The marine cyanobacterium, Prochlorococcus, is the most abundant photosynthetic organism in the world. Prochlorococcus is composed of two main clades, High Light (HL) and Low Light (LL). Within the clades, further subdivisions exist as differentiated populations adapted to their environment (ecotypes). Although these organisms can be found in most global surface oceans, the ecotypes are not equally distributed latitudinally nor vertically. Furthermore, the inter-specific relationship between the ecotypes and how the proportion of each one corresponds with different environmental conditions are not well understood. Therefore, in this study I investigate the intra- and interseasonal environmental conditions (e.g. temperature, light, and nutrients) that affect the distribution of Prochlorococcus ecotypes in the North Pacific Ocean. Using python coding, I analyzed the correlation of environmental data to datasets from a series of cruises that contain optical and molecular properties of Prochlorococcus. I utilized sequenced and mapped community samples of RNA from a published dataset, also known as metatranscriptomes, to identify present Prochlorococcus ecotypes and their associated relative abundances. Additionally, I used flow cytometry data to analyze forward scatter (cell size) and red fluorescence (chlorophyll) of each Prochlorococcus cell that is captured in a sample. Expected results of this study are that 1) light and temperature will be the most important factors determining the distribution shifts between the HL and LL clades, 2) temperature will be the most important factor differentiating the HL ecotypes, and 3) nutrient levels will be the most important factor differentiating the LL ecotypes. This study will enhance our understanding of how environmental conditions influence Prochlorococcus ecotypes in the North Pacific, though the findings may not represent global patterns. Furthermore, the results suggest that Prochlorococcus strains more susceptible to environmental changes may experience ecological shifts, an issue likely to intensify as climate change impacts the ocean.

Planktonic protists (unicellular eukaryotes) play essential roles in open-ocean biogeochemical cycles and food webs, functioning as phototrophs, heterotrophs, or mixotrophs depending on the species. However, cultured representatives of protists from the Pacific Ocean are scarce, limiting our understanding of protists within the largest ocean on Earth. In this study, we analyze seven cultured protist strains isolated from the tropical Pacific Ocean from the upper ocean from 30 °N to 4 °S and from 120 to 140 °W, including seven haptophytes, five pelagophytes, and four dinoflagellates. We examine transcriptomes from laboratory cultures of these isolates. We construct a phylogenetic tree of the isolates based on single-copy marker genes to infer evolutionary relationships. We examine correlations between phylogenetic relatedness and the latitude and depth of isolation. An additional objective of this work is to resolve the species-/strain-level taxonomy of these isolates, enabling their integration into the Marine Functional Eukaryotic Reference Taxa database. This will improve our ability to characterize marine protist diversity and function in metagenomes and -transcriptomes.

Ocean microbial communities are sensitive to temperature fluctuations. This study examines the direct and indirect effects of temperature on bacterial abundance off the coast of Guam (4°N-16°N, 149°E). Water samples were collected at 10m depth and the Deep Chlorophyll Maximum (DCM) to assess bacterial abundance and its relationship with temperature and chlorophyll concentration. Results show a strong positive correlation between bacterial abundance and temperature at 10m, suggesting warmer conditions enhance microbial growth. However, no significant correlation was found at the DCM, indicating other factors, such as mixing and nutrient availability, influence deeper bacterial communities. A notable anomaly at 8°N was linked to strong currents that redistributed bacteria, concentrating them at about 200m. These findings highlight the interplay between temperature, primary production, and ocean currents in regulating microbial abundance, offering insight into how microbial ecosystems may respond to climate-driven ocean changes.

Key drivers for primary productivity vary on latitudinal scales, such as nutrient and light. Nutrient variation can be seen at differing latitudes, such as lower nitrate to phosphate concentrations in the tropics (23°26’N to 23°26’S) compared to higher nitrate to phosphate concentrations in temperate regions (35° to 50° N and S) (Lønborg et al., 2021). With consistent differences by latitude of nutrient concentrations and abundance, it prompts the question of whether a nutrient-dependent entity such as phytoplankton biomass can be attributed to latitude change. To determine a correlation between nutrient availability and phytoplankton biomass, limiting nutrients and nutrient variations by latitude were investigated within the mixed layer determined by thermoclines from 4°N to 16°N along 149°E in Guam. The limiting nutrient of phytoplankton biomass was determined using on-deck incubators consisting of three conditions: + 1uM nitrate, +0.2uM phosphate, and a control with no added nutrients. Total chlorophyll served as a proxy for phytoplankton biomass, and was measured for three incubation sets from three different sampling stations. Nutrient concentrations were collected at every degree from 4°N to 16°N and compared by latitude to determine a relation between nutrient variability to latitude. Chlorophyll rate of change and mean total chlorophyll from nitrate incubations were significantly greater than phosphate and control incubations, pointing to nitrate as the limiting nutrient of primary productivity. No statistical correlation was established between nutrient variability and latitude, but there was a statistical correlation between size-fractionated chlorophyll and N:P ratios at the same latitude, signaling a latitudinal correlation. I hypothesized that the intensity of bottom-up control on primary productivity will increase with increasing latitude across a ten degree transect due to concentration variation of the limiting nutrient of chlorophyll.