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Significance Prochlorococcus is the smallest and most abundant photosynthetic organism on Earth and is thought to be confined to low-latitude regions by its requirement for warm waters. Latitudinal transects in the North Pacific, however, demonstrate that the poleward decrease of this species occurs across a wide range of temperatures. An additional mechanism is likely required. We use theory, computational models, and additional observational data to suggest that the poleward decrease is caused by an ecological interaction: a shared predator which consumes both Prochlorococcus and similar-sized heterotrophic bacteria. Understanding the fate of this organism requires a knowledge of the interconnected ecosystem of other organisms, where both direct and indirect interactions control community structure.

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tsakalakis, I., Follows, M. J., Dutkiewicz, S., Follett, C. L., & Vallino, J. J. Diel light cycles affect phytoplankton competition in the global ocean. Global Ecology and Biogeography, 31(9), (2022): 1838-1849, https://doi.org/10.1111/geb.13562., Aim Light, essential for photosynthesis, is present in two periodic cycles in nature: seasonal and diel. Although seasonality of light is typically resolved in ocean biogeochemical–ecosystem models because of its significance for seasonal succession and biogeography of phytoplankton, the diel light cycle is generally not resolved. The goal of this study is to demonstrate the impact of diel light cycles on phytoplankton competition and biogeography in the global ocean. Location Global ocean. Major taxa studied Phytoplankton. Methods We use a three-dimensional global ocean model and compare simulations of high temporal resolution with and without diel light cycles. The model simulates 15 phytoplankton types with different cell sizes, encompassing two broad ecological strategies: small cells with high nutrient affinity (gleaners) and larger cells with high maximal growth rate (opportunists). Both are grazed by zooplankton and limited by nitrogen, phosphorus and iron. Results Simulations show that diel cycles of light induce diel cycles in limiting nutrients in the global ocean. Diel nutrient cycles are associated with higher concentrations of limiting nutrients, by 100% at low latitudes (−40° to 40°), a process that increases the relative abundance of opportunists over gleaners. Size classes with the highest maximal growth rates from both gleaner and opportunist groups are favoured by diel light cycles. This mechanism weakens as latitude increases, because the effects of the seasonal cycle dominate over those of the diel cycle. Main conclusions Understanding the mechanisms that govern phytoplankton biogeography is crucial for predicting ocean ecosystem functioning and biogeochemical cycles. We show that the diel light cycle has a significant impact on phytoplankton competition and biogeography, indicating the need for understanding the role of diel processes in shaping macroecological patterns in the global ocean., Simons Collaboration on Computational Biogeochemical Modeling of Marine Ecosystems supported M.J.F. and S.D. on CBIOMES grant #549931; C.L.F. on CBIOMES grants #827829 and #553242; and J.J.V. and I.T. on CBIOMES grant #549941. The National Science Foundation supported I.T. and J.J.V. on award #1558710 and J.J.V. on awards #1637630, #1655552 and #1841599.

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Cael, B. B., Bisson, K., Conte, M., Duret, M. T., Follett, C. L., Henson, S. A., Honda, M. C., Iversen, M. H., Karl, D. M., Lampitt, R. S., Mouw, C. B., Muller-Karger, F., Pebody, C. A., Smith, K. L., & Talmy, D. Open ocean particle flux variability from surface to seafloor. Geophysical Research Letters, 48(9), (2021): e2021GL092895, https://doi.org/10.1029/2021GL092895., The sinking of carbon fixed via net primary production (NPP) into the ocean interior is an important part of marine biogeochemical cycles. NPP measurements follow a log-normal probability distribution, meaning NPP variations can be simply described by two parameters despite NPP's complexity. By analyzing a global database of open ocean particle fluxes, we show that this log-normal probability distribution propagates into the variations of near-seafloor fluxes of particulate organic carbon (POC), calcium carbonate, and opal. Deep-sea particle fluxes at subtropical and temperate time-series sites follow the same log-normal probability distribution, strongly suggesting the log-normal description is robust and applies on multiple scales. This log-normality implies that 29% of the highest measurements are responsible for 71% of the total near-seafloor POC flux. We discuss possible causes for the dampening of variability from NPP to deep-sea POC flux, and present an updated relationship predicting POC flux from mineral flux and depth., B. B. Cael and S. A. Henson acknowledge support from the National Environmental Research Council (NE/R015953/1) and the Horizon 2020 Framework Programme (820989, project COMFORT). The work reflects only the authors' views; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains. S. A. Henson also acknowledges support from a European Research Council Consolidator grant (GOCART, agreement number 724416). C. L. Follett acknowledges support from the Simons Foundation (grants #827829 and #553242). M. H. Iversen acknowledges support from the DFG-Research Center/Cluster of Excellence “The Ocean Floor – Earth's Uncharted Interface”: EXC-2077-390741603 and the HGF Young Investigator Group SeaPump “Seasonal and regional food web interactions with the biological pump”: VH-NG-1000. M. C. Honda acknowledges financial support from the Ministry of Education, Culture, Sports, Science, and Technology – Japan (grants #: KAKENHI JP18H04144 and JP19H05667). M. Conte acknowledges support from the US National Science Foundation, Division of Ocean Sciences for support for the Oceanic Flux Program time-series since inception, most recently by NSF OCE grant 1829885. D. M. Karl acknowledges support from the Gordon and Betty Moore Foundation (#3794) and the Simons Foundation (SCOPE #329108).

The sinking of carbon fixed via net primary production (NPP) into the ocean interior is an important part of marine biogeochemical cycles. NPP measurements follow a log‐normal probability distribution, meaning NPP variations can be simply described by two parameters despite NPP’s complexity. By analyzing a global database of open ocean particle fluxes, we show that this log‐normal probability distribution propagates into the variations of near‐seafloor fluxes of particulate organic carbon (POC), calcium carbonate, and opal. Deep‐sea particle fluxes at subtropical and temperate time‐series sites follow the same log‐normal probability distribution, strongly suggesting the log‐normal description is robust and applies on multiple scales. This log‐normality implies that 29% of the highest measurements are responsible for 71% of the total near‐seafloor POC flux. We discuss possible causes for the dampening of variability from NPP to deep‐sea POC flux, and present an updated relationship predicting POC flux from mineral flux and depth.

In the North Pacific Ocean, nutrient rich surface waters flow south from the subpolar gyre through a transitional region and into the subtropics. Along the way, nutrients are used, recycled, and exported, leading to lower biomass and a commensurate change in ecosystem structure moving southward. We focus on the region between the two gyres (the Transition Zone) using a coupled biophysical ocean model, remote sensing, floats, and cruise data to explore the nature of the physical, biogeochemical, and ecological fields in this region. Nonlinear interactions between biological processes and the meridional gradient in nutrient supply lead to sharp shifts across this zone. These transitions between a southern region with more uniform biological and biogeochemical properties and steep meridional gradients to the north are diagnosed from extrema in the first derivative of the properties with latitude. Some transitions like that for chlorophyll a (the transition zone chlorophyll front [TZCF]) experience large seasonal excursions while the location of the transitions in other properties moves very little. The seasonal shifts are not caused by changes in the horizontal flow field, but rather by the interaction of seasonal, depth related, forcing with the mean latitudinal gradients. Focusing on the TZCF as a case study, we express its phase velocity in terms of vertical nutrient flux and internal ecosystem processes, demonstrating their nearly equal influence on its motion. This framework of propagating biogeochemical transitions can be systematically expanded to better understand the processes that structure ecosystems and biogeochemistry in the North Pacific and beyond.

The sinking of carbon fixed via net primary production (NPP) into the ocean interior is an important part of marine biogeochemical cycles. NPP measurements follow a log-normal probability distribution, meaning NPP variations can be simply described by two parameters despite NPP's complexity. By analyzing a global database of open ocean particle fluxes, we show that this log-normal probability distribution propagates into the variations of near-seafloor fluxes of particulate organic carbon (POC), calcium carbonate, and opal. Deep-sea particle fluxes at subtropical and temperate time-series sites follow the same log-normal probability distribution, strongly suggesting the log-normal description is robust and applies on multiple scales. This log-normality implies that 29% of the highest measurements are responsible for 71% of the total near-seafloor POC flux. We discuss possible causes for the dampening of variability from NPP to deep-sea POC flux, and present an updated relationship predicting POC flux from mineral flux and depth.

Diatom–diazotroph associations (DDAs) are symbioses where trichome-forming cyanobacteria support the host diatom with fixed nitrogen through dinitrogen (N2) fixation. It is inferred that the growth of the trichomes is also supported by the host, but the support mechanism has not been fully quantified. Here, we develop a coarse-grained, cellular model of the symbiosis between Hemiaulus and Richelia (one of the major DDAs), which shows that carbon (C) transfer from the diatom enables a faster growth and N2 fixation rate by the trichomes. The model predicts that the rate of N2 fixation is 5.5 times that of the hypothetical case without nitrogen (N) transfer to the host diatom. The model estimates that 25% of fixed C from the host diatom is transferred to the symbiotic trichomes to support the high rate of N2 fixation. In turn, 82% of N fixed by the trichomes ends up in the host. Modeled C fixation from the vegetative cells in the trichomes supports only one-third of their total C needs. Even if we ignore the C cost for N2 fixation and for N transfer to the host, the total C cost of the trichomes is higher than the C supply by their own photosynthesis. Having more trichomes in a single host diatom decreases the demand for N2 fixation per trichome and thus decreases their cost of C. However, even with five trichomes, which is about the highest observed for Hemiaulus and Richelia symbiosis, the model still predicts a significant C transfer from the diatom host. These results help quantitatively explain the observed high rates of growth and N2 fixation in symbiotic trichomes relative to other aquatic diazotrophs. Keywords: DDA; nitrogen fixation; diatom; diazotroph; carbon; nitrogen; growth rate; photosynthesis; cell flux model, Simons CBIOMES Postdoctoral Fellowship (award no. 553242)

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in O'Brien, R. E., Ridley, K. J., Canagaratna, M. R., Jayne, J. T., Croteau, P. L., Worsnop, D. R., Budisulistiorini, S. H., Surratt, J. D., Follett, C. L., Repeta, D. J., & Kroll, J. H. Ultrasonic nebulization for the elemental analysis of microgram-level samples with offline aerosol mass spectrometry. Atmospheric Measurement Techniques, 12(3), (2019):1659-1671, doi:10.5194/amt-12-1659-2019., The elemental composition of organic material in environmental samples – including atmospheric organic aerosol, dissolved organic matter, and other complex mixtures – provides insights into their sources and environmental processing. However, standard analytical techniques for measuring elemental ratios typically require large sample sizes (milligrams of material or more). Here we characterize a method for measuring elemental ratios in environmental samples, requiring only micrograms of material, using a small-volume nebulizer (SVN). The technique uses ultrasonic nebulization of samples to generate aerosol particles (100–300 nm diameter), which are then analyzed using an aerosol mass spectrometer (AMS). We demonstrate that the technique generates aerosol from complex organic mixtures with minimal changes to the elemental composition of the organic material and that quantification is possible using internal standards (e.g., NH154NO3). Sample volumes of 2–4 µL with total solution concentrations of at least 0.2 g L−1 form sufficient particle mass for elemental ratio measurement by the AMS, despite only a small fraction (∼ 0.1 %) of the sample forming fine particles after nebulization (with the remainder ending up as larger droplets). The method was applied to aerosol filter extracts from the field and laboratory, as well as to the polysaccharide fraction of dissolved organic matter (DOM) from the North Pacific Ocean. In the case of aerosol particles, the mass spectra and elemental ratios from the SVN–AMS agree with those from online AMS sampling. Similarly, for DOM, the elemental ratios determined from the SVN–AMS agree with those determined using combustion analysis. The SVN–AMS provides a platform for the rapid quantitative analysis of the elemental composition of complex organic mixtures and non-refractory inorganic salts from microgram samples with applications that include analysis of aerosol extracts and terrestrial, aquatic, and atmospheric dissolved organic mat, This work was supported by National Oceanic and Atmospheric Administration grant nos. NA13OAR4310072 and NA140AR4310132. Kelsey J. Ridley acknowledges support from the NSF Graduate Research Fellowship Program. Sri Hapsari Budisulistiorini and Jason D. Surratt acknowledges support from the U.S. Environmental Protection Agency award no. 835404, Electric Power Research Institute (EPRI), and National Oceanic and Atmospheric Administration grant no. NA13OAR4310064. Special thanks are due to David Karl and Eric Grabowski, University of Hawaii, for the CHNS elemental analysis of DOM. Daniel J. Repeta acknowledges support from the Gordan and Betty Moore Foundation award 6000 and the Simons Foundation SCOPE award 329108.

We describe the basis of a theory for interpreting measurements of two key biogeochemical fluxes—primary production by phytoplankton (p, μg C · L−1 · day−1) and biological carbon export from the surface ocean by sinking particles (f, mg C · m−2 · day−1)—in terms of their probability distributions. Given that p and f are mechanistically linked but variable and effectively measured on different scales, we hypothesize that a quantitative relationship emerges between collections of the two measurements. Motivated by the many subprocesses driving production and export, we take as a null model that large‐scale distributions of p and f are lognormal. We then show that compilations of p and f measurements are consistent with this hypothesis. The compilation of p measurements is extensive enough to subregion by biome, basin, depth, or season; these subsets are also well described by lognormals, whose log‐moments sort predictably. Informed by the lognormality of both p and f we infer a statistical scaling relationship between the two quantities and derive a linear relationship between the log‐moments of their distributions. We find agreement between two independent estimates of the slope and intercept of this line and show that the distribution of f measurements is consistent with predictions made from the moments of the p distribution. These results illustrate the utility of a distributional approach to biogeochemical fluxes. We close by describing potential uses and challenges for the further development of such an approach.

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 9 (2018): 358, doi:10.3389/fmicb.2018.00358., Viruses are key players in ocean ecology and biogeochemistry, not only because of their functional roles but also partially due to their sheer abundance (Fuhrman, 1999; Wilhelm and Suttle, 1999). Because viruses cannot replicate without their hosts' machinery, their abundance is inextricably related to that of their (mostly microbial) hosts. The relationship between viral and microbial abundances is thus of great interest.

Viruses are key players in ocean ecology and biogeochemistry, not only because of their functional roles but also partially due to their sheer abundance (Fuhrman, 1999; Wilhelm and Suttle, 1999). Because viruses cannot replicate without their hosts’ machinery, their abundance is inextricably related to that of their (mostly microbial) hosts. The relationship between viral and microbial abundances is thus of great interest.

In the North Pacific Subtropical Gyre (NPSG), an annual pulse of sinking organic carbon is observed at 4000 m between July and August, driven by large diatoms found in association with nitrogen fixing, heterocystous, cyanobacteria: Diatom-Diazotroph Associations (DDAs). Here we ask what drives the bloom of DDAs and present a simplified trait-based model of subtropical phototroph populations driven by observed, monthly averaged, environmental characteristics. The ratio of resource supply rates favors nitrogen fixation year round. The relative fitness of DDA traits is most competitive in early summer when the mixed layer is shallow, solar irradiance is high, and phosphorus and iron are relatively abundant. Later in the season, as light intensity drops and phosphorus is depleted, the traits of small unicellular diazotrophs become more competitive. The competitive transition happens in August, at the time when the DDA export event occurs. This seasonal dynamic is maintained when embedded in a more complex, global-scale, ecological model, and provides predictions for the extent of the North Pacific DDA bloom. The model provides a parsimonious and testable hypothesis for the stimulation of DDA blooms., Simons Foundation (Award 329108), National Science Foundation (U.S.) (Grant OCE-1434007), Gordon and Betty Moore Foundation (Grant GBMF3778), Gordon and Betty Moore Foundation (Grant GBMF3794)

Viruses are key players in ocean ecology and biogeochemistry, not only because of their functional roles but also partially due to their sheer abundance (Fuhrman, 1999; Wilhelm and Suttle, 1999). Because viruses cannot replicate without their hosts' machinery, their abundance is inextricably related to that of their (mostly microbial) hosts. The relationship between viral and microbial abundances is thus of great interest. Keywords: marine viruses; virus; virioplankton; viral abundance; virus-like particles; virus to microbe ratio; virus to bacterium ratio; virus-microbe relationship, National Science Foundation (U.S.) (Grant GRFP-2388357), National Science Foundation (U.S.) (Grant OCE-1315201), United States. National Aeronautics and Space Administration (Grant NNX16AR47G)

Thesis: Ph. D., Joint Program in Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2014., 122, Cataloged from PDF version of thesis., Includes bibliographical references (pages 149-158)., Understanding the fate of primary production in the ocean is a challenging task because once produced, organic material is oxidized over timescales which range from minutes, to millions of years. This timescale diversity is matched by an equal heterogeneity in both the local physical and chemical environment. In this thesis we explore the relationship between the distinct reservoirs of organic carbon in the ocean and their underlying complexity. First, we show how the heterogeneity of portions of the carbon cycle can be packaged in terms of age structured models and their accompanying age and rate distributions. We further relate the moments of the rate distributions to bulk reservoir properties like average age and flux. Explicit relationships are then derived for the specific case of a single turnover time and a lognormal distribution. We apply these ideas to the problem of dissolved organic carbon (DOC) cycling in the ocean. Current models of bulk concentration and isotope data suggest a microbially sourced DOC reservoir consisting of two components. A nearly homogeneous background component with a long turnover time (> 6000 years) is joined by a component of fast turnover time (~ 1 year) and equal concentration in the surface ocean. We confirm the presence of isotopically enriched, modern DOC co-cycling with an isotopically depleted older fraction in the upper ocean. However, our results show that up to 30% of the deep DOC reservoir is modern and supported by a 1 Pg per year carbon flux, ten times higher than inferred from bulk isotope measurements. Isotopically depleted material turns over at an apparent time scale of 30, 000 years, far slower than indicated by bulk isotope measurements. These results are consistent with global DOC measurements and explain both the fluctuations in deep DOC concentration and the anomalous radiocarbon values of DOC in the Southern Ocean. Finally, the thesis explores methods for determining the validity of diffusion limitation as, by Christopher L. Follett., Ph. D.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2014, Understanding the fate of primary production in the ocean is a challenging task because once produced, organic material is oxidized over timescales which range from minutes, to millions of years. This timescale diversity is matched by an equal heterogeneity in both the local physical and chemical environment. In this thesis we explore the relationship between the distinct reservoirs of organic carbon in the ocean and their underlying complexity. First, we show how the heterogeneity of portions of the carbon cycle can be packaged in terms of age structured models and their accompanying age and rate distributions. We further relate the moments of the rate distributions to bulk reservoir properties like average age and flux. Explicit relationships are then derived for the specific case of a single turnover time and a lognormal distribution. We apply these ideas to the problem of dissolved organic carbon (DOC) cycling in the ocean. Current models of bulk concentration and isotope data suggest a microbially sourced DOC reservoir consisting of two components. A nearly homogeneous background component with a long turnover time (> 6000 years) is joined by a component of fast turnover time (~1 year) and equal concentration in the surface ocean. We confirm the presence of isotopically enriched, modern DOC co-cycling with an isotopically depleted older fraction in the upper ocean. However, our results show that up to 30% of the deep DOC reservoir is modern and supported by a 1 Pg per year carbon flux, ten times higher than inferred from bulk isotope measurements. Isotopically depleted material turns over at an apparent time scale of 30;000 years, far slower than indicated by bulk isotope measurements. These results are consistent with global DOC measurements and explain both the fluctuations in deep DOC concentration and the anomalous radiocarbon values of DOC in the Southern Ocean. Finally, the thesis explores methods for determining the validity of diffusion limitation as th, Linden Fellowship, and the Jule Charney Prize at MIT. This work was funded by National Science Foundation grants OCE-0930866 and EAR-0420592 on which Daniel Rothman was the primary investigator. Additional work was funded by National Science Foundation Science and Technology Center for Microbial Oceanography Research and Education (C-MORE) grant number CCF-424599 and the Gordon and Betty Moore Foundation Microbial Oceanography Initiative grant number 3298 for which Daniel Repeta (Woods Hole Oceanographic Institution) was the primary investigator.

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 5 (2014): 111, doi:10.3389/fmicb.2014.00111., Production of dissolved organic matter (DOM) by marine phytoplankton supplies the majority of organic substrate consumed by heterotrophic bacterioplankton in the sea. This production and subsequent consumption converts a vast quantity of carbon, nitrogen, and phosphorus between organic and inorganic forms, directly impacting global cycles of these biologically important elements. Details regarding the chemical composition of DOM produced by marine phytoplankton are sparse, and while often assumed, it is not currently known if phylogenetically distinct groups of marine phytoplankton release characteristic suites of DOM. To investigate the relationship between specific phytoplankton groups and the DOM they release, hydrophobic phytoplankton-derived dissolved organic matter (DOMP) from eight axenic strains was analyzed using high-performance liquid chromatography coupled to mass spectrometry (HPLC-MS). Identification of DOM features derived from Prochlorococcus, Synechococcus, Thalassiosira, and Phaeodactylum revealed DOMP to be complex and highly strain dependent. Connections between DOMP features and the phylogenetic relatedness of these strains were identified on multiple levels of phylogenetic distance, suggesting that marine phytoplankton produce DOM that in part reflects its phylogenetic origin. Chemical information regarding the size and polarity ranges of features from defined biological sources was also obtained. Our findings reveal DOMP composition to be partially conserved among related phytoplankton species, and implicate marine DOM as a potential factor influencing microbial diversity in the sea by acting as a link between autotrophic and heterotrophic microbial community structures., This research was supported by grants to Daniel J. Repeta and Sallie W. Chisholm from the Gordon and Betty Moore Foundation and funding to Daniel J. Repeta, Edward F. DeLong, and Sallie W. Chisholm from the National Science Foundation Science and Technology Center Award 0424599.