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Bio‐optical properties of marine phytoplankton retrieved through satellite remote sensing are used to estimate ocean productivity and carbon cycling. Daily activity of phytoplankton is attuned to the predictable light fluctuations of the diel cycle. Field and laboratory studies have documented diel changes in phytoplankton growth, division, and respiration, carbon and pigment content, cell size, photosynthetic efficiency and rate, and DNA replication and transcription. Many of these physiological changes can alter cellular optical properties and contribute to diel variations in bulk absorption and scattering properties. Consequently, understanding phytoplankton contributions to diel optical cycles is essential for improving algorithms that convert remote sensing data to biological rates and stocks. Here, we describe time‐resolved cellular, photophysiological, and bio‐optical properties for three cultured phytoplankton ranging in cell size from ~ 1 to 6 μm: Ostreococcus lucimarinus, Synechococcus (WH8102), and Thalassiosira pseudonana, all of which can significantly contribute to phytoplankton abundance and/or biomass in the open and coastal ocean. This work is the first to characterize complete diel cycles in absorption and attenuation for O. lucimarinus and backscattering for all these species. Results show that the percent increase from the minimum to maximum values over the diel cycle ranged between ~ 24–121%, ~ 31–235%, and ~ 25–198% for particulate absorption, attenuation, and backscattering, respectively. Diel changes in bio‐optical properties also differed in both timing and magnitude across phytoplankton species, demonstrating the importance of contextualizing remote sensing observations with phytoplankton community composition. [ABSTRACT FROM AUTHOR]

Viral lysis of phytoplankton is one of the most common forms of death on Earth. Building on an assay used extensively to assess rates of phytoplankton loss to predation by grazers, lysis rates are increasingly quantified through dilution-based techniques. In this approach, dilution of viruses and hosts are expected to reduce infection rates and thus increase host net growth rates (i.e., accumulation rates). The difference between diluted and undiluted host growth rates is interpreted as a measurable proxy for the rate of viral lytic death. These assays are usually conducted in volumes ≥ 1 L. To increase throughput, we implemented a miniaturized, high-throughput, high-replication, flow cytometric microplate dilution assay to measure viral lysis in environmental samples sourced from a suburban pond and the North Atlantic Ocean. The most notable outcome we observed was a decline in phytoplankton densities that was exacerbated by dilution, instead of the increased growth rates expected from lowered virus-phytoplankton encounters. We sought to explain this counterintuitive outcome using theoretical, environmental, and experimental analyses. Our study shows that, while die-offs could be partly explained by a 'plate effect' due to small incubation volumes and cells adhering to walls, the declines in phytoplankton densities are not volume-dependent. Rather, they are driven by many density- and physiology-dependent effects of dilution on predation pressure, nutrient limitation, and growth, all of which violate the original assumptions of dilution assays. As these effects are volume-independent, these processes likely occur in all dilution assays that our analyses show to be remarkably sensitive to dilution-altered phytoplankton growth and insensitive to actual predation pressure. Incorporating altered growth as well as predation, we present a logical framework that categorizes locations by the relative dominance of these mechanisms, with general applicability to dilution-based assays. [ABSTRACT FROM AUTHOR]

The abundance and productivity of phytoplankton constrain energy transfer through marine food webs and the export of organic carbon to the deep ocean. Bio‐optical measurements correlate well with phytoplankton carbon (Cphyto), but the effect of taxonomic variability on this relationship is still uncertain. Here, we explore how changes in phytoplankton community composition influence the relationship between the particulate backscatter coefficient (bbp) and Cphyto and present a new approach to estimate phytoplankton biomass more accurately using bbp. We found that using a fixed scaling factor for the conversion of bbp to Cphyto could lead to the underestimation or overestimation of biomass, depending on the dominant taxonomic group in the phytoplankton community. In addition, we demonstrate how a simple ratio of absorption at two wavelengths can be used to provide a coarse approximation of phytoplankton community composition when scaling bbp to Cphyto, thereby improving the estimation of net primary production. [ABSTRACT FROM AUTHOR]

Episodic deep mixing events are one component of the biological carbon pump that physically transports organic carbon into the mesopelagic. Episodic deep mixing also disrupts summertime thermal stratification thereby changing the light field and nutrient concentrations available for phytoplankton growth. Phytoplankton survival and growth below the mixed layer following restratification depends on how rapidly cells can employ a variety of photoacclimation processes in response to the environmental changes. To compare the relative timescales of summertime episodic deep mixing events with the timescales of phytoplankton photoacclimation processes, we first analyzed autonomous float data to survey the frequency and magnitude of deep mixing events in the western North Atlantic Ocean. Next, we simulated a sustained deep mixing event in the laboratory and measured rates of acclimation processes ranging from light harvesting to growth in a model diatom and green alga. In both algae increases in chlorophyll (Chl) were coupled to growth, but growth of the green alga lagged the diatom by about a day. In float profiles, significant increases in Chl and phytoplankton carbon (Cphyto) were detected below the mixed layer following episodic deep mixing events. These events pose a previously unrecognized source of new production below the mixed layer that can significantly boost the amount of carbon available for export to the deep ocean. [ABSTRACT FROM AUTHOR]

Diatoms are major producers of carbon and energy in aquatic ecosystems, however their ability to rapidly and successfully acclimate to wide ranging irradiances is poorly understood at the biochemical level. We applied complementary transcriptomic and metabolomic approaches using the model centric diatom Thalassiosira pseudonana to understand mechanisms regulating cell acclimation and growth under high and low light intensities. The integration of these omics data revealed specific mechanisms that shifted carbon and energy fluxes in T. pseudonana depending on light-driven growth rate. To support a growth rate of >1 d−1 in high light, cells upregulated metabolic pathways involved in the production of long chain fatty acids, glycolic acid, and carbohydrates. Under low light, cells maintained a growth rate of ≤0.2 d−1 by conserving photosynthetic energy through upregulation of carbon retention pathways (e.g., gluconeogenesis, glyoxylate cycle). The few significant metabolites detected in low light acclimated cells were associated with light harvesting, energy storage, and signalling. In particular, our metabolite data revealed that eicosanoic acid, a metabolite commonly involved in cellular signalling, may poise light limited cells for fast metabolic remodelling with improving light conditions. The combined physiological, gene expression and metabolite profiling data revealed key metabolic switch points that underpin diatom success and could be leveraged in cell-based models for predictions and manipulations of cell growth or bio-production. • T. pseudonana exhibited significant metabolic shifts in response to changes in light. • Central metabolism shifts toward biosynthesis for rapid cell growth under high light. • Cells conserve photosynthetic energy and reduce energy-loss under low light. • Under low light, gluconeogenesis and glyoxylate cycle sustained steady growth rate. • Combined transcriptomics and metabolomics data revealed light-dependent shifts to support growth. [ABSTRACT FROM AUTHOR]

Acetaldehyde, methanol, acetone, and isoprene are important reactive volatile organic compounds (VOCs) in the oceans that partition to the atmosphere in significant amounts. Reports of potentially high rates of VOC turnover in the North Atlantic suggested that both biotic and abiotic processes are involved. The biological basis for VOC cycling by ocean plankton is unknown, but is potentially important because of VOC contributions to carbon cycle budgets and atmospheric chemistry. We designed dynamic stripping chambers that coupled to a proton transfer reaction mass spectrometer to measure VOC production and consumption by cultured phytoplankton and bacterioplankton. The diatom, Thalassiosira pseudonana, produced acetaldehyde in a light-dependent manner. Acetaldehyde was oxidized by the chemoheterotrophic bacterium, Pelagibacter, at rates that suggest that most acetaldehyde is recycled in the ocean before escaping to the atmosphere. These results show that field observations of acetaldehyde turnover reported previously could be explained by biological activity. Rates of production by phytoplankton cultures of methanol, acetone, and isoprene were also measured. These findings support the conclusion that VOCs are a conduit for carbon transfer directly from phytoplankton to bacterioplankton, with the remainder available for escape to the atmosphere. [ABSTRACT FROM AUTHOR]

Recent advances in sorting flow cytometry (SFCM) and sensitive biochemical techniques have opened the door for a variety of new incubation-free measurements to assess cell physiology. We adapted highly sensitive bioluminescent methodologies to measure cell NAD(H) and adenosine triphosphate (ATP) contents as predictors of phytoplankton primary production and growth rate. We first applied these methods to algae in taxonomically distinct groups: the chlorophyte Dunaliella tertiolecta, the diatom Thalassiosira pseudonana and a model cyanobacterium Synechococcus sp. WH8102. We found that NAD(H) content [NAD(H)phyto] increased significantly with increasing net primary productivity (NPP) and growth rate. In contrast, and in agreement with previous studies, phytoplankton ATP content (ATPphyto) was highly correlated with carbon biomass. Furthermore, ATPphyto was not correlated with NPP or growth rate. As a field test of the method, we then measured NAD(H)phyto in phytoplankton samples collected by SFCM in the North Pacific. These measurements yielded an estimate of NPP closely matching that from the traditional incubation-dependent 14C uptake technique. The methods described here enable low-biomass, incubation-free estimates of phytoplankton NPP, growth rate, and biomass. [ABSTRACT FROM AUTHOR]

The reproductive strategy of diatoms includes asexual and sexual phases, but in many species, including the model centric diatom Thalassiosira pseudonana, sexual reproduction has never been observed. Furthermore, the environmental factors that trigger sexual reproduction in diatoms are not understood. Although genome sequences of a few diatoms are available, little is known about the molecular basis for sexual reproduction. Here we show that ammonium reliably induces the key sexual morphologies, including oogonia, auxospores, and spermatogonia, in two strains of T. pseudonana, T. weissflogii, and Cyclotella cryptica. RNA sequencing revealed 1,274 genes whose expression patterns changed when T. pseudonana was induced into sexual reproduction by ammonium. Some of the induced genes are linked to meiosis or encode flagellar structures of heterokont and cryptophyte algae. The identification of ammonium as an environmental trigger suggests an unexpected link between diatom bloom dynamics and strategies for enhancing population genetic diversity. [ABSTRACT FROM AUTHOR]

The Carbon, Absorption, and Fluorescence Euphotic-resolving (CAFE) net primary production model is an adaptable framework for advancing global ocean productivity assessments by exploiting state-of-the-art satellite ocean color analyses and addressing key physiological and ecological attributes of phytoplankton. Here we present the first implementation of the CAFE model that incorporates inherent optical properties derived from ocean color measurements into a mechanistic and accurate model of phytoplankton growth rates ( μ) and net phytoplankton production (NPP). The CAFE model calculates NPP as the product of energy absorption ( QPAR), and the efficiency ( ϕ μ) by which absorbed energy is converted into carbon biomass ( CPhyto), while μ is calculated as NPP normalized to CPhyto. The CAFE model performance is evaluated alongside 21 other NPP models against a spatially robust and globally representative set of direct NPP measurements. This analysis demonstrates that the CAFE model explains the greatest amount of variance and has the lowest model bias relative to other NPP models analyzed with this data set. Global oceanic NPP from the CAFE model (52 Pg C m−2 yr−1) and mean division rates (0.34 day−1) are derived from climatological satellite data (2002-2014). This manuscript discusses and validates individual CAFE model parameters (e.g., QPAR and ϕ μ), provides detailed sensitivity analyses, and compares the CAFE model results and parameterization to other widely cited models. [ABSTRACT FROM AUTHOR]

The 14C-uptake method is the most common approach employed for estimating primary production in the ocean. Normalizing 14C-uptake to chlorophyll a and time yields a value termed the assimilation number, which is thought to reflect phytoplankton physiology. It is often assumed that the measured rate of 14C-uptake is between net and gross primary production, depending on the time scale of the incubation. Recent studies employing multiple oxygen and carbon isotopic methods to measure photosynthesis of phytoplankton grown over a range of steady-state division rates have provided mechanistic insights on the relationship between 14C-uptake and gross-to-net primary production. Results from these studies show that short-term (<12 h) "photosynthesis-irradiance" measurements are not a reliable means of estimating net production, gross production or nutrient limitation, but can provide important information on the photoacclimation state of the phytoplankton. Long-term (24 h) incubations yield assimilation numbers that are in good agreement with net production rates, but are independent of nutrient-limited division rates. Despite complications in interpreting 14C-uptake data, we suggest that these measurements are important for understanding phytoplankton physiology and carbon cycles while, at the same time, efforts are needed to establish new incubation-free methods for measuring phytoplankton division rate and biomass. [ABSTRACT FROM AUTHOR]

We compare the nutrient-dependent photosynthetic efficiencies of the chlorophyte, Dunaliella tertiolecta, with those of the marine diatom, Thalassiosira weissflogii. Despite considerable evolutionary and physiological differences, these two species appear to use nearly identical growth strategies under a wide range of nutrient limitation., Using a variety of physiological measurements, we find that, for both species and across all growth rates, 75% of the gross photosynthetic electron flow is invested in carbon fixation and only 30% is retained as net carbon accumulation. A majority of gross photosynthesis (70%) is ultimately used as reductant for biosynthetic pathways and for the generation of ATP., In both species, newly formed carbon products exhibit much shorter half-lives at slow growth rates than at fast growth rates. We show that this growth rate dependence is a result of increased polysaccharide storage during the S phase of the cell cycle., We present a model of carbon utilization that incorporates this growth rate-dependent carbon allocation and accurately captures ( r2 = 0.94) the observed time-resolved carbon retention. Together, our findings suggest a common photosynthetic optimization strategy in evolutionarily distinct phytoplankton species and contribute towards a systems-level understanding of carbon flow in photoautotrophs. [ABSTRACT FROM AUTHOR]

The chl-specific short-term C-based production ( P) measurement is a widely used tool to understand phytoplankton responses to environmental stresses. However, among the metabolic consequences of these stresses is variability in lifetimes of newly fixed carbon that cause P to range between chl-specific net primary production ( NPP *) and chl-specific gross photosynthetic electron flow that is available for carbon reduction () depending on growth rate. To investigate the basis for this discrepancy, photosynthate utilization was characterized in Dunaliella tertiolecta Butcher grown at three different growth rates in N-limited chemostats. P was measured throughout a 2 min to 24 h time course and showed clear growth-rate-dependent differences in lifetimes of newly fixed carbon. C pulse-chase experiments revealed differences in patterns of carbon utilization between growth rates. At high growth rate, the majority of C was initially fixed into polysaccharide and lipid, but the relative contribution of each labeled biochemical pool to the total label changed over 24 h. In fast-growing cells, labeled polysaccharides decreased 50%, while labeled lipids increased over the first 4 h. At low growth rate, C was initially incorporated primarily into protein, but the contribution of labeled protein to the total label increased over the next 24 h. Together, time-resolved measurements of P and cellular NAD and NADP content suggest an enhanced role for alternative dissipation pathways at very low growth rate. Findings of this study contribute to an integrated understanding of growth-rate-dependent shifts in metabolic processes from photosynthesis to net growth. [ABSTRACT FROM AUTHOR]

Abstract: Submarine volcanic eruptions can result in both real and apparent changes in marine algal communities, e.g., increases in phytoplankton biomass and/or growth rates that can cover thousands of square kilometers. Satellite ocean color monitoring detects these changes as increases in chlorophyll and particulate backscattering. Detailed, high resolution analysis is needed to separate the optical effects of volcanic products from the response of the marine algal community. It is possible to calculate an index, which maps the magnitude of improbable change (relative to long term average conditions) following known volcanic eruptions by using low resolution, initial estimates of chlorophyll and backscatter along with an archived history of satellite data. We apply multivariate probability analysis to changes in global satellite ocean chlorophyll and particulate backscatter data to create a new metric for observing apparent biological responses to submarine eruptions. Several examples are shown, illustrating the sensitivity of our improbability mapping index to known submarine volcanic events, yielding a potentially robust method for the detection of new events in remote locations. [Copyright &y& Elsevier]