Repetti, S.I., Orizar, I.D.S, Lewandowska, A.M. “Salinity modifies the effect of resource availability on phytoplankton communities.” Poster, October 2021. Baltic Sea Science Congress 2021, Aarhus University
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Repetti, S.I. Orizar, I.D.S., Blomster, J. & Lewandowska, A.M. “Winners vs Losers: Integrating phenotypic and transcriptomic responses of microalgae to salinity change.” Poster, August 2023. 6th Workshop on Trait-Based Approaches to Ocean Life, Copenhagen, Denmark, and Baltic Sea Science Congress 2023, Helsinki, Finland.
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Masters Thesis
Battle in the Brine: Salinity modifies the effect of resource availability on competition within phytoplankton communities Completed as part of Masters of Science (Environmental Change and Global Sustainability) at The University of Helsinki Grade: 5 Abstract: My master’s thesis aims to determine the effect of salinity on phytoplankton traits related to nutrient acquisition, and particularly how this interacts with resource availability. Salinity is an important driver structuring phytoplankton communities in the Baltic Sea. Salinity can also influence nutrient uptake by increasing metabolic rates required for osmotic adjustment. Thus, interaction between salinity and nutrient availability is expected to change community structure by altering phytoplankton traits determining resource competition. This is a particularly relevant area of study for the Baltic Sea due to predicted future freshening of the sea’s upper layer. We performed a microcosm experiment using artificial communities of 10 diverse phytoplankton species grown under different combinations of salinity (0, 5, 12 and 24), Nitrogen to Phosphorus molar ratio (N:P ratio = 2, 10, 16 and 80) and light (10 and 130 μmol photon m-2 s-1) conditions. A three-way interaction among these environmental parameters influenced phytoplankton traits associated with resource competition, as well as the presence and proportions of phytoplankton taxa. Light limitation inhibited community growth under all salinity conditions, but allowed diatom Phaeodactylum tricornutum to dominate. Community growth rate was higher under high light, but also more variable between salinity conditions. The strongest negative effects of nutrient limitation (N, P, and both nutrients together), both on growth rate and taxonomic diversity, were observed in the highest salinity treatment. In the freshwater treatment with the highest proportion of green algae Monoraphidium sp., N-limitation did not inhibit phytoplankton community growth and P- limitation had a more profound negative effect on community performance. Decreasing salinity appeared to decrease community C:N and C:P ratios. This shift is in opposition to the increasing C:N and C:P predicted as a consequence of other climate change-related drivers. Our results emphasise the importance of a trade-off between salinity and resource limitation in functioning of phytoplankton communities and suggest that future freshening of the Baltic Sea is likely to modify phytoplankton community composition and performance. Associated publications:
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Honours thesis
A Little Bit of Green: Genome dynamics in green algae Completed as part of Bachelor of Science (Degree with Honours) at The University of Melbourne. Grade: 88% Abstract: The green algae (Chlorophyta) include a diverse range of organisms that differ considerably in both morphology and the structure of their genomes. Their common origin, as well as the common origins of their organelles, means that the diversity of Chlorophyta genomes reflects evolutionary forces acting differently on various lineages and, potentially, differently on the three genomes – nuclear, chloroplast and mitochondrion – within a single lineage. My project aimed to examine the evolutionary forces shaping genomes within the Chlorophyta by characterising and analysing two genomes: the nuclear genome of unidentified pedinophyte YPF701, and the mitochondrial genome of the siphonous green seaweed Ostreobium quekettii. Both genomes are significant due to their positions phylogenetically. YPF701 at the base of the core Chlorophyta can provide insights into gene family evolution that occurred as this group diverged, while the O. quekettii mitochondrial genome represents only the second mitochondrial genome sequenced in the Bryopsidales. Both projects involved combining long and short read sequencing data to assemble the genomes as well as a variety of bioinformatic tools to analyse and compare them with other Chlorophyta. The nuclear genome of pedinophyte YPF701 is a fairly small (26-34 Mb) genome that shows evidence of gene family loss along the pedinophyte lineage. My project created a more contiguous hybrid nuclear genome assembly for YPF701 that can be used to examine gene family evolution, as well as the nature of noncoding regions in this lineage. The O. quekettii mitochondrial genome is the largest green algal mitochondrial genome sequenced thus far (241,739 bp), and is approximately three times larger than its economical plastid genome. The genome encodes genes typical of green algal mitochondrial genomes. Most of this excess size is explained by the expansion of intergenic DNA and proliferation of introns. Several theories can explain the evolution of both genomes described in this study, which ultimately reflect an interplay of mutation, natural selection and genetic drift. Associated publications:
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