Elucidating the influence of ocean mixed layer depth on phytoplankton physiology, virus production, and cloud formation
Creators: name: Diaz, Benjamin Peter
Publisher: name: No Publisher Supplied
Year: 2022

description: Marine phytoplankton are a diverse group of planktonic photoautotrophs which constitute the base of the marine food web and supply about half of the oxygen in Earth’s atmosphere. They grow and accumulate seasonally to high cell concentrations in the sunlit regions of the ocean in mesoscale events called “blooms”, which span hundreds of thousands of square kilometers. The annual formation and demise of phytoplankton blooms are closely tied to changes in mixed layer depth (MLD), which is the uppermost region that is homogeneously mixed by currents, wind, and convective heating/cooling. It is known that mixed layer (ML) shallowing in the spring induces rapid phytoplankton accumulation, which ultimately turns negative throughout the summer as the MLD remains stably shallow. Traditionally, nutrient limitation and grazing were considered driving factors of phytoplankton senescence, even though the intracellular mechanisms and pathways utilized by phytoplankton, as well as the effect of virus infection of phytoplankton, had not been examined on a bloom wide scale and in response to short term MLD changes. As the ML deepens in the beginning of the winter, phytoplankton communities experience less sunlight per day, replenishment of nutrients from colder water below, and dilution of phytoplankton and their predators (grazers) as well as pathogens (viruses). Throughout the annual bloom cycle, aerosols are continuously formed at the air-sea interface. In ocean regions with senescing phytoplankton communities, aerosol have higher organic carbon entrainment, which can impact their ability to form clouds. The cloud forming activity of aerosolized dissolved organic matter associated with virus infected phytoplankton communities remains undescribed. My thesis addresses how ocean physics impacts phytoplankton physiology, virus infection and aerosol composition using multidisciplinary oceanic observations along with lab-based model systems.Chapter 1 investigated the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing altered community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring MLs were characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring had high levels of oxidative stress. Prolonged stratification into early autumn manifested in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renewed during ML deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure. Chapter 2 characterized the phytoplankton community response to mixed layer shallowing (from 233 m to 5 m) over the course of two days during the late spring in the Northwest Atlantic using metatranscriptomics. Most phytoplankton genera downregulated core photosynthesis, carbon storage, and carbon fixation genes as the system transitioned from a deep to a shallow mixed layer and shifted towards catabolism of stored carbon supportive of rapid cell growth. In contrast, phytoplankton genera exhibited divergent transcriptional strategies for photosystem light harvesting complex genes during this transition. Active infection, taken as the ratio of virus to host transcripts, increased in the Bacillariophyta (diatom) phylum and decreased in the Chlorophyta (green algae) phylum upon mixed layer shallowing. A conceptual model is proposed to provide ecophysiological context for our findings, in which integrated light limitation and lower division rates during transient deep mixing disrupts resource-driven, oscillating transcript levels related to photosynthesis, carbon fixation, and carbon storage. Chapter 2’s findings highlight shared and unique transcriptional response strategies within phytoplankton communities acclimating to the dynamic light environment associated with transient deep mixing and shallowing events during the annual North Atlantic bloom.Chapter 3 determined the effect of transparent exopolymer particles and viral-mediated release of organic matter on the cloud condensation activity of aerosolized phytoplankton exudates. Dissolved organic matter derived from five common host-virus combinations comprising diatoms, coccolithophores, and chlorophytes, all taxa commonly found in the North Atlantic, were aerosolized and analyzed for their cloud forming activity. Overall, viral infection and subsequent aerosolization resulted in organic aerosols with less cloud condensation nucleation activity compared to uninfected seawater or healthy phytoplankton exudate. Viral infection increased the critical activation diameter compared to unamended seawater in three out of five host/virus combinations tested. Compared to bulk seawater and healthy cultures, the dissolved organic material derived from infected cultures had a higher organic carbon content, average molar mass, and had generally lower hygroscopicity than organic material derived from uninfected phytoplankton. Xanthan gum, a proxy for transparent exopolymers commonly found in infected and stressed phytoplankton, altered the surface tension and average molar mass only of infected diatom cultures. The findings in Chapter 3 call attention to how virus infection can alter the physical properties of associated aerosols, with implications for cloud formation modeling and climate.My thesis concludes with some interpretive thoughts and general outcomes on the major findings in the context of MLD changes and addresses key open questions, the potential impacts on a changing ocean, and future research directions of research.