Award: OCE-2049386

Marine phytoplankton form the base of marine food webs and are responsible for nearly half of the global net primary production - the process that produces oxygen through the chemical conversion of carbon dioxide into organic. Diatoms, a group of unicellular, eukaryotic phytoplankton responsible for ~40% of the marine primary productivity, uniquely produce a cell wall comprised of silicon dioxide, or silica. The ballasted nature of this "glass house" facilitates sinking through the biological pump, which can enhance carbon sequestration. As the most abundant predatory entities in the ocean, viruses play a critical role in shaping microbial ecosystems, and as seen by the COVID-19 pandemic, can impact humanity on a global scale. For decades, viral infection in the ocean has predominantly been considered a mechanism that redirects ("shunts") particulate matter away from higher trophic levels and recycled as dissolved organic matter by heterotrophic bacteria. However, some studies suggest viruses may also act as "shuttles", facilitating carbon export by stimulating sinking. The goal of this project was to characterize the these two diametrically opposing outcomes in diatoms by comparing and contrasting the impact of viral infection on processes that facilitate sinking - mineral ballast production and particle aggregation - to those that stimulate recycling - bacterial-mediated remineralization of diatom silica and organic matter. Using a combination of laboratory culture-based studies and field observations of natural communities, we discovered that environmental conditions, specifically nutrient limitation, play a critical role in driving infection dynamics. While silicon limitation of diatom growth facilitated infection and accelerated mortality (Kranzler et al. 2019), iron limitation delayed infection and reduced viral production (Kranzler et al. 2021). These two conditions highlight the dynamic nature of infection and demonstrate the importance of nutrient availability in tipping the balance of infection toward the shunt (e.g. faster mortality under silicon limitation) or shuttle (e.g. delayed infection under iron limitation). At the same time, we also found that iron-limitation of diatoms decreased cellular carbon and nitrogen assimilation, but did not impact biogenic silica production relative to iron-replete diatoms. Although this manifests as an increase in ballast mineral in iron-limited diatoms, it suggests that although iron-limited diatoms may be efficient sinkers, they may not be efficient carbon exporters (Maniscalco et al. 2022). Phytoplankton are an important source of organic matter for heterotrophic bacteria, which in turn are critical to carbon cycling and nutrient recycling. For diatoms, bacteria play an additional role in facilitating the dissolution of silica into silicate that can be reused by other diatoms for growth. We discovered that the host physiological response to different viruses leads to distinct alterations to the landscape of organic matter that is released upon host lysis (Edwards et al., 2024). Small, utilizable DOM produced during infection by a DNA virus immediately stimulated bacterial growth, but less bioavailable DOM produced during infection by an RNA virus required additional enzymatic processing prior to consumption (Kranzler et al., 2025). Enzymatic activity has been shown in other studies to decrease the flux of carbon out of the surface ocean suggesting that, relative to DNA viral infection, RNA viral infection may tip the balance toward the viral shunt by increasing enzymatic activity and processing of organic matter. This revealed that not all viral infections are created equal and that infection by different viruses can influence the fate of diatom organic matter. During the life of this award, we published nine manuscripts (two more are in preparation) and three book chapters. Results were communicated at six national and international conferences, three workshops, and five institutional seminars. Seven graduate students were supported by aspects of this project and four undergraduate students gained valuable hands-on experience, two of which served as co-authors on manuscripts. To facilitate ocean literacy and enhance the broader impact of this work, we expanded the "Tools of Science", a freely available series of educational videos and lesson plans designed to help young learners explore the nature and process of science through real research investigations. A new video on the role of "Creativity" in science was produced alongside a lesson plan that teaches students the value of convergent research and the importance of phytoplankton and viruses to Earth's biogeochemical cycles. For decades, the role of viruses as shunts has dominated microbial ecology and marine virology. This project led to new discoveries on the role of viruses as shuttles and provided insight into the complexity and dynamic nature of the relationship between diatom hosts and viruses. This work will be critical to ongoing ecosystems modeling efforts that seek to constrain and parameterize the relevant factors involved in net primary production and biological pump efficiency, advancing our understanding of host-virus interactions in this ecologically important group of phytoplankton. Last Modified: 04/18/2025 Submitted by: KimberleeThamatrakoln