Follett
Christopher L.
Follett
Christopher L.
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ArticleDiel light cycles affect phytoplankton competition in the global ocean(Wiley, 2022-07-02) Tsakalakis, Ioannis ; Follows, Michael J. ; Dutkiewicz, Stephanie ; Follett, Christopher L. ; Vallino, Joseph J.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.
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ArticleUltrasonic nebulization for the elemental analysis of microgram-level samples with offline aerosol mass spectrometry(European Geosciences Union, 2019-03-14) O'Brien, Rachel E. ; Ridley, Kelsey J. ; Canagaratna, Manjula R. ; Jayne, John T. ; Croteau, Philip L. ; Worsnop, Douglas R. ; Budisulistiorini, Sri Hapsari ; Surratt, Jason D. ; Follett, Christopher L. ; Repeta, Daniel J. ; Kroll, Jesse H.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 matter.
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ArticleMarine virus-like particles and microbes : a linear interpretation(Frontiers Media, 2018-03-01) Cael, B. Barry ; Carlson, Michael C. G. ; Follett, Christopher L. ; Follows, Michael J.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.
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ArticleOpen ocean particle flux variability from surface to seafloor(American Geophysical Union, 2021-04-18) Cael, B. Barry ; Bisson, Kelsey ; Conte, Maureen H. ; Duret, Manon T. ; Follett, Christopher L. ; Henson, Stephanie A. ; Honda, Makio C. ; Iversen, Morten H. ; Karl, David M. ; Lampitt, Richard S. ; Mouw, Colleen B. ; Muller-Karger, Frank E. ; Pebody, Corinne ; Smith, Kenneth L., Jr. ; Talmy, DavidThe 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.
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ArticleCan rates of ocean primary production and biological carbon export be related through their probability distributions?(John Wiley & Sons, 2018-05-08) Cael, B. Barry ; Bisson, Kelsey ; Follett, Christopher L.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.
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ArticleClosely related phytoplankton species produce similar suites of dissolved organic matter(Frontiers Media, 2014-03-28) Becker, Jamie W. ; Berube, Paul M. ; Follett, Christopher L. ; Waterbury, John B. ; Chisholm, Sallie W. ; DeLong, Edward F. ; Repeta, Daniel J.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.
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ThesisHeterogeneous reservoirs in the marine carbon cycle(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2014-06) Follett, Christopher L.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 the mechanism behind the power-law slowdown in organic remineralization in sediment. We find that diffusion limitation connects the decay behavior of organic material to the correlations found between mineral surface area and organic matter content in sediments.
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ArticleDisentangling top-down drivers of mortality underlying diel population dynamics of Prochlorococcus in the North Pacific Subtropical Gyre(Nature Research, 2024-03-07) Beckett, Stephen J. ; Demory, David ; Coenen, Ashley R. ; Casey, John R. ; Dugenne, Mathilde ; Follett, Christopher L. ; Connell, Paige ; Carlson, Michael C. G. ; Hu, Sarah K. ; Wilson, Samuel T. ; Muratore, Daniel ; Rodriguez-Gonzalez, Rogelio A. ; Peng, Shengyun ; Becker, Kevin W. ; Mende, Daniel R. ; Armbrust, E. Virginia ; Caron, David A. ; Lindell, Debbie ; White, Angelicque E. ; Ribalet, Francois ; Weitz, Joshua S.Photosynthesis fuels primary production at the base of marine food webs. Yet, in many surface ocean ecosystems, diel-driven primary production is tightly coupled to daily loss. This tight coupling raises the question: which top-down drivers predominate in maintaining persistently stable picocyanobacterial populations over longer time scales? Motivated by high-frequency surface water measurements taken in the North Pacific Subtropical Gyre (NPSG), we developed multitrophic models to investigate bottom-up and top-down mechanisms underlying the balanced control of Prochlorococcus populations. We find that incorporating photosynthetic growth with viral- and predator-induced mortality is sufficient to recapitulate daily oscillations of Prochlorococcus abundances with baseline community abundances. In doing so, we infer that grazers in this environment function as the predominant top-down factor despite high standing viral particle densities. The model-data fits also reveal the ecological relevance of light-dependent viral traits and non-canonical factors to cellular loss. Finally, we leverage sensitivity analyses to demonstrate how variation in life history traits across distinct oceanic contexts, including variation in viral adsorption and grazer clearance rates, can transform the quantitative and even qualitative importance of top-down controls in shaping Prochlorococcus population dynamics.