The microscopic cyanobacteria Prochlorococcus is the smallest and most abundant photosynthetic organism in the ocean, playing a large role in the global cycling of nutrients. Prochlorococcus form ecotypes, which are genetically variable lineages with differing physiological traits adapted to various environmental niches that enable them to be so widespread. They reside in oligotrophic (nutrient depleted) waters where certain required nutrients like phosphate can be scarce, and use a high affinity phosphate uptake pathway to take in phosphate. However, this pathway cannot completely differentiate a phosphate molecule from the physiochemically similar arsenate molecule, leading to the uptake of arsenic into cells, especially when the phosphate to arsenic ratio is low. Recent work performed by my mentors has studied the two pathways Prochlorococcus uses to detoxify arsenic, the efflux detoxification pathway, which was previously known, and the recently discovered putative methylation pathway, and they have found that strains of Prochlorococcus have different genomic capacities for arsenic detoxification. We are currently testing that genomic capacity for arsenic detoxification correlates to relative arsenic tolerance among lab strains of Prochlorococcus by batch culturing and comparing growth inhibition rates of four Prochlorococcus strains that differ in the pathway genes they contain, MED4, NATL2A, AS9601 and SS120. By inoculating the cultures in three different conditions with variable arsenate concentrations and calculating the inhibition rates, we will answer if Prochlorococcus genomic capacity correlates to relative arsenic tolerance. This would have impacts on how we look at the global circulation of arsenic because the two arsenic pathways have varying outputs that affect the circulation of arsenic and can influence the bioaccumulation of arsenic in upper trophic levels.

Pseudo-nitzschia is a diverse and cosmopolitan genus of diatoms composed of 37 species and known for its production of the neurotoxin domoic acid (DA). The phylogeny of the Pseudo-nitzschia genus is complex due to the presence of cryptic species complexes and the frequent discovery of new species. In previous phylogenetic studies, large subunits of ribosomal RNA (LSU) were used as a genetic marker to estimate how Pseudo-nitzschia species were related to one another. We sought to reevaluate the phylogeny of Pseudo-nitzschia both through recreating a LSU tree that accounts for recent discoveries made about the genus, as well as using the rbcL gene, which encodes the large subunit of the carbon fixation enzyme, ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO), in order to better understand the evolutionary relationships within the genus. Both trees provided better resolution in the branching pattern of Pseudo-nitzschia species and identified the presence of a third clade within the genus. Each clade had distinct morphological and physiological characteristics, such as cell size, frustule morphology, pigment content and DA production, which further supported the branching order. Metatranscriptomic reads obtained from four stations along a transect ranging from the Washington coast to the open ocean were placed on the rbcL tree and reflected a biogeographical distinction between the clades. The resolution provided in these phylogenies reflects the importance of continued identification and sequencing of new species. Furthermore, the phylogeny of the rbcL shows that it is an excellent marker for both phylogenic comparison as well as community profiling in the environment.

Cryopreservation is a technique that uses very low temperatures in order to store and preserve biological material for an extended period of time without deterioration. With the use of cryoprotectants, a substance that prevents the damage of freezing, cells are protected from the normally lethal effects of freezing. In order to understand the effects of cryopreservation in marine microorganisms, we are developing this technique with Phaeodactylum tricornutum, a marine pennate diatom with a sturdy body type. Diatoms are important drivers for biogeochemical cycles and the versatility of Phaeodactylum tricornutum in different environments allow it to be used as a model organism in studies about global phytoplankton primary production. By using a cryoprotectant such as DMSO, and a two-step freezing protocol, we allow the cells to experience freezing at a gradual rate giving it as stress-free of an environment as possible. Once frozen, the Phaeodactylum will be preserved until the thawing process begins, using a protocol that allows the rate of thawing to be gradual as well. This technique could allow for successful preservation and could be applied to many aspects of biology, in our case a laboratory model for ecological work.