Phosphorus physiology and environmental forcing of oceanic cyanobacteria, primarily Trichodesmium spp.

The biological transformation of dinitrogen gas (N2) into combined forms(termed N2 fixation) by certain genera of oceanic cyanobacteria represents the largest incoming flux of nitrogen to the global ocean. As such, biological nitrogen fixation plays a significant role in the regulation of oceanic productivity and the export of carbon and nitrogen out of the sun-lit surface waters. Currently, our knowledge of the biogeochemical and ecological significance of N2-fixing organisms is restricted by our relative inability to define mechanistically the relevant chemical, biological and physical controls of the production and abundance of biological N2 fixation in the marine environment. The four chapters that form the main body of this dissertation touch upon specific aspects of the controls of the production and abundance of N2-fixing organisms, particularly that of the cyanobacterial genus Trichodesmium. Chapter II defines the range of intracellular elemental composition exhibited by Trichodesmium and identifies phosphorus (P) as a key factor limiting N2 fixation by populations of this organism residing in specific regions of the Atlantic and Pacific oceans. Chapter III explores vertical migration as a physiological adaptation relevant to the growth of this same cyanobacterium, Trichodesmium. Results from this work indicate that vertical migration may allow a subset of the population to exploit the separation of light and nutrients under stratified oligotrophic conditions, thus potentially supplementing their P requirements for growth. Chapter IV examines time-series records in an attempt to define the physical characteristics of the environment that may regulate surface blooms of cyanobacteria. These analyses indicate that season, sea surface temperature and mixed layer depth are the most constrained predictors of blooms of N2-fixing cyanobacteria in the North Pacific. Lastly, Chapter V identifies a novel region of the ocean inhabited by N2-fixing organisms

New insights into bacterial acquisition of phosphorus in the surface ocean

Since 1958 when Alfred C. Redfield (1) recognized the similarity between the ratios of elements in living biomass and those dissolved in the surrounding seawater, we have understood that microorganisms largely control the concentrations, distribution, and molecular makeup of nutritional resources in the ocean. The primary elemental ingredients for life, carbon (C), nitrogen (N), and phosphorus (P), are assembled, disassembled, transformed, and consumed by marine microorganisms, resulting in a steady cycling of elements between intracellular, inorganic, and organic reservoirs. Of these pools, dissolved organic matter (DOM) represents the largest C, N, and P reservoir in the surface ocean of most marine habitats, greatly exceeding the respective concentrations of inorganic pools or that found in living organisms. DOM is a source of energy and elements, fueling heterotrophic and autotrophic growth alike (2), yet we understand very little about the biomolecular strategies marine microbes employ to use organic substrates in the global ocean. What is the molecular composition of organic matter, where do these compounds originate, and how much of this is bioavailable? How do microbes hydrolyze and transport constituents of DOM into the cell? What are the factors that regulate enzyme expression and control the decomposition of organic matter? These are but a few of the questions that must be addressed to fundamentally and mechanistically understand how microorganisms assimilate, transform, and turn over elemental resources in the ocean. In this issue of PNAS, Luo et al. (3) use a bioinformatics approach to investigate the diversity and localization of bacterial phosphatases, enzymes specialized for the hydrolysis of a reactive fraction of DOM, P-linked esters. Most notably, their study indicates that a significant fraction of bacteria may transport intact organophosphate compounds across the cell membrane for intracellular depolymerization, a finding counter to the prevailing concept of phosphatases as being largely extracellular. These disparate modes of DOM hydrolysis (extracellular versus intracellular) would have fundamentally different impacts on the ratios of elements in dissolved and particulate matter

Physiological Response of Crocosphaera watsonii to Enhanced and Fluctuating Carbon Dioxide Conditions

We investigated the effects of elevated pCO₂ on cultures of the unicellular N₂-fixing cyanobacterium Crocosphaera watsonii WH8501. Using CO₂-enriched air, cultures grown in batch mode under high light intensity were exposed to initial conditions approximating current atmospheric CO₂ concentrations (~400 ppm) as well as CO₂ levels corresponding to low- and high-end predictions for the year 2100 (~750 and 1000 ppm). Following acclimation to CO₂ levels, the concentrations of particulate carbon (PC), particulate nitrogen (PN), and cells were measured over the diurnal cycle for a six-day period spanning exponential and early stationary growth phases. High rates of photosynthesis and respiration resulted in biologically induced pCO₂ fluctuations in all treatments. Despite this observed pCO₂ variability, and consistent with previous experiments conducted under stable pCO₂ conditions, we observed that elevated mean pCO₂ enhanced rates of PC production, PN production, and growth. During exponential growth phase, rates of PC and PN production increased by ~1.2- and ~1.5-fold in the mid- and high-CO₂ treatments, respectively, when compared to the low-CO₂ treatment. Elevated pCO₂ also enhanced PC and PN production rates during early stationary growth phase. In all treatments, PC and PN cellular content displayed a strong diurnal rhythm, with particulate C:N molar ratios reaching a high of 22:1 in the light and a low of 5.5:1 in the dark. The pCO₂ enhancement of metabolic rates persisted despite pCO₂ variability, suggesting a consistent positive response of Crocosphaera to elevated and fluctuating pCO₂ conditions

Chasing after non-cyanobacterial nitrogen fixation in marine pelagic environments

Traditionally, cyanobacterial activity in oceanic photic layers was considered responsible for the marine pelagic dinitrogen (N2) fixation. Other potentially N2-fixing bacteria and archaea have also been detected in the pelagic water column, however, the activity and importance of these non-cyanobacterial diazotrophs (NCDs) remain poorly constrained. In this perspective we summarize the N2 fixation rates from recently published studies on photic and aphotic layers that have been attributed to NCD activity via parallel molecular measurements, and discuss the status, challenges, and data gaps in estimating non-cyanobacterial N2 fixation NCNF in the ocean. Rates attributed to NCNF have generally been near the detection limit thus far (<1 nmol N L−1 d−1). Yet, if considering the large volume of the dark ocean, even low rates of NCNF could make a significant contribution to the new nitrogen input to the ocean. The synthesis here shows that nifH transcription data for NCDs have been reported in only a few studies where N2 fixation rates were detected in the absence of diazotrophic cyanobacteria. In addition, high apparent diversity and regional variability in the NCDs complicate investigations of these communities. Future studies should focus on further investigating impacts of environmental drivers including oxygen, dissolved organic matter, and dissolved inorganic nitrogen on NCNF. Describing the ecology of NCDs and accurately measuring NCNF rates, are critical for a future evaluation of the contribution of NCNF to the marine nitrogen budget

Flexible elemental stoichiometry in Trichodesmium spp. and its ecological implications

We conducted laboratory experiments to assess the bioelemental plasticity of cultures of Trichodesmium IMS101 under phosphorus (P)-replete, P-restricted, and light-limited conditions. The results reveal a high degree of stoichiometric flexibility. Specifically, Trichodesmium IMS101 is capable of growth with carbon (C) : nitrogen (N) : P ratios of C₅₈₅±₅₆ : N₉₀±₁₀ : P₁, approximately six times higher than would be predicted by the Redfield reference ratio (C₁₀₆ : N₁₆ : P₁), thus signifying low cellular P quotas relative to C and N. Luxury consumption of P occurs rapidly after periods of prolonged P restriction, under both light and dark conditions, resulting in substantial increases in P quotas and reductions of C : N : P ratios (C₉₆±₈ : N₁₆±₁ : P₁). Comparisons of laboratory culture data to our field observations from the Northwest Atlantic and the North Pacific indicate that, while natural populations of Trichodesmium exhibit persistently low P content relative to C and N (C₂₉₀±₁₅ : N ₅₃±₃ : P₁), the highest and lowest C : P and N : P ratios recorded in the laboratory are rarely observed in nature. We have also performed laboratory experiments intended to simulate the energetic and nutritional extremes that would occur as naturally migrating populations of Trichodesmium sink out of the euphotic zone into P-rich regions of the upper disphotic zone. The duration of dark survival for this isolate is on the order of 3–6 d, after which time cells are unable to recover from light deprivation. This finding provides a constraint on the temporal scale of vertical migration.Individual authors are also free to post PDF copies of their work published in ASLO journals on their individual or institutional websites. It is not necessary to request permission from ASLO so long as such postings are not used for commercial purposes

Small Phytoplankton Drive High Summertime Carbonand Nutrient Export in the Gulf of California and Eastern Tropical North Pacific

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom‐dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton‐dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton‐ and diazotroph‐dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton‐dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including \u3c50 \u3eµm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method

SeaFlow Data V1, High-Resolution Abundance, Size and Biomass of Small Phytoplankton in the North Pacific

SeaFlow is an underway flow cytometer that provides continuous shipboard observations of the abundance and optical properties of small phytoplankton (\u3c5 μm in equivalent spherical diameter, ESD). Here we present data sets consisting of SeaFlow-based cell abundance, forward light scatter, and pigment fluorescence of individual cells, as well as derived estimates of ESD and cellular carbon content of picophytoplankton, which includes the cyanobacteria Prochlorococcus, Synechococcus and small-sized Crocosphaera (\u3c5 μm ESD), and picophytoplankton and nanophytoplankton (2–5 μm ESD). Data were collected in surface waters (≈5 m depth) from 27 oceanographic cruises carried out in the Northeast Pacific Ocean between 2010 and 2018. Thirteen cruises provide high spatial resolution (≈1 km) measurements across 32,500 km of the Northeast Pacific Ocean and 14 near-monthly cruises beginning in 2015 provide seasonal distributions at the long-term sampling site (Station ALOHA) of the Hawaii Ocean Time-Series. These data sets expand our knowledge of the current spatial and temporal distributions of picophytoplankton in the surface ocean

Small phytoplankton drive high summertime carbon and nutrient export in the Gulf of California and Eastern Tropical North Pacific

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 29 (2015): 1309–1332, doi:10.1002/2015GB005134.Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.NSF Grant Numbers: OCE-0726290, OCF-0962362, OCE-0726543, OCE-0726422; EU Grant Number: FP7-MC-IIF-220485; MEC Grant Number: CTM2007-31241-E/MAR; ICREA Academia; MERS Grant Number: 2014 SGR – 1356; Spain's Ministerio de Educación y Ciencia Grant Numbers: AP-2009-4733, BES-2004-3348; NASA New Investigator Award Grant Number: NNX10AQ81G; Sloan Research Fellowship2016-02-2

Summer surface waters in the Gulf of California: Prime habitat for biological N₂ fixation

We report significant rates of dinitrogen (N₂) fixation in the central basins of the Gulf of California (GC) during July–August 2005. Mixing model estimates based upon δ¹⁵N values of particulate matter in the surface mixed layer indicate that N₂ fixation provides as much as 35% to 48% of the phytoplankton-based nitrogen demand in the central Guaymas and Carmen basins. Microscopic analyses identify the responsible genera as the N₂-fixing endosymbiont, Richelia intracellularis, with lesser contributions from the large nonheterocystous diazotroph Trichodesmium. Analyses of remotely sensed chlorophyll a and sea surface temperature indicate that primary production levels are elevated in regions of the GC where oceanographic conditions are ideal in summertime for the growth of N₂-fixing organisms. These findings suggest that biological N₂ fixation must be taken into account when assessing past and present nitrogen dynamics in this environmentally important region.Keywords: nitrogen fixation, Gulf of California, remote sensin

High N2 Fixation in and Near the Gulf Stream Consistent with a Circulation Control on Diazotrophy

The stoichiometry of physical nutrient supply may provide a constraint on the spatial distribution and rate of marine nitrogen (N2) fixation. Yet agreement between the N2 fixation rates inferred from nutrient supply and those directly measured has been lacking. The relative transport of phosphate and nitrate across the Gulf Stream suggests that 3–6 Tg N year−1 must be fixed to maintain steady nutrient stoichiometry in the North Atlantic subtropical gyre. Here we show direct measurements of N2 fixation consistent with these estimates, suggesting elevated N2 fixation in and near the Gulf Stream. At some locations across the Gulf Stream, we measured diazotroph abundances and N2 fixation rates that are 1–3 orders of magnitude greater than previously measured in the central North Atlantic subtropical gyre. In combination, rate measurements and gene abundances suggest that biogeochemical budgets can be a robust predictive tool for N2 fixation hot spots in the global ocean