Journal of Phycology

We used common garden growth experiments to study genetic variation among geographic isolates (Greenland, Massachusetts, and Connecticut, USA) of the filamentous brown seaweed Pilayella littoralis, including the free-living form unique to Nahant Bay, Massachusetts. Ecotypic variation for temperature growth maximum was demonstrated for a west Greenland isolate (10{degrees} C versus 15{degrees} C for other attached isolates) and between Nahant Bay attached (narrower phenotypic plasticity) and free-living forms (broader phenotypic plasticity) of the species. Morphological and reproductive characteristics of attached and free-living isolates remained distinctive under identical culture conditions after four years. The attached forms were characteristically cabled, twisted, and clumped; unilocular reproductive cells were common and plurilocular reproductive cells were present. The free-living form was characteristically loosely branched and ball-like; only vegetative reproduction occurred, with a few unilocular reproductive cells observed in one experiment. Free-living and attached isolates cultured using no water movement and turbulent conditions to mimic surf and surge conditions did not develop forms that resembled each other after eight months. We additionally used starch gel electrophoresis to study genetic variability in attached and free-living forms of P. littoralis from Nahant Bay. Free-living and attached populations were not different at the isozyme level because a limited number of isozymes were resolved (six out of 39 enzymes tested). One isozyme (PGI) was polymorphic, with two alleles present. The two alleles shared in the attached and free-living populations suggest that the free-living form is not one large identical clone. For attached and free-living P. littoralis, both transplant and growth studies in the laboratory provide convincing evidence of ecotypic differentiation.

Marine non-cyanobacterial diazotrophs (NCDs) are recognized as globally distributed, however, few representatives have been isolated in pure cultures. As a result, understanding the physiology, growth rate, substrate preference and dinitrogen (N2) fixation capabilities proves difficult. Thalassolituus haligoni. sp. nov., BB40 was isolated from a fjord-like inlet within Kjipuktuk (Halifax), Nova Scotia. The fully sequenced genome displayed all necessary genes required for N2 fixation, and various carbon uptake pathways. The gram-negative flagellated rod shape bacterium displayed significantly higher growth rates in medium amended with nitrate (NO3-) or ammonia (NH3), compared to dissolved N2, as the sole nitrogen source. Biological N2 fixation rates were detectable across all conditions, measuring a range from 9.34 x 10-6 to 1.4 x 10-1 fmol N cell-1 day-1. Growth of the isolate was successful between 4 {degrees}C up to 35 {degrees}C, with a Topt of 20 {degrees}C for N2, and between 27 - 30 {degrees}C for fixed nitrogen (NO3- and NH3). The closest relatives to T. haligoni, were found to be the uncultured Arc-gamma-03 (99% average nucleotide identity (ANI)) and Oceanobacter antarcticus (81% ANI). T. haligoni also displays versatile capabilities for growth on various carbon, and nitrogen sources, and antibiotics. Collectively this study provides an in-depth physiological assessment of an Oceanospirillales diazotrophic species which we presently have limited knowledge of.

The upside-down jellyfish, Cassiopea, is an increasingly popular model organism gaining prominence for both its endosymbiotic dinoflagellates from the family Symbiodiniaceae and its behavioral changes of bell pulsations associated with environmental cues. Pulsation provides a unique window into the hosts response to environmental conditions, a typically difficult to access component of other symbiotic cnidarians. Pulsation has also been hypothesized to play a regulatory role on the endosymbiotic assemblage, but the magnitude of this regulatory effect is not well understood. Here, we used two light-acclimation experiments to help disentangle the complex phenotypic responses of the cnidarian host and its endosymbiotic dinoflagellates. The first experiment examined the phenotypic plasticity (size, behavior, color) of Cassiopea sp. in response to repeated ambient light acclimation trials to determine the rate and magnitude of phenotypic plasticity. The second experiment compared the acclimation response of jellyfish across three experimental groups to test whether a variable environment and resulting short acclimation times destabilized the host-endosymbiont relationship. Our goal was to identify covarying host-endosymbiont phenotypes to gain new insights into the dynamics of this relationship. We employed flow cytometric phenotypic profiling for high-throughput phenotypic characterization of endosymbiotic dinoflagellates in addition to pulse-amplitude modulated (PAM) fluorometry to characterize photosynthetic efficiency (Fv/Fm). Host phenotypes responded predictably to light-dark cycles, and stabilized after nine to twelve days of exposure to consistent light conditions. However, disruption of this acclimation period affected both the hosts circadian rhythm and the endosymbionts phenotypic profile. We also found evidence that phenotypic responses of the host and endosymbionts were generally decoupled, indicating a stronger regulatory response of light conditions on phenotypes than possible host-regulatory strategies on the endosymbiotic assemblage. This study provides unique insights into the acclimation strategies of upside-down jellyfish, an emerging model for the study of cnidarian-dinoflagellate symbiosis. HighlightsO_LICassiopea behavior and color respond predictably to changing light conditions C_LIO_LIInadequate acclimation time destabilizes the hosts circadian rhythm and causes unique phenotypic characteristics of the endosymbionts C_LIO_LILight may be a stronger influence on host and endosymbiont phenotypes than host-endosymbiont relationships C_LI

Diverse phytoplankton modulate the coupling between the ocean carbon and nutrient cycles through life-history traits such as cell size, elemental quotas, and ratios. Biodiversity is mostly considered at broad functional levels, but major phytoplankton lineages are themselves highly diverse. As an example, Synechococcus is found in nearly all ocean regions and contain extensive intraspecific variation. Here, we grew four closely related Synechococcus isolates in semi-continuous cultures across a range of temperatures (16-25{degrees}C) to quantify for the relative role of intraspecific trait variation vs. environmental change. We report differences in cell size (p<0.01) as a function of strain and clade (p<0.01). The carbon (QC), nitrogen (QN), and phosphorus (QP) cell quotas all increased with cell size. Furthermore, cell size has an inverse relationship to growth rate. Within our experimental design, temperature alone had a weak physiological effect on cell quota and elemental ratios. Instead, we find systemic intraspecific variance of C:N:P, with cell size and N:P having an inverse relationship. Our results suggest a key role for intraspecific life history traits in determining elemental quotas and stoichiometry. Thus, the extensive biodiversity harbored within many lineages may modulate the impact of environmental change on ocean biogeochemical cycles.

O_LITrait variation within and between phytoplankton species can shape community dynamics and ecosystem processes. A better understanding of this variability will improve the prediction of future ocean scenarios. We investigated thermal response variation within and among three Ostreococcus species from the Baltic Sea area and eastern North Sea. C_LIO_LIThirty-one strains were exposed to a range of temperatures. Cell counts, mean size, and chlorophyll a (Chla) content were measured by flow cytometry. Maximum growth rates were estimated using the modified Gompertz model, thermal performance curves were fitted and parameters of interest extracted. Generalized additive models assessed size and Chla responses across temperature. C_LIO_LIThermal performance traits varied substantially within and among species. There were trends for higher Topt and peak rate in Ostreococcus tauri, thermal breadth and Ea varied mainly within species. For size and Chla Ostreococcus mediterraneus showed high intraspecific variability and delayed responses at colder temperatures. Ostreococcus tauri exhibited more linear responses, the hybrid species was intermediate. C_LIO_LIHigh intra- and interspecific variation in Ostreococcus indicates evolutionary potential to buffer temperature changes and provide ecosystem resilience. Temperature induced shifts in community composition could impact nutrient cycling and energy transport, highlighting the need to account for such variability in ecosystem models. C_LI

Diatoms are among the most successful marine eukaryotic phytoplankton groups. Their diversity has been investigated in the worlds oceans through expeditions and observations carried out from the equator to the poles. Traditionally, diatom species have been distinguished based on morphological characters of their frustules, but high-throughput sequencing offers new, high-resolution data that can be used to re-examine spatial and/or temporal patterns of diversity. Here we investigated diatoms genetic diversity using metabarcoding (18S V4 rRNA gene) obtained along the years 2011 to 2013 at two coastal time series stations (SOMLIT-Astan and LTER-MareChiara) installed respectively off Roscoff in the Western English Channel and in the Gulf of Naples in the Mediterranean Sea. Diatom species pools detected were different, which fitted with previous observations and with our expectations, since these stations are installed in two contrasted pelagic habitats (permanently-mixed versus stratified in summer). However, this analysis also shows a pool of common ASVs among which some are persistent and dominant in both sites. The observed synchronous variations in relative read abundances of shared ASVs assigned to Chaetoceros socialis, C. tenuissimus, Cyclotella, Mediolabrus comicus and Leptocylindrus aporus at the two geographically distant sites could indicate that internal controls of growth rate and sexual reproduction, rather that external environmental parameters are at work.

Dinophysis are specialist mixotrophs that must balance prey capture, cell division, and sexual recombination during blooms, yet relatively little is known about the occurrence and role of sex in their ecology. Here, mating was investigated in D. acuminata through continuous automated microscopy of cells in culture and during natural blooms. Both in culture and in situ, vegetative division and mating were phased on a diel timescale, occurring primarily at night and near dawn, regardless of prey availability. When prey were available, feeding occurred primarily during daylight hours. The sequencing of division and mating phases, the correlation of their daily amplitudes, and the timing of their establishment after a two-day light block suggests linkage of these processes. Mating, though frequent (up to 14% day-1 in culture), was also not associated with zygote accumulation or resting stage formation but rapid cell reproduction via meiosis. Confinement of division and mating to nighttime and early morning may minimize conflict with photosynthesis-related metabolism and/or predator exposure. Sexual reproduction was the dominant mode of proliferation during the observed blooms, accounting for 71% and 64% of new cell production in 2015 and 2021, respectively. Because it is dependent on encounter of a compatible gamete, sexual reproduction is increasingly accessible as blooms intensify. This sexual mode of proliferation may also alleviate populations susceptibility to pathogens, parasites, and other threats via genetic recombination, an example of Red Queen dynamics.

The green algae C. reinhardtii serves as a useful model for studying photosynthetic cells and has been extensively utilized for investigating various physiological processes. Currently, limited information is available regarding the molecular mechanisms controlling oil accumulation in microalgae. C4 photosynthesis metabolic pathways are essential for high rates of CO2 fixation in plants. High rates of photosynthesis are crucial for the biomass accumulation of algae. Surprisingly, C4 pathway enzymes and their regulatory factors have not been studied at the molecular level in any green algae, except for our efforts, which focused on the molecular characterization of phosphoenolpyruvate carboxylase (PEPC) genes (Ppc) in C. reinhardtii (Mamedov et al., 2005; Moellering et al., 2007) and phosphoenolpyruvate carboxykinase (Cebarailoglu, 2017). In this study, we isolated and cloned two pyruvate orthophosphate dikinase (PPDK) genes from the green microalga C. reinhardtii for the first time and performed expression analysis under different conditions. We demonstrate that both ppdk genes encode functional PPDK enzymes in C. reinhardtii and that both genes are responsive to changes in carbon dioxide or ammonium concentration during growth. Phylogenetic analysis suggests that C. reinhardtii PPDK2 is evolutionarily closer to PPDKs from plants rather than to protozoan and bacterial enzymes. Furthermore, alignment data indicate that the global structure and key amino acid residues involved in catalysis and substrate binding are well conserved in both PPDK enzymes in plants, C. reinhardtii, bacteria, and protozoa.

Ectocarpus subulatus is one of the few brown algae found in river habitats. Its ability to tolerate freshwater is due, in part, to its uncultivated microbiome. We investigated this phenomenon by modifying the microbiome of laboratory-grown E. subulatus using mild antibiotic treatments, which affected its ability to grow in low salinity. The acclimation to low salinity of fresh water-tolerant and intolerant holobionts was then compared. Salinity had a significant impact on bacterial gene expression as well as the expression of algae- and bacteria-associated viruses in all holobionts, albeit in different ways for each holobiont. On the other hand, gene expression of the algal host and metabolite profiles were affected almost exclusively in the fresh water intolerant holobiont. We found no evidence of bacterial protein production that would directly improve algal stress tolerance. However, we identified vitamin K synthesis as one possible bacterial service missing specifically in the fresh water-intolerant holobiont in low salinity. We also noticed an increase in bacterial transcriptomic activity and the induction of microbial genes involved in the biosynthesis of the autoinducer AI-1, a compound that regulates quorum sensing. This could have caused a shift in bacterial behavior in the intolerant holobiont, resulting in virulence or dysbiosis. Originality-Significance StatementThe importance of symbiotic microbes for the health and stress resistance of multicellular eukaryotes is widely acknowledged, but understanding the mechanisms underlying these interactions is challenging. They are especially difficult to separate in systems with one or more uncultivable components. We bridge the gap between fully controlled, cultivable model systems and purely environmental studies through the use of a multi-omics approach and metabolic models on experimentally modified "holobiont" systems. This allows us to generate two promising working hypotheses on the mechanisms by which uncultivated bacteria influence their brown algal hosts fresh water tolerance.

Cysts serve as a seed source for the initiation and recurrence of a harmful algal bloom (HAB) caused by dinoflagellates. And the influence of calcium on cyst formation has been relatively understudied. In the present study, we investigated the effects of calcium (Ca2+) on the growth and encystment of Scrippsiella trochoidea. We incubated S. trochoidea in modified f/2 media in flasks which were divided into five groups and treated with different Ca2+ concentrations (0, 0.2, 0.4, 0.6, and 0.8 g{middle dot}L-1). We revealed that cell density increased with increasing Ca2+ concentrations; however, cell density was reduced when Ca2+ concentrations exceeded 0.2 g{middle dot}mL-1. Additionally, the number of cysts and the cyst formation rate similarly increased as Ca2+ concentrations increased, but these were reduced when Ca2+ concentrations exceeded 0.4 g{middle dot}mL-1. Lastly, S. trochoidea absorbed Ca2+ from the water when cysts were formed and under high Ca2+ concentrations, more calcareous thorn cysts formed.

Summary Marker pigments are used as a proxy for biomass of distinct phytoplankton classes in different oceanic regions. However, sometimes disagreements are observed between microscopy and accessory-pigment based approaches in distinct regions mainly due to changing environmental factors governing diversity and structure of community composition. In this study, concordance between microscopy and HPLC-CHEMTAX methods were investigated first time in coastal waters of Erdemli, Turkey, in the Levantin Basin of the northeastern Mediterranean Sea by weekly intervals during 2015-2016. According to our results, marker pigment of diatoms, fucoxanthin, which was the most prominent pigment in the study area during most of the year, was a better indicator of diatom abundance than diatom carbon biomass. CHEMTAX derived values of diatom chlorophyll a (Chl a) were not in concert with either abundance or carbon biomass of this group. Contribution of dinoflagellates and cryptophytes to the phytoplankton community was underestimated with pigment based approach. Accessory pigment of cyanophytes, zeaxanthin, was also an important pigment in the samples. Biomass of haptophytes seemed to be overestimated by HPLC-CHEMTAX analysis. In contrast to diatoms, CHEMTAX derived chlorophyll a values of cryptophytes were correlated with abundance of this group but not with alloxanthin. Inclusion of live counts of nanoplanktic cryptophytes, haptophytes and prasinophytes provided a better correlation between microscopy and pigment based results. According to CHEMTAX analysis, nanoplankton and picoplankton constituted [~]55% of Chl a in the region.

Diatom blooms dominate nutrient-rich ecosystems. Less is known about the ecology and bloom dynamics of diatom populations in oligotrophic ecosystems. Here, we investigated seasonal succession of planktonic diatoms in the Gulf of Aqaba (GoA) at the northern Red Sea. The GoA is a subtropical ecosystem alternating between stratified, oligotrophic profiles during summer, and deeply mixed, mesotrophic during winter. Diatom density and diversity were lower during the stratified season, dominated by pennate species, and increased at mid-winter as nitrate exceeded [~]0.5 {micro}mol L-1. Diatom density lagged after total phytoplankton and entailed a transition to centric-diatom dominance, suggesting both higher nutrient requirements for diatom growth and ecophysiological differences between morphotypes. Ephemeral blooms were detected at the mixing-to-stratification transition. Under milder conditions, mixing was shallow and diatoms reached [~]98 individuals. mL-1. Small-centric Thalassiosiraceae and several pennates dominated. However, during the following colder year, mixing depth reached [~]700 m. Consequently, nutrient concentrations were higher and diatoms reached [~]390 individuals. mL-1. This enabled emergence of chain-forming species (namely Chaetoceros and Leptocylindrus) along small-centric and pennates, and high spore abundance was detected. Restratification led to rapid bloom decline. These results illustrate diatom community succession and bloom development as a function of nutrient availability in subtropical ecosystems.

(1) Picophytoplankton are important primary producers, but not always adequately recognized, e.g. due to methodological limitations. (2) In this study, we combined flow cytometry and metabarcoding to investigate seasonal and spatial patterns of picophytoplankton abundance and community composition in the Elbe estuary. (3) Picophytoplankton (mostly picoeukaryotes such as Mychonastes and Minidiscus) contributed on average 70 % (SD = 14 %) to the total phytoplankton counts. In the summer picocyanobacteria (e.g. Synechococcus) played a more significant role. The contributions of picophytoplankton to the total phytoplankton were particularly high from summer to winter as well as in the mid estuary. However, at salinities of around 10 PSU in the mixing area the proportion of picophytoplankton was comparably low (average 49 %, SD = 13 %). (4) Our results indicate that picophytoplankton prevail in the Elbe estuary year-round with respect to cell counts. Picophytoplankton could occupy important niche positions to maintain primary production under extreme conditions where larger phytoplankton might struggle (e.g. at high or low temperature and high turbidity), and also benefit from high nutrient availability here. However, we did not find evidence that they played a particularly significant role at the salinity interface. Our study highlights the importance of including picophytoplankton when assessing estuarine phytoplankton as has been suggested for other ecosystems such as oceans.

Cyanobacteria play essential roles in marine primary productivity and the global carbon/nitrogen biogeochemical cycle. Increasing urea emissions and decreasing pH value in the ocean caused by human activities are changing the community structure and competitive interactions of marine phytoplankton, which will have a profound impact on the marine ecosystem and global biogeochemical cycle. Here, we report that a coastal Synechococcus strain exhibited better adaptability to extreme low pH conditions when it uses urea as nitrogen source compared to using other inorganic nitrogen. Very low pH values can also alleviate damage by high concentrations of urea to cyanobacteria. Urease plays an essential role in this process. Synechococcus mutants with inactivated urease cannot adapt well to highly acidic environments, while heterologous expression of urease homologs from acidophilic Helicobacter pylori can help the cyanobacterial mutants to restore their adaptability to acidification. A TARA Oceans database analysis indicates that the distribution of cyanobacteria with the urease gene is closely related to estuaries and nearshore waters with potentially high urea inputs. In summary, we report for the first time that the use of urea and adaptation to acid stress are highly interactive in marine phytoplankton. Future work should determine whether this interaction is likely to allow phytoplankton that utilize urea to have a competitive advantage in the future ocean with high urea emissions and environmentally relevant pH scenarios.

One challenge to large-scale microalgae cultivation, e.g., for biodiesel production, is the seasonal low-temperature conditions. We argue that seasonally varying selection in natural environments has prevented algae from better adapting to cold temperatures, and that laboratory evolution offers a promising approach for obtaining cold-adapted algal materials. We conducted a population-level artificial selection experiment with the unicellular green microalgae Chlorella sorokiniana at both a benign temperature (25) and a mildly cold temperature (15). Four artificial selection regimes were established: random selection, selection for high biomass (i.e., cell yield), selection for high lipid production, and rotation between high-biomass and high-lipid selection. We did not observe significant differences among the four selection regimes in evolutionary changes of algal cell yield or lipid yield, suggesting that natural selection at the individual level had dominated the evolutionary changes in our experiment. Compared with the ancestral strain, selection lines that had evolved at 15 typically exhibited increased cell yield and reduced lipid content per cell, indicating a trade-off relationship. However, substantial increases in cell yield may compensate for the reduction in lipid content per cell. Notably, three out of 16 selection lines showed > 1-fold increase in cell yield, and one exhibited > 1-fold increase in population-level lipid yield. Selection lines that had evolved at 25 displayed even greater increases in both cell and lipid yields, with a positive relationship between cell yield and lipid content per cell. Our results demonstrated the potential for laboratory evolution to obtain algal materials suitable for biofuel production under seasonal low-temperature conditions. HighlightsO_LIAlgae from natural environments not well-adapted to seasonal cold conditions. C_LIO_LILaboratory evolution under constant conditions with Chlorella sorokiniana. C_LIO_LIBoth cell yield and lipid production at a low temperature increased. C_LI

Amino acid catabolism is a vital metabolic process in bacteria, providing energy, carbon and potentially nitrogen as resources, and affecting global cycles of these elements. The ability of a bacterium to catabolize an amino acid is often inferred from the presence of the relevant catabolic pathways in its genome, yet the "gene=function" inference is not straightforward. Here, we use growth assays in 96 well plates on individual amino acids and their combinations to directly measure the ability of a model marine bacterium, Alteromonas macleodii ATCC 27126, to utilize these resources for growth. With the exception of aspartate and glutamate, which did not support growth in any of our experiments, ATCC 27126 grew on all other amino acids. However, the probability of growth, together with growth yield and rate, differed depending on the entry point of the catabolic pathway to central carbon metabolism, with robust growth occurring only on amino acids catabolized into pyruvate or acetyl CoA. Growth on combinations of two amino acids revealed reproducible patterns, the clearest being inhibition of growth on other amino acids by asparagine, aspartate and their degradation product, oxaloacetate. Finally, growth was different in test tubes compared with 96 well plates. Our results reveal hidden complexity in amino acid utilization and suggest a "TCA-centric" viewpoint for amino acid utilization, perhaps reflecting the high metabolic flexibility of pyruvate and specific regulatory aspects of the TCA cycle in Alteromonas.

Seagrass meadow ecosystems offer several valuable ecosystem services in coastal regions around the world. Recent studies have suggested that one such important service is reduction of pathogenic bacteria and specifically Vibrio spp. in adjacent waters. The specific mechanisms of pathogen reduction remain unclear, although increased sedimentation has been suggested as one likely process for pathogens to be quenched from the water column. Whether Vibrio spp. persist in the sediment or in other compartments of the seagrass meadow is currently not known, but it has been shown that marine surface biofilms can function as reservoirs of pathogenic vibrios. This general feature may also apply to seagrass and sediment surfaces. In this study, we investigated the relative abundance and community ecology of Vibrio spp. bacteria in Baltic Sea seagrass meadows using both culturing and culture-independent methods. While we did not detect a significant reduction of Vibrio spp., the highest relative abundances of Vibrio spp. were observed in the water column above unvegetated sites as compared to seagrass meadows. We also detected high relative abundances of Vibrio spp. on seagrass roots, supporting previous observations that marine surfaces are selectively colonized by Vibrio spp., implying that these habitats are important for the persistence and possibly release of Vibrio spp. into the water column. Our results emphasize the need to understand the interactions of pathogenic bacteria with coastal habitats, including interactions with host organisms such as seagrasses that provide biofilm microenvironments, in order to understand how diseases associated with these organisms develop.

Dinoflagellates are a diverse group of microplankton that include free-living, symbiotic, and parasitic species. Amoebophrya, a basal lineage of parasitic dinoflagellates, infects a variety of marine microorganisms, including harmful-bloom-forming algae. Although there are currently three published Amoebophrya genomes, this genus has considerable genomic diversity. We add to the growing genomic data for Amoebophrya with an annotated genome assembly for Amoebophrya sp. ex Karlodinium veneficum. This species appears to translate all three canonical stop codons contextually. Stop codons are present in the open reading frames of about half of the predicted gene models, including genes essential for cellular function. The in-frame stop codons are likely translated by suppressor tRNAs that were identified in the assembly. We also assembled the mitochondrial genome, which has remained elusive in the previous Amoebophrya genome assemblies. The mitochondrial genome assembly consists of many fragments with high sequence identity in the genes but low sequence identity in intergenic regions. Nuclear and mitochondrially-encoded proteins indicate that Amoebophrya sp. ex K. veneficum does not have a bipartite electron transport chain, unlike previously analyzed Amoebophrya species. This study highlights the importance of analyzing multiple genomes from highly diverse genera such as Amoebophrya. SummaryThis new long-read assembly demonstrates the remarkable diversity found within Amoebophrya. Despite being assigned the rank of genus, the available genome assemblies indicate significant variation in gene content, AT content, genetic codes, and potentially mitochondrial biology. Furthermore, this study contributes to the expanding list of organisms that contextually translate all three canonical stop codons. Although the mechanisms underlying such a genetic code remain elusive, the relative ease of culturing Amoebophrya suggests it may be useful as a model organism for future research on this subject.

Cyanate (OCN-) is potentially an important organic nitrogen source in aquatic environments given the prevalence and activity of cyanate lyase genes in microalgae. However, the conditions under which these genes are expressed and the actual capacity of microalgae to assimilate cyanate remain underexplored. Here, we studied the nitrogen metabolism of the cosmopolitan picoalga Pelagomonas calceolata (Pelagophyceae, Stramenopiles) in environmental metatranscriptomes and transcriptomes from culture experiments under different nitrogen sources and concentrations. We observed that cyanate lyase is over-expressed in nitrate-poor oceanic regions, suggesting that cyanate is an important molecule contributing to the persistence of P. calceolata in oligotrophic environments. In the laboratory, we confirmed that this gene is over-expressed in low-nitrate medium together with several genes involved in nitrate recycling from endogenous molecules. Non-axenic cultures of P. calceolata were capable of growing on various nitrogen sources, including nitrate, urea and cyanate, but not ammonium. RNA sequencing of these cultures revealed that cyanate lyase was under-expressed in the presence of cyanate, indicating that this gene in not involved in the catabolism of extracellular cyanate to ammonia. Conversely, axenic P. calceolata cultures were not able to grow on cyanate, suggesting that the bacterial community consumes cyanate and provides an available form of nitrogen for growth of the alga. Based on environmental datasets and laboratory experiments, we propose that cyanate lyase is important in nitrate-poor environments to reduce the toxicity of intracellular cyanate produced by endogenous nitrogenous compound recycling, rather than being used to metabolise imported extracellular cyanate as an alternative nitrogen source.

Temperature and light play a crucial role in regulating phytoplankton blooms in the Ocean. To assess the importance of these two parameters experimentally, microcosms were conducted on seven picoplankton communities (<3 m) sampled in December, March, June and September 2015 and 2016 in the North Western Mediterranean Sea. Each community was exposed to 4 realistic seasonal conditions (December, March, June and September). Metabarcoding was used to investigate the eukaryotic diversity in the 56 microcosms experiments in parallel to high-frequency monitoring of environmental diversity in the sea. The three major lineages identified were the Stramenopiles, Alveolata and Archaeplastida. Overall, the five-day incubations were not sufficient to reshape the initial microbial communities completely. The microcosm outcome was strongly influenced by the dynamics of phytoplankton starting communities. In pre-bloom conditions, phytoplanktonic species were the most sensitive to temperature and light conditions. During a bloom, species belonging to diatoms or Chlorodendrophyceae usually did not respond to light and temperature in microcosms and continued to bloom independently of the applied seasonal condition. Together, these results suggest that light and temperature seasonal conditions play a crucial role in regulating phytoplankton dynamics in pre-bloom conditions and biotic interactions may be preponderant in bloom and post-bloom conditions.