The V-type ATPase enhances photosynthesis in marine phytoplankton and further links phagocytosis to symbiogenesis.

Diatoms, dinoflagellates, and coccolithophores are dominant groups of marine eukaryotic phytoplankton that are collectively responsible for the majority of primary production in the ocean.1 These phytoplankton contain additional intracellular membranes around their chloroplasts, which are derived from ancestral engulfment of red microalgae by unicellular heterotrophic eukaryotes that led to secondary and tertiary endosymbiosis.2 However, the selectable evolutionary advantage of these membranes and the physiological significance for extant phytoplankton remain poorly understood. Since intracellular digestive vacuoles are ubiquitously acidified by V-type H+-ATPase (VHA),3 proton pumps were proposed to acidify the microenvironment around secondary chloroplasts to promote the dehydration of dissolved inorganic carbon (DIC) into CO2, thus enhancing photosynthesis.4,5 We report that VHA is localized around the chloroplasts of centric diatoms and that VHA significantly contributes to their photosynthesis across a wide range of oceanic irradiances. Similar results in a pennate diatom, dinoflagellate, and coccolithophore, but not green or red microalgae, imply the co-option of phagocytic VHA activity into a carbon-concentrating mechanism (CCM) is common to secondary endosymbiotic phytoplankton. Furthermore, analogous mechanisms in extant photosymbiotic marine invertebrates6,7,8 provide functional evidence for an adaptive advantage throughout the transition from endosymbiosis to symbiogenesis. Based on the contribution of diatoms to ocean biogeochemical cycles, VHA-mediated enhancement of photosynthesis contributes at least 3.5 Gtons of fixed carbon per year (or 7% of primary production in the ocean), providing an example of a symbiosis-derived evolutionary innovation with global environmental implications.

Elongation enhances encounter rates between phytoplankton in turbulence.

Phytoplankton come in a stunning variety of shapes but elongated morphologies dominate-typically 50% of species have aspect ratio above 5, and bloom-forming species often form chains whose aspect ratios can exceed 100. How elongation affects encounter rates between phytoplankton in turbulence has remained unknown, yet encounters control the formation of marine snow in the ocean. Here, we present simulations of encounters among elongated phytoplankton in turbulence, showing that encounter rates between neutrally buoyant elongated cells are up to 10-fold higher than for spherical cells and even higher when cells sink. Consequently, we predict that elongation can significantly speed up the formation of marine snow compared to spherical cells. This unexpectedly large effect of morphology in driving encounter rates among plankton provides a potential mechanistic explanation for the rapid clearance of many phytoplankton blooms.

Dense intracellular ion pools in unicellular organisms.

Uptake and concentration of inorganic ions are part of the complex cellular processes required for cell homeostasis, as well as for mineral formation by organisms. These ion transport mechanisms include distinct cellular compartments and chemical phases that play various roles in the physiology of organisms. Here, the prominent cases of dense ion pools in unicellular organisms are briefly reviewed. The specific observations that were reported for different organisms are consolidated into a wide perspective that emphasizes general traits. It is suggested that the intracellular ion pools can be divided into three types: a high cytoplasmic concentration, a labile storage compartment that hosts dense ion-rich phases, and a mineral-forming compartment in which a stable long-lived structure is formed. Recently, many labile pools were identified in various organisms using advanced techniques, bringing many new questions about their possible roles in the formation of the stable mineralized structures.

Teamwork in the viscous oceanic microscale.

Nutrient acquisition is crucial for oceanic microbes, and competitive solutions to solve this challenge have evolved among a range of unicellular protists. However, solitary solutions are not the only approach found in natural populations. A diverse array of oceanic protists form temporary or even long-lasting attachments to other protists and marine aggregates. Do these planktonic consortia provide benefits to their members? Here, we use empirical and modeling approaches to evaluate whether the relationship between a large centric diatom, Coscinodiscus wailesii, and a ciliate epibiont, Pseudovorticella coscinodisci, provides nutrient flux benefits to the host diatom. We find that fluid flows generated by ciliary beating can increase nutrient flux to a diatom cell surface four to 10 times that of a still cell without ciliate epibionts. This cosmopolitan species of diatom does not form consortia in all environments but frequently joins such consortia in nutrient-depleted waters. Our results demonstrate that symbiotic consortia provide a cooperative alternative of comparable or greater magnitude to sinking for enhancement of nutrient acquisition in challenging environments.

Morphological bases of phytoplankton energy management and physiological responses unveiled by 3D subcellular imaging.

Eukaryotic phytoplankton have a small global biomass but play major roles in primary production and climate. Despite improved understanding of phytoplankton diversity and evolution, we largely ignore the cellular bases of their environmental plasticity. By comparative 3D morphometric analysis across seven distant phytoplankton taxa, we observe constant volume occupancy by the main organelles and preserved volumetric ratios between plastids and mitochondria. We hypothesise that phytoplankton subcellular topology is modulated by energy-management constraints. Consistent with this, shifting the diatom Phaeodactylum from low to high light enhances photosynthesis and respiration, increases cell-volume occupancy by mitochondria and the plastid CO2-fixing pyrenoid, and boosts plastid-mitochondria contacts. Changes in organelle architectures and interactions also accompany Nannochloropsis acclimation to different trophic lifestyles, along with respiratory and photosynthetic responses. By revealing evolutionarily-conserved topologies of energy-managing organelles, and their role in phytoplankton acclimation, this work deciphers phytoplankton responses at subcellular scales.

Phytoplankton composition in a eutrophic estuary: Comparison of multiple taxonomic approaches and influence of environmental factors.

To assess the comparability between taxonomic identification methods for phytoplankton, multiple approaches were used to characterize phytoplankton community composition within the Neuse River Estuary (NRE), North Carolina. Small subunit 18S rRNA gene sequencing and accessory pigment analysis displayed similar trends, indicating chlorophytes were the dominant microalgal group during most of the year, whereas results from microscopic cell counts, biovolume analysis and metatranscriptomics suggested diatom and dinoflagellate-dominated communities. Spatial environmental gradients drove variation in taxonomic composition due to preferences for specific environmental conditions among different microalgal groups. Cryptophytes were a greater proportion of the phytoplankton community within high nutrient, fresher environments whereas diatoms and dinoflagellates dominated higher salinity sections of the estuary. This study provides a detailed examination of phytoplankton communities associated with environmental gradients present in the NRE. The high level of taxonomic resolution offered by DNA sequencing (i.e., species to sub-species level) provides a better understanding of population dynamics at the base of estuarine food webs.

Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre.

Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation (cp) and backscattering (bbp) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in cp and bbp to daytime particle growth and division of cells, with particles <7µm driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacterium Crocosphaera (∼4-7µm) to be an important driver of cp at the time of sampling, whereas Prochlorococcus dynamics (∼0.5µm) were essential to reproducing temporal variability in bbp. This study is a step towards improved characterization of the particle size range represented by in situ bulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.

Chlorophyll absorption and phytoplankton size information inferred from hyperspectral particulate beam attenuation.

Electromagnetic theory predicts spectral dependencies in extinction efficiency near a narrow absorption band for a particle with an index of refraction close to that of the medium in which it is immersed. These absorption band effects are anticipated in oceanographic beam-attenuation (beam-c) spectra, primarily due to the narrow red peak in absorption produced by the phytoplankton photopigment, chlorophyll a (Chl a). Here we present a method to obtain Chl a absorption and size information by analyzing an eigendecomposition of hyperspectral beam-c residuals measured in marine surface waters by an automatic underway system. We find that three principal modes capture more than 99% of the variance in beam-c residuals at wavelengths near the Chl a red absorption peak. The spectral shapes of the eigenvectors resemble extinction efficiency residuals attributed to the absorption band effects. Projection of the eigenvectors onto the beam-c residuals produces a time series of amplitude functions with absolute values that are strongly correlated to concurrent Chl a absorption line height (aLH) measurements (r values of 0.59 to 0.83) and hence provide a method to estimate Chl a absorption. Multiple linear regression of aLH on the amplitude functions enables an independent estimate of aLH, with RMSE of 3.19⋅10-3m-1 (3.3%) or log10-RMSE of 18.6%, and a raw-scale R2 value of 0.90 based on the Tara Oceans Expedition data. Relationships between the amplitude functions and the beam-c exponential slopes are in agreement with theory relating beam-c to the particle size distribution. Compared to multispectral analysis of beam-c slope, hyperspectral analysis of absorption band effects is anticipated to be relatively insensitive to the addition of nonpigmented particles and to monodispersion.

Candidatus Abditibacter, a novel genus within the Cryomorphaceae, thriving in the North Sea.

Coastal phytoplankton blooms are frequently followed by successive blooms of heterotrophic bacterial clades. The class Flavobacteriia within the Bacteroidetes has been shown to play an important role in the degradation of high molecular weight substrates that become available in the later stages of such blooms. One of the flavobacterial clades repeatedly observed over the course of several years during phytoplankton blooms off the coast of Helgoland, North Sea, is Vis6. This genus-level clade belongs to the family Cryomorphaceae and has been resistant to cultivation to date. Based on metagenome assembled genomes, comparative 16S rRNA gene sequence analyses and fluorescence in situ hybridization, we here propose a novel candidate genus Abditibacter, comprising three novel species Candidatus Abditibacter vernus, Candidatus Abditibacter forsetii and Candidatus Abditibacter autumni. While the small genomes of the three novel photoheterotrophic species encode highly similar gene repertoires, including genes for degradation of proteins and algal storage polysaccharides such as laminarin, two of them - Ca. A. vernus and Ca. A. forsetii - seem to have a preference for spring blooms, while Ca. A. autumni almost exclusively occurs in late summer and autumn.

Cell-by-cell estimation of PAH sorption and subsequent toxicity in marine phytoplankton.

Polycyclic Aromatic Hydrocarbons (PAHs) have elicited increasing concern due to their ubiquitous occurrence in coastal marine environments and resultant toxicity in organisms. Due to their lipophilic nature, PAHs tend to accumulate in phytoplankton cells and thus subsequently transfer to other compartments of the marine ecosystem. The intrinsic fluorescence properties of PAHs in the ultraviolet (UV)/blue spectral range have recently been exploited to investigate their uptake modes, localization, and aggregation in various biological tissues. Here, we quantitatively evaluate the sorption of two model PAHs (phenanthrene and pyrene) in three marine phytoplankton species (Chaetoceros tenuissimus, Thalassiosira sp. and Proteomonas sp.) using a combined approach of UV excitation flow cytometry and fluorescence microscopy. Over a 48-h exposure to a gradient of PAHs, Thalassiosira sp. showed the highest proportion of PAH-sorbed cells (29% and 97% of total abundance for phenanthrene and pyrene, respectively), which may be attributed to its relatively high total lipid content (33.87 percent dry weight). Moreover, cell-specific pulse amplitude modulation (PAM) microscope fluorometry revealed that PAH sorption significantly reduced the photosynthetic quantum efficiency (Fv/Fm) of individual phytoplankton cells. We describe a rapid and precise hybrid method for the detection of sorption of PAHs on phytoplankton cells. Our results emphasize the ecologically relevant sub-lethal effects of PAHs in phytoplankton at the cellular level, even at concentrations where no growth inhibition was apparent. This work is the first study to address the cell-specific impacts of fluorescent toxicants in a more relevant toxicant-sorbed subpopulation; these cell-specific impacts have to date been unidentified in traditional population-based phytoplankton toxicity assays.

Hemolytic and cytotoxic activity from cultures of Aureococcus anophagefferens-a causative species of brown tides in the north-western Bohai Sea, China.

Brown tides were first observed in 2009 in the north-western Bohai Sea (Qinhuangdao sea area), China, and blooms have occurred at different scales in late spring every year since then. Although the detrimental effects on marine organisms of the causative phytoplankton species Aureococcus anophagefferens have been extensively studied, the mechanism remains poorly understood. We used erythrocytes and adrenal gland chromaffin tumor cells (PC12) to explore the hemolytic activity and cytotoxicity, respectively, of chloroform and methanol extracts of cultured A. anophagefferens isolated from the north-western Bohai Sea area. The methanol extracts showed no hemolytic or cytotoxic activity. Chloroform extracts had a potent hemolytic effect on rabbit erythrocytes; thin layer chromatography (TLC) indicated that the hemolysin was a kind of glycolipid compound. Erythrocyte lysis assay showed that erythrocytes of sea bream were sensitive to the hemolysin, whereas those of human and chicken erythrocytes were insensitive. The hemolytic effects were elevated as temperatures rose from 4 °C to 37 °C. Hemolytic blocking experiments showed that sphingomyelin and d-xylose can inhibit hemolysis significantly, while osmotic protectants with different hydrated molecular diameters had no inhibition, and the hemolysins had no obvious phospholipase activity. The chloroform extracts of A. anophagefferens had significant inhibitory effects on the viability of PC12 cells, and can induce efflux of lactic dehydrogenase (LDH) of PC12 cells and lead to their necrosis.

Phytoplankton biomass and size structure in Xiangshan Bay, China: Current state and historical comparison under accelerated eutrophication and warming.

To explore the effects of coastal eutrophication and warming on phytoplankton biomass and cell size, we analyzed current and historical data for size-fractionated chlorophyll a (chla) in Xiangshan Bay, China. Results showed that micro- and nanophytoplankton overwhelmingly dominated (>84%) in all seasons. The contribution of micro-chla was significantly lower in warm than in cold seasons, whereas contribution of pico-chla showed the opposite result. Overall, the micro-chla contribution increased with decreasing pico-chla contribution from the stable, clear, eutrophic upper bay to the turbulent, turbid lower bay, indicating that phytoplankton size structure on a spatial scale was largely shaped by water column stability and light rather than by nutrients. Since the 1980s, phytoplankton biomass, primary productivity, and micro-chla contribution in Xiangshan Bay have increased sharply with increasing nutrient amounts and temperature. Additionally, it seems that algal bloom seasonality has shifted forward from spring to winter since the power plant operations in 2006.

Gyrotactic phytoplankton in laminar and turbulent flows: A dynamical systems approach.

Gyrotactic algae are bottom heavy, motile cells whose swimming direction is determined by a balance between a buoyancy torque directing them upwards and fluid velocity gradients. Gyrotaxis has, in recent years, become a paradigmatic model for phytoplankton motility in flows. The essential attractiveness of this peculiar form of motility is the availability of a mechanistic description which, despite its simplicity, revealed predictive, rich in phenomenology, easily complemented to include the effects of shape, feedback on the fluid and stochasticity (e.g., in cell orientation). In this review we consider recent theoretical, numerical and experimental results to discuss how, depending on flow properties, gyrotaxis can produce inhomogeneous phytoplankton distributions on a wide range of scales, from millimeters to kilometers, in both laminar and turbulent flows. In particular, we focus on the phenomenon of gyrotactic trapping in nonlinear shear flows and in fractal clustering in turbulent flows. We shall demonstrate the usefulness of ideas and tools borrowed from dynamical systems theory in explaining and interpreting these phenomena.

Size differences predict niche and relative fitness differences between phytoplankton species but not their coexistence.

Here we aim to incorporate trait-based information into the modern coexistence framework that comprises a balance between stabilizing (niche-based) and equalizing (fitness) mechanisms among interacting species. Taking the modern coexistence framework as our basis, we experimentally tested the effect of size differences among species on coexistence by using fifteen unique pairs of resident vs. invading cyanobacteria, resulting in thirty unique invasibility tests. The cyanobacteria covered two orders of magnitude differences in size. We found that both niche and fitness differences increased with size differences. Niche differences increased faster with size differences than relative fitness differences and whereas coexisting pairs showed larger size differences than non-coexisting pairs, ultimately species coexistence could not be predicted on basis of size differences only. Our findings suggest that size is more than a key trait controlling physiological and population-level aspects of phytoplankton, it is also relevant for community-level phenomena such as niche and fitness differences which influence coexistence and biodiversity.

Picophytoplankton size and biomass around equatorial eastern Indian Ocean.

The cellular size and biomass of picophytoplankton were studied by flow cytometer during spring monsoon (March-May of 2015) in equatorial eastern Indian Ocean. We established an empirical relationship between forward scatter and cellular size to address the size and biomass of picophytoplankton. Results indicated that mean cell diameter of Prochlorococcus (0.60 µm) was the smallest, and then followed by Synechococcus (0.98 µm) and picoeukaryotic phytoplankton (1.05 µm). Thereafter, the biomass converted by abundance reached 0.64 µg·C·L-1 for Prochlorococcus, 0.34 µg·C·L-1 for Synechococcus, and 0.20 µg·C·L-1 for picoeukaryotic phytoplankton. Additionally, the distinct biomass contribution of picophytoplankton appeared to be affected by abundance, but not changes in cellular size. Vertically, the cellular sizes of picophytoplankton were remarkably small in upper waters, which was predominantly controlled by the nutrient availability. In contrast, they were larger in deeper waters, which was primarily attributed to the combined effects of low temperature and reduced light availability. Spatially, under the influence of high nutrient concentration induced by the different circulations and coastal upwelling, slightly high carbon biomass of picophytoplankton was observed around the coastal zones of Sri Lanka island and Sumatra, as well as the southern Bay of Bengal.

Asymmetric Versus Symmetric Filter Wheels and Associated Processing Algorithms: Results from Asynchronous Fluorescence Imaging Photometer Measurements of Phytoplankton.

The use of rotating filter wheels is common in photometric applications. Traditional filter wheel designs typically exhibit a number of filter openings spaced evenly about the circumference of the wheel. In this work we examine a number of shortcomings of this traditional filter design in measurements of phytoplankton fluorescence made with our fluorescence imaging photometer (FIP). We present an alternative asymmetric wheel design that offers a number of advantages over the traditional design as well as a new processing algorithm designed to accommodate convolution of signals from adjacent channels inherent in measurements collected with the asymmetric design. This approach eliminates the need for a separate signal to establish timing and wheel position, unambiguously establishes filter order even when the direction of rotation is unknown, allows for better estimates of signal baseline, and is more resilient to effects of vibration and other dynamic processes that could occur on the time scale of wheel rotation. We demonstrate performance improvements for phytoplankton fluorescence measurements associated with the new wheel design and algorithm compared with previously published methods using the FIP. Both the improved image processing algorithm and filter wheel design were found to reduce noise in our measurements significantly.

In situ measurements of micronutrient dynamics in open seawater show that complex dissociation rates may limit diatom growth.

In this first in situ study of the dynamic availability of phytoplankton micronutrients, a SeaExplorer glider was combined with Diffusive Gradients in Thin Films and deployed in the Mediterranean Sea. On the basis of their labile metal complex pools, we discovered that Fe and Co can be potentially limiting and Cu co-limiting to diatom growth, contrary to the generally accepted view that phosphorus (phosphate) is the growth limiting element in the Mediterranean Sea. For flagellates and picoplankton, phosphorus remains the main element limiting growth. Our in situ measurements showed that organic complexes of Fe and Cu (>98% of total dissolved concentration), dissociate slower than inorganic complexes of Co, Cd and Ni (>99% of total dissolved concentration being free ions and inorganic complexes). This strengthens the potential growth limiting effect of Fe and Cu versus phosphate, which is present as a free ion and, thus, directly available for plankton.

The metabolite dimethylsulfoxonium propionate extends the marine organosulfur cycle.

Algae produce massive amounts of dimethylsulfoniopropionate (DMSP), which fuel the organosulfur cycle1,2. On a global scale, several petagrams of this sulfur species are produced annually, thereby driving fundamental processes and the marine food web1. An important DMSP transformation product is dimethylsulfide, which can be either emitted to the atmosphere3,4 or oxidized to dimethylsulfoxide (DMSO) and other products5. Here we report the discovery of a structurally unusual metabolite, dimethylsulfoxonium propionate (DMSOP), that is synthesized by several DMSP-producing microalgae and marine bacteria. As with DMSP, DMSOP is a low-molecular-weight zwitterionic metabolite that carries both a positively and a negatively charged functional group. Isotope labelling studies demonstrate that DMSOP is produced from DMSP, and is readily metabolized to DMSO by marine bacteria. DMSOP was found in near nanomolar amounts in field samples and in algal culture media, and thus represents-to our knowledge-a previously undescribed biogenic source for DMSO in the marine environment. The estimated annual oceanic production of oxidized sulfur from this pathway is in the teragram range, similar to the calculated dimethylsulfide flux to the atmosphere3. This sulfoxonium metabolite is therefore a key metabolite of a previously undescribed pathway in the marine sulfur cycle. These findings highlight the importance of DMSOP in the marine organosulfur cycle.

Characteristics of phytoplankton assemblages in the southern Yellow Sea, China.

The characteristics of phytoplankton assemblages were analyzed by the Utermöhl method in the southern Yellow Sea in summer, 2008. A total of 113 species (including varieties and forms), belonging to 51 genera and 3 phyla, were identified. Diatom was the most dominant group. The dominant species included Eucampia zoodiacus, Leptocylindrus danicus, Chaetoceros affinis, Thalassionema nitzschioides, Skeletonema costatum, Paralia sulcata and Chaetoceros tortissimus, which were eurytopic and temperate species. The cell abundance of phytoplankton ranged from 0.04 to 620.08 cells·mL-1 with an average of 27.52 cells·mL-1. Horizontally, the cell abundance showed a decreasing trend from the south to the north. In terms of vertical distribution, the values in surface and subsurface water layers were higher than those in bottom water layers. Results of correlation analysis showed that phytoplankton cell abundance was positively correlated with water temperature, ammonia and nitrite concentrations, and negatively correlated with salinity and silicate concentration.