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Thebiogeochemical model described in Sharoni & Halevy (2021) "Geologic controls on phytoplankton elemental composition", PNAS, DOI:https://doi.org/10.1073/pnas.2113263118. CNPorg_results.xlsxis anExcel file with the results of the model, i.e.,the model C:Porg and N:Porg trajectories (medians and 95% confidence intervals) over Phanerozoic time.

Significance The elemental composition of marine phytoplankton reflects their quality as a food source and regulates the flow of carbon, oxygen, and nutrients between the ocean, atmosphere, and rock reservoirs. Despite this importance, no systematic estimate exists of the geologic history of phytoplankton elemental composition, which is often regarded to have been constant through time, limiting our understanding of evolutionary patterns and global biogeochemistry. Here, using a biogeochemical model forced by geologic and evolutionary events (e.g., the colonization of the continents by plants), we find that the nutrient content of phytoplankton has increased over the last 550 million y of Earth history. This increase in the organic matter nutrient content has likely been important for the evolution of marine fauna., Planktonic organic matter forms the base of the marine food web, and its nutrient content (C:N:Porg) governs material and energy fluxes in the ocean. Over Earth history, C:N:Porg had a crucial role in marine metazoan evolution and global biogeochemical dynamics, but the geologic history of C:N:Porg is unknown, and it is often regarded constant at the “Redfield” ratio of ∼106:16:1. We calculated C:N:Porg through Phanerozoic time by including nutrient- and temperature-dependent C:N:Porg parameterizations in a model of the long-timescale biogeochemical cycles. We infer a decrease from high Paleozoic C:Porg and N:Porg to present-day ratios, which stems from a decrease in the global average temperature and an increase in seawater phosphate availability. These changes in the phytoplankton’s growth environment were driven by various Phanerozoic events: specifically, the middle to late Paleozoic expansion of land plants and the Triassic breakup of the supercontinent Pangaea, which increased continental weatherability and the fluxes of weathering-derived phosphate to the oceans. The resulting increase in the nutrient content of planktonic organic matter likely impacted the evolution of marine fauna and global biogeochemistry.

Spatial characteristics of phytoplankton blooms often reflect the horizontal transport properties of the oceanic turbulent flow in which they are embedded. Classically, bloom response to horizontal stirring is regarded in terms of generation of patchiness following large-scale bloom initiation. Here, using satellite observations from the North Pacific Subtropical Gyre and a simple ecosystem model, we show that the opposite scenario of turbulence dispersing and diluting fine-scale (∼1–100 km) nutrient-enriched water patches has the critical effect of regulating the dynamics of nutrients–phytoplankton–zooplankton ecosystems and enhancing accumulation of photosynthetic biomass in low-nutrient oceanic environments. A key factor in determining ecological and biogeochemical consequences of turbulent stirring is the horizontal dilution rate, which depends on the effective eddy diffusivity and surface area of the enriched patches. Implementation of the notion of horizontal dilution rate explains quantitatively plankton response to turbulence and improves our ability to represent ecological and biogeochemical processes in oligotrophic oceans., The degree to which horizontal transport affects phytoplankton ecosystems remains understudied. Here, the authors combine satellite observations, ARGO float data and a simple ecosystem model to explore the impact of horizontal stirring on naturally-stimulated fine-scale phytoplankton blooms.

Sea spray aerosols (SSA) are amongst the largest contributors to global aerosol mass, with a profound effect on climate through their radiative and microphysical properties. The contribution of oceanic microbial activity to SSA properties is yet to be fully established. We assessed the SSA emission of the calcite units (coccoliths) that compose the exoskeletons of the cosmopolitan bloom-forming coccolithophore, Emiliania huxleyi. We show that while emission of airborne coccoliths occurs in steady-state conditions, it increases by an order of magnitude during E. huxleyi infection by its specific E. huxleyi virus (EhV). The seawater to airborne coccolith ratio during viral infection was 1:108, from which we can estimate ambient airborne coccolith concentrations as a function of oceanic bloom properties. The unique aerodynamic structure of the emitted coccoliths yields a characteristic settling velocity ~25 times smaller compared to sea salt particles of the same size, resulting in an enrichment of the coccolith fraction in the air. The predicted enrichment is established in lab results, which show that coccoliths are key contributors to coarse mode SSA surface area, comparable with sea salt aerosols. This study demonstrates the coupling between key oceanic microbial interactions and fundamental atmospheric processes like SSA formation and emission.

SummaryPhytoplankton blooms are ephemeral events of exceptionally high primary productivity that regulate the flux of carbon across marine food webs [1–3]. Quantification of bloom turnover [4] is limited by a fundamental difficulty to decouple between physical and biological processes as observed by ocean color satellite data. This limitation hinders the quantification of bloom demise and its regulation by biological processes [5, 6], which has important consequences on the efficiency of the biological pump of carbon to the deep ocean [7–9].Here, we address this challenge and quantify algal blooms’ turnover using a combination of satellite and in situ data, which allows identification of a relatively stable oceanic patch that is subject to little mixing with its surroundings. Using a newly developed multisatellite Lagrangian diagnostic, we decouple the contributions of physical and biological processes, allowing quantification of a complete life cycle of a mesoscale (∼10–100 km) bloom of coccolithophores in the North Atlantic, from exponential growth to its rapid demise. We estimate the amount of organic carbon produced during the bloom to be in the order of 24,000 tons, of which two-thirds were turned over within 1 week. Complimentary in situ measurements of the same patch area revealed high levels of specific viruses infecting coccolithophore cells, therefore pointing at the importance of viral infection as a possible mortality agent. Application of the newly developed satellite-based approaches opens the way for large-scale quantification of the impact of diverse environmental stresses on the fate of phytoplankton blooms and derived carbon in the ocean.

Marine viruses constitute a major ecological and evolutionary driving force in the marine ecosystems. However, their dispersal mechanisms remain underexplored. Here we follow the dynamics of Emiliania huxleyi viruses (EhV) that infect the ubiquitous, bloom-forming phytoplankton E. huxleyi and show that EhV are emitted to the atmosphere as primary marine aerosols. Using a laboratory-based setup, we showed that the dynamic of EhV aerial emission is strongly coupled to the host–virus dynamic in the culture media. In addition, we recovered EhV DNA from atmospheric samples collected over an E. huxleyi bloom in the North Atlantic, providing evidence for aerosolization of marine viruses in their natural environment. Decay rate analysis in the laboratory revealed that aerosolized viruses can remain infective under meteorological conditions prevailing during E. huxleyi blooms in the ocean, allowing potential dispersal and infectivity over hundreds of kilometers. Based on the combined laboratory and in situ findings, we propose that atmospheric transport of EhV is an effective transmission mechanism for spreading viral infection over large areas in the ocean. This transmission mechanism may also have an important ecological impact on the large-scale host–virus “arms race” during bloom succession and consequently the turnover of carbon in the ocean.

Using shipboard and satellite measurements we explore the environmental factors affecting the number concentration of aerosols with diameter 100