comparative Analysis of Membrane Vesicles from Three Piscirickettsia salmonis Isolates Reveals Differences in Vesicle Characteristics', PLOS ONE, vol. 11, no. 10.View/Download from: UTS OPUS or Publisher's site
Varkey, D, Mazard, S, Ostrowski, M, Tetu, SG, Haynes, P & Paulsen, IT 2016, 'Effects of low temperature on tropical and temperate isolates of marine Synechococcus', ISME JOURNAL, vol. 10, no. 5, pp. 1252-1263.View/Download from: UTS OPUS or Publisher's site
Tetu, SG, Johnson, DA, Varkey, D, Phillippy, K, Stuart, RK, Dupont, CL, Hassan, KA, Palenik, B & Paulsen, IT 2013, 'Impact of DNA damaging agents on genome-wide transcriptional profiles in two marine Synechococcus species', FRONTIERS IN MICROBIOLOGY, vol. 4.View/Download from: UTS OPUS or Publisher's site
© Springer International Publishing AG 2017. Viruses, bacteria, archaea and single celled eukaryotes, collectively known as microbes, dominate the biomass and metabolism of ocean ecosystems. Marine microbes are highly abundant and critical to human survival, but the vast majority of taxa have not yet been cultured. The use of environmental nucleic acid sequencing as a cultivation-independent approach to microbial oceanography has therefore significantly expanded our understanding of the diversity, evolution, biogeography and important biogeochemical roles of marine microorganisms. Here we provide illustrative examples of how genomic, transcriptomic and proteomic approaches have been applied to marine microbes to advance our understanding of their ecology. A remaining challenge is the need to link phenotypes to their environment, requiring a better understanding of genomic features that influence transcription (e.g. promoters and methylation) as well as post-translational modifications, and how such regulatory processes are impacted by extracellular abiotic and biotic processes. In addition, the expansion of available protein and taxonomic databases will greatly increase our capacity to link microbial function to specific taxa.