The molecular ecophysiology of programmed cell death in marine phytoplankton

Algal blooms in the ocean: hot spots for chemically mediated microbial interactions

The cycling of major nutrients in the ocean is affected by large-scale phytoplankton blooms, which are hot spots of microbial life. Diverse microbial interactions regulate bloom dynamics. At the single-cell level, interactions between microorganisms are mediated by small molecules in the chemical crosstalk that determines the type of interaction, ranging from mutualism to pathogenicity. Algae interact with viruses, bacteria, parasites, grazers and other algae to modulate algal cell fate, and these interactions are dependent on the environmental context. Recent advances in mass spectrometry and single-cell technologies have led to the discovery of a growing number of infochemicals - metabolites that convey information - revealing the ability of algal cells to govern biotic interactions in the ocean. The diversity of infochemicals seems to account for the specificity in cellular response during microbial communication. Given the immense impact of algal blooms on biogeochemical cycles and climate regulation, a major challenge is to elucidate how microscale interactions control the fate of carbon and the recycling of major elements in the ocean. In this Review, we discuss microbial interactions and the role of infochemicals in algal blooms. We further explore factors that can impact microbial interactions and the available tools to decipher them in the natural environment.

Purple acid phosphatase promoted hydrolysis of organophosphate pesticides in microalgae

When organophosphate pesticides (OPs) are not used and handled in accordance with the current rules and standards, it results in serious threats to the aquatic environment and human health. Phaeodactylum tricornutum is a prospective microalgae-based system for pollutant removal and carbon sequestration. Genetically engineered P. tricornutum, designated as the OE line (endogenously expressing purple acid phosphatase 1 [PAP1]), can utilize organic phosphorus for cellular metabolism. However, the competencies and mechanisms of the microalgae-based system (namely the OE line of P. tricornutum) for metabolizing OPs remain to be addressed. In this study, the OE line exhibited the effective biodegradation competencies of 72.12% and 68.2% for 30 mg L-1 of dichlorvos and 50 mg L-1 of glyphosate, accompanied by synergistic accumulations of biomass (0.91 and 0.95 g L-1) and lipids (32.71% and 32.08%), respectively. Furthermore, the biodiesel properties of the lipids from the OE line manifested a high potential as an alternative feedstock for microalgae-based biofuel production. A plausible mechanism of OPs biodegraded by overexpressed PAP1 is that sufficient inorganic P for adenosine triphosphate and concurrent carbon flux for the reduced form of nicotinamide adenine dinucleotide phosphate biosynthesis, which improved the OP tolerance and biodegradation competencies by regulating the antioxidant system, delaying programmed cell death and accumulating lipids via the upregulation of related genes. To sum up, this study demonstrates a potential strategy using a genetically engineered strain of P. tricornutum to remove high concentrations of OPs with the simultaneous production of biomass and biofuels, which might provide novel insights for microalgae-based pollutant biodegradation.

SLC24A-mediated calcium exchange as an indispensable component of the diatom cell density-driven signaling pathway

Diatom bloom is characterized by a rapid increase of population density. Perception of population density and physiological responses can significantly influence their survival strategies, subsequently impacting bloom fate. The population density itself can serve as a signal, which is perceived through chemical signals or chlorophyll fluorescence signals triggered by high cell density, and their intracellular signaling mechanisms remain to be elucidated. In this study, we focused on the model diatom, Phaeodactylum tricornutum, and designed an orthogonal experiment involving varying cell densities and light conditions, to stimulate the release of chemical signals and light-induced chlorophyll fluorescence signals. Utilizing RNA-Seq and Weighted Gene Co-expression Network Analysis, we identified four gene clusters displaying density-dependent expression patterns. Within these, a potential hub gene, PtSLC24A, encoding a Na+/Ca2+ exchanger, was identified. Based on molecular genetics, cellular physiology, computational structural biology, and in situ oceanic data, we propose a potential intracellular signaling mechanism related to cell density in marine diatoms using Ca2+: upon sensing population density signals mediated by chemical cues, the membrane-bound PtSLC24A facilitates the efflux of Ca2+ to maintain specific intracellular calcium levels, allowing the transduction of intracellular density signals, subsequently regulating physiological responses, including cell apoptosis, ultimately affecting algal blooms fate. These findings shed light on the calcium-mediated intracellular signaling mechanism of marine diatoms to changing population densities, and enhances our understanding of diatom bloom dynamics and their ecological implications.

Tea polyphenols inhibit blooms caused by eukaryotic and prokaryotic algae

With changes in global climate, blooms are becoming more frequent and difficult to control. Therefore, the selection of algal suppressor agents with effective inhibition and environmental safety is of paramount importance. One of the main treatment strategies is to inhibit the release of harmful algal toxins. Tea polyphenols (TP) are natural products that have been widely used in medicine, the environment, and other fields due to their antibacterial and antioxidant properties. To investigate their potential application in the treatment of algal blooms, TP were applied to three different microalgae. TP exhibited strong inhibitory effects towards all three microalgae. They stimulate the accumulation of ROS in algal cells, leading to lipid peroxidation and subsequent damage to the cell membrane, resulting in the rupture and necrosis of Cyclotella sp. and Chlorella vulgaris cells. Remarkably, it was observed that lower concentrations of TP exhibited the ability to induce apoptosis in M. aeruginosa cells without causing any structural damage. This outcome is particularly significant as it reduces the potential risk of microcystin release resulting from cell rupture. Overall, blooms dominated by different algae can be treated by adjusting the concentration of TP, a new algal suppressor, indicating strong potential treatment applications.

Identification and Verification of Candidate miRNA Biomarkers with Application to Infection with Emiliania huxleyi Virus

The interactions of Emiliania huxleyi and its specific lytic virus (EhV) have a profound influence on marine biogeochemical carbon-sulfur cycles and play a prominent role in global climate change. MicroRNAs (miRNAs) have emerged as promising candidates with extensive diagnostic potential due to their role in virus-host interactions. However, the application of miRNA signatures as diagnostic markers in marine viral infection has made limited progress. Based on our previous small-RNA sequencing data, one host miRNA biomarker that is upregulated in early infection and seven viral miRNA biomarkers that are upregulated in late infection were identified and verified using qRT-PCR and a receiver operating characteristic curve analysis in pure culture, mixed culture, and natural seawater culture. The host ehx-miR20-5p was able to significantly differentiate infection groups from the control in the middle (24 h post-infection, hpi) and late infection (48 hpi) phases, while seven virus-derived miRNA biomarkers could diagnose the early and late stages of EhV infection. Functional enrichment analysis showed that these miRNAs participated in numerous essential metabolic pathways, including gene transcription and translation, cell division-related pathways, protein-degradation-related processes, and lipid metabolism. Additionally, a dual-luciferase reporter assay confirmed the targeted relationship between a viral ehv-miR7-5p and the host dihydroceramide desaturase gene (hDCD). This finding suggests that the virus-derived miRNA has the ability to inhibit the host sphingolipid metabolism, which is a specific characteristic of EhV infection during the late stage. Our data revealed a cluster of potential miRNA biomarkers with significant regulatory functions that could be used to diagnose EhV infection, which has implications for assessing the infectious activity of EhV in a natural marine environment.

The concepts and origins of cell mortality

Organismal death is foundational to the evolution of life, and many biological concepts such as natural selection and life history strategy are so fashioned only because individuals are mortal. Organisms, irrespective of their organization, are composed of basic functional units-cells-and it is our understanding of cell death that lies at the heart of most general explanatory frameworks for organismal mortality. Cell death can be exogenous, arising from transmissible diseases, predation, or other misfortunes, but there are also endogenous forms of death that are sometimes the result of adaptive evolution. These endogenous forms of death-often labeled programmed cell death, PCD-originated in the earliest cells and are maintained across the tree of life. Here, we consider two problematic issues related to PCD (and cell mortality generally). First, we trace the original discoveries of cell death from the nineteenth century and place current conceptions of PCD in their historical context. Revisions of our understanding of PCD demand a reassessment of its origin. Our second aim is thus to structure the proposed origin explanations of PCD into coherent arguments. In our analysis we argue for the evolutionary concept of PCD and the viral defense-immunity hypothesis for the origin of PCD. We suggest that this framework offers a plausible account of PCD early in the history of life, and also provides an epistemic basis for the future development of a general evolutionary account of mortality.

Aerobic bacteria produce nitric oxide via denitrification and promote algal population collapse

Microbial interactions govern marine biogeochemistry. These interactions are generally considered to rely on exchange of organic molecules. Here we report on a novel inorganic route of microbial communication, showing that algal-bacterial interactions between Phaeobacter inhibens bacteria and Gephyrocapsa huxleyi algae are mediated through inorganic nitrogen exchange. Under oxygen-rich conditions, aerobic bacteria reduce algal-secreted nitrite to nitric oxide (NO) through denitrification, a well-studied anaerobic respiratory mechanism. The bacterial NO is involved in triggering a cascade in algae akin to programmed cell death. During death, algae further generate NO, thereby propagating the signal in the algal population. Eventually, the algal population collapses, similar to the sudden demise of oceanic algal blooms. Our study suggests that the exchange of inorganic nitrogen species in oxygenated environments is a potentially significant route of microbial communication within and across kingdoms.

Virus infection of phytoplankton increases average molar mass and reduces hygroscopicity of aerosolized organic matter

Viral infection of phytoplankton is a pervasive mechanism of cell death and bloom termination, which leads to the production of dissolved and colloidal organic matter that can be aerosolized into the atmosphere. Earth-observing satellites can track the growth and death of phytoplankton blooms on weekly time scales but the impact of viral infection on the cloud forming potential of associated aerosols is largely unknown. Here, we determine the influence of viral-derived organic matter, purified viruses, and marine hydrogels on the cloud condensation nuclei activity of their aerosolized solutions, compared to organic exudates from healthy phytoplankton. Dissolved organic material derived from exponentially growing and infected cells of well-characterized eukaryotic phytoplankton host-virus systems, including viruses from diatoms, coccolithophores and chlorophytes, was concentrated, desalted, and nebulized to form aerosol particles composed of primarily of organic matter. Aerosols from infected phytoplankton cultures resulted in an increase in critical activation diameter and average molar mass in three out of five combinations evaluated, along with a decrease in organic kappa (hygroscopicity) compared to healthy cultures and seawater controls. The infected samples also displayed evidence of increased surface tension depression at realistic cloud water vapor supersaturations. Amending the samples with xanthan gum to simulate marine hydrogels increased variability in organic kappa and surface tension in aerosols with high organic to salt ratios. Our findings suggest that the pulses of increased dissolved organic matter associated with viral infection in surface waters may increase the molar mass of dissolved organic compounds relative to surface waters occupied by healthy phytoplankton or low phytoplankton biomass.

Algal cell viability assessment: The role of environmental factors in phytoplankton population dynamics

The viability of algal cells is one of the most fundamental issues in marine ecological research. In this work, a method was designed to identify algal cell viability based on digital holography and deep learning, which divided algal cells into three categories: active, weak, and dead cells. This method was applied to measure algal cells in surface waters of the East China Sea in spring, revealing about 4.34 %-23.29 % weak cells and 3.98 %-19.47 % dead cells. Levels of nitrate and chlorophyll a were the main factors affecting the viability of algal cells. Furthermore, algal viability changes during the heating and cooling were observed in laboratory experiments: high temperatures led to an increase in weak algal cells. This may provide an explanation for why most harmful algal blooms occur in warming months. This study provided a novel insight into how to identify the viability of algal cells and understand their significance in the ocean.

Paraquat induces different programmed cell death patterns in Microcystis aeruginosa and Chlorella luteoviridis

Although programmed cell death (PCD) has been reported in phytoplankton, knowledge of the characterization of the PCD pathway and cascade process in different phytoplankton species is still limited. In this study, PCD progression in cyanobacterium Microcystis aeruginosa and green algae Chlorella luteoviridis by paraquat-induced oxidative stress was monitored. The results showed that paraquat-induced PCD in the two species belonged to the caspase-dependent pathway. Dose- and time-dependent PCD characteristics in the two strains under paraquat included the increase in caspase-like activity, DNA fragmentation, and chromatin condensation. However, the signaling pathway and cascade events of PCD in M. aeruginosa and C. luteoviridis differed. In M. aeruginosa, the free Ca²⁺ concentration was rapidly increased at 8 h, followed by a significant elevation of the reactive oxygen species (ROS) level at 24 h, and eventual cell death. In C. luteoviridis, the mitochondrial apoptosis pathway, revealed by the depolarization of the mitochondrial membrane potential at 1 h and increase in the ROS level and caspase-like activity at 8 h, might contribute to cell death. In addition, the dynamics of ROS levels and metacaspase activity were synchronized, suggesting that paraquat-triggered PCD was ROS-mediated in both M. aeruginosa and C. luteoviridis. These results provide insights into PCD patterns in prokaryotic cyanobacteria and eukaryotic green algae under similar stress.

Do photosynthetic cells communicate with each other during cell death? From cyanobacteria to vascular plants

As in metazoans, life in oxygenic photosynthetic organisms relies on the accurate regulation of cell death. During development and in response to the environment, photosynthetic cells activate and execute cell death pathways that culminate in the death of a specific group of cells, a process known as regulated cell death (RCD). RCD control is instrumental, as its misregulation can lead to growth penalties and even the death of the entire organism. Intracellular molecules released during cell demise may act as 'survival' or 'death' signals and control the propagation of cell death to surrounding cells, even in unicellular organisms. This review explores different signals involved in cell-cell communication and systemic signalling in photosynthetic organisms, in particular Ca2+, reactive oxygen species, lipid derivates, nitric oxide, and eATP. We discuss their possible mode-of-action as either 'survival' or 'death' molecules and their potential role in determining cell fate in neighbouring cells. By comparing the knowledge available across the taxonomic spectrum of this coherent phylogenetic group, from cyanobacteria to vascular plants, we aim at contributing to the identification of conserved mechanisms that control cell death propagation in oxygenic photosynthetic organisms.

Cell death responses to acute high light mediated by non-photochemical quenching in the dinoflagellate Karenia brevis

Programmed cell death (PCD) can be induced in microalgae by many abiotic challenges via generation of reactive oxygen species (ROS). Marine phytoplankton live in a highly variable light environment, yet the potential for excess photosynthetically available radiation to trigger PCD has not been examined. On the other hand, photoprotective non-photochemical quenching (NPQ) is hypothesized to counteract intracellular ROS, potentially preventing cell death. The main objective of this study is to investigate high-light-induced death processes and their relationship with photosynthesis in bloom-forming dinoflagellate Karenia brevis. Here, we characterized the prevalence of ROS, caspase-like enzyme activity and cell death as well as photosynthetic status under acute irradiance of 500, 750 or 1000 µmol m^-2 s^-1. PCD only occurred at the largest light shift. Although depressed photosynthetic capacities and oxidative stress were apparent across the stress gradient, they did not necessarily lead to cell death. NPQ exhibited dose-dependent activation with increasing light stress, which enabled cells to resist or delay PCD. These results highlight the important role of the balance between ROS generation and NPQ activation on determining cell fates in Karenia under acute irradiance stress. This research also provides insights into potential survival strategies and mechanisms of cell loss under a changeable light environment.

Toxicity of 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) on the green microalgae Chlorella sp. and the role of cellular oxidative stress

Polybrominated diphenyl ethers (PBDEs) are toxic to marine organisms including the major primary producer phytoplankton, while the toxic mechanisms haven't yet been fully clarified. Therefore, we comprehensively studied the toxic mechanisms of BDE-47 on the marine chlorophyte Chlorella sp., with a focus on the role of cellular oxidative stress. The results indicate that BDE-47 stress resulted in the inhibition of population growth as well as cell death and programmed cell death. The antioxidant system was activated in both low and high BDE-47 treatments, but only microalgal cells in the high BDE-47 treatment showed cellular oxidative stress. By adding ROS inhibitor, the relief of photosynthetic inhibition, Ca²⁺ overproduction and cell death was found. Therefore, we conclude that photosynthetic damage, cell death and cellular oxidative stress were the major mechanisms of BDE-47 toxicity to Chlorella sp., and that cellular oxidative stress played an important role in mediating the other mechanisms.

Algicidal Bacteria: A Review of Current Knowledge and Applications to Control Harmful Algal Blooms

Interactions between bacteria and phytoplankton in aqueous ecosystems are both complex and dynamic, with associations that range from mutualism to parasitism. This review focuses on algicidal interactions, in which bacteria are capable of controlling algal growth through physical association or the production of algicidal compounds. While there is some evidence for bacterial control of algal growth in the field, our understanding of these interactions is largely based on laboratory culture experiments. Here, the range of these algicidal interactions is discussed, including specificity of bacterial control, mechanisms for activity, and insights into the chemical and biochemical analysis of these interactions. The development of algicidal bacteria or compounds derived from bacteria for control of harmful algal blooms is reviewed with a focus on environmentally friendly or sustainable methods of application. Potential avenues for future research and further development and application of bacterial algicides for the control of algal blooms are presented.

Microbial metabolites in the marine carbon cycle

One-quarter of photosynthesis-derived carbon on Earth rapidly cycles through a set of short-lived seawater metabolites that are generated from the activities of marine phytoplankton, bacteria, grazers and viruses. Here we discuss the sources of microbial metabolites in the surface ocean, their roles in ecology and biogeochemistry, and approaches that can be used to analyse them from chemistry, biology, modelling and data science. Although microbial-derived metabolites account for only a minor fraction of the total reservoir of marine dissolved organic carbon, their flux and fate underpins the central role of the ocean in sustaining life on Earth.

Increased nitrate concentration differentially affects cell growth and expression of nitrate transporter and other nitrogen-related genes in the harmful dinoflagellate Prorocentrum minimum

The molecular mechanisms through which dinoflagellates adapt to nitrate fluctuations in aquatic environments remain poorly understood. Here, we sequenced the full-length cDNA of a nitrate transporter (NRT) gene from the harmful marine dinoflagellate Prorocentrum minimum Schiller. The cDNA length was 2431 bp. It encoded a 529-amino acid protein, which was phylogenetically clustered with proteins from other dinoflagellates. Nitrate supply promoted cell growth up to a certain concentration (∼1.76 mM) but inhibited it at higher concentrations. Interestingly, at the inhibitory concentrations, nitrite levels in the medium were considerably increased. Nitrate concentration affected the expression of PmNRT, nitrite transporter (PmNiRT), nitrate reductase (PmNR), and nitrite reductase (PmNiR). Specifically, PmNRT was upregulated after 24 h, with ∼6-fold change compared with the control level, in both nitrate-depleted and nitrate-repleted cultures. In addition, PmNR transcript levels increased to the maximum of 4-fold at 48 h but decreased thereafter. In contrast, PmNiR levels remained unchanged in both nitrate-repleted and nitrate-depleted cultures. Therefore, P. minimum likely copes with nitrate fluctuations in its environment by regulating a set of genes responsible for nitrate uptake.

The Influence of Genes on the “Killer Plasmid” of Dinoroseobacter shibae on Its Symbiosis With the Dinoflagellate Prorocentrum minimum

The marine bacterium Dinoroseobacter shibae shows a Jekyll-and-Hyde behavior in co-culture with the dinoflagellate Prorocentrum minimum: In the initial symbiotic phase it provides the essential vitamins B12 (cobalamin) and B1 (thiamine) to the algae. In the later pathogenic phase it kills the dinoflagellate. The killing phenotype is determined by the 191 kb plasmid and can be conjugated into other Roseobacters. From a transposon-library of D. shibae we retrieved 28 mutants whose insertion sites were located on the 191 kb plasmid. We co-cultivated each of them with P. minimum in L1 medium lacking vitamin B12. With 20 mutant strains no algal growth beyond the axenic control lacking B12 occurred. Several of these genes were predicted to encode proteins from the type IV secretion system (T4SS). They are apparently essential for establishing the symbiosis. With five transposon mutant strains, the initial symbiotic phase was intact but the later pathogenic phase was lost in co-culture. In three of them the insertion sites were located in an operon predicted to encode genes for biotin (B7) uptake. Both P. minimum and D. shibae are auxotrophic for biotin. We hypothesize that the bacterium depletes the medium from biotin resulting in apoptosis of the dinoflagellate.

Determination of Caspase-Like Activities in Roots by the Use of Fluorogenic Substrates

Activity of proteases in tissues can be influenced by various intrinsic and extrinsic factors. One of the activities that is regularly monitored in organisms ranging from prokaryotes to metazoans is the -aspase-like activity: activity of proteases, which cleave their substrates after the negatively charged amino acid residues, especially the aspartic acid. This activity is also known as the caspase-like activity, since the caspases, metazoan cysteine proteases, are one of the best characterized proteases with Asp-directed activities. Plants do not contain caspases; however, various plant proteases have been shown to exhibit caspase-like activity including saspases, phytaspases, and legumains (VPEs). The activity of these proteases can change in plants in response to stress. Here we present a simple method for monitoring of the caspase-like protease activity in roots, which have been treated with allelopathic extracts, using a set of commercially available caspase substrates. We show that activity towards some, but not all, caspase substrates is upregulated in treated but not control samples. The protocol can be used also for other plant tissues as well as for other stressors.

Seasonal mixed layer depth shapes phytoplankton physiology, viral production, and accumulation in the North Atlantic

Seasonal shifts in phytoplankton accumulation and loss largely follow changes in mixed layer depth, but the impact of mixed layer depth on cell physiology remains unexplored. Here, we investigate the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing alter community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring mixed layers are characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring have high levels of oxidative stress. Prolonged stratification into early autumn manifests in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renews during mixed layer deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure.

Phytoplankton are important primary producers. Here the authors investigate phytoplankton physiological changes associated with bloom phases and mixing regimes in the North Atlantic, finding that stratification and deep mixing shape accumulation rates by altering physiology and viral production.

Programmed cell death induced by modified clay in controlling Prorocentrum donghaiense bloom

Modified clay (MC), an effective material used for the emergency elimination of algal blooms, can rapidly reduce the biomass of harmful algal blooms (HABs) via flocculation. After that, MC can still control bloom population through indirect effects such as oxidative stress, which was initially proposed to be related to programmed cell death (PCD) at molecular level. To further study the MC induced cell death in residual bloom organisms, especially identifying PCD process, we studied the physiological state of the residual Prorocentrum donghaiense. The experimental results showed that flocculation changed the physiological state of the residual cells, as evidenced by growth inhibition and increased reactive oxygen species production. Moreover, this research provides biochemical and ultrastructural evidence showing that MC induces PCD in P. donghaiense. Nuclear changes were observed, and increased caspase-like activity, externalization of phosphatidylserine and DNA fragmentation were detected in MC-treated groups and quantified. And the mitochondrial apoptosis pathway was activated in both MC-treated groups. Besides, the features of MC-induced PCD in a unicellular organism were summarized and its concentration dependent manner was proved. All our preliminary results elucidate the mechanism through which MC can further control HABs by inducing PCD and suggest a promising application of PCD in bloom control.

Marine Polymer-Gels' Relevance in the Atmosphere as Aerosols and CCN

Marine polymer gels play a critical role in regulating ocean basin scale biogeochemical dynamics. This brief review introduces the crucial role of marine gels as a source of aerosol particles and cloud condensation nuclei (CCN) in cloud formation processes, emphasizing Arctic marine microgels. We review the gel’s composition and relation to aerosols, their emergent properties, and physico-chemical processes that explain their change in size spectra, specifically in relation to aerosols and CCN. Understanding organic aerosols and CCN in this context provides clear benefits to quantifying the role of marine nanogel/microgel in microphysical processes leading to cloud formation. This review emphasizes the DOC-marine gel/aerosolized gel-cloud link, critical to developing accurate climate models.

Biochemical Characterization of a Novel Redox-Regulated Metacaspase in a Marine Diatom

Programmed cell death (PCD) in marine microalgae was suggested to be one of the mechanisms that facilitates bloom demise, yet its molecular components in phytoplankton are unknown. Phytoplankton are completely lacking any of the canonical components of PCD, such as caspases, but possess metacaspases. Metacaspases were shown to regulate PCD in plants and some protists, but their roles in algae and other organisms are still elusive. Here, we identified and biochemically characterized a type III metacaspase from the model diatom Phaeodactylum tricornutum, termed PtMCA-IIIc. Through expression of recombinant PtMCA-IIIc in E. coli, we revealed that PtMCA-IIIc exhibits a calcium-dependent protease activity, including auto-processing and cleavage after arginine. Similar metacaspase activity was detected in P. tricornutum cell extracts. PtMCA-IIIc overexpressing cells exhibited higher metacaspase activity, while CRISPR/Cas9-mediated knockout cells had decreased metacaspase activity compared to WT cells. Site-directed mutagenesis of cysteines that were predicted to form a disulfide bond decreased recombinant PtMCA-IIIc activity, suggesting its enhancement under oxidizing conditions. One of those cysteines was oxidized, detected in redox proteomics, specifically in response to lethal concentrations of hydrogen peroxide and a diatom derived aldehyde. Phylogenetic analysis revealed that this cysteine-pair is unique and widespread among diatom type III metacaspases. The characterization of a cell death associated protein in diatoms provides insights into the evolutionary origins of PCD and its ecological significance in algal bloom dynamics.

Cell Death and Metabolic Stress in Gymnodinium catenatum Induced by Allelopathy

Allelopathy between phytoplankton species can promote cellular stress and programmed cell death (PCD). The raphidophyte Chattonella marina var. marina, and the dinoflagellates Margalefidinium polykrikoides and Gymnodinium impudicum have allelopathic effects on Gymnodinium catenatum; however, the physiological mechanisms are unknown. We evaluated whether the allelopathic effect promotes cellular stress and activates PCD in G. catenatum. Cultures of G. catenatum were exposed to cell-free media of C. marina var. marina, M. polykrikoides and G. impudicum. The mortality, superoxide radical (O₂^●-) production, thiobarbituric acid reactive substances (TBARS) levels, superoxide dismutase (SOD) activity, protein content, and caspase-3 activity were quantified. Mortality (between 57 and 79%) was registered in G. catenatum after exposure to cell-free media of the three species. The maximal O₂^●- production occurred with C. marina var. marina cell-free media. The highest TBARS levels and SOD activity in G. catenatum were recorded with cell-free media from G. impudicum. The highest protein content was recorded with cell-free media from M. polykrikoides. All cell-free media caused an increase in the activity of caspase-3. These results indicate that the allelopathic effect in G. catenatum promotes cell stress and caspase-3 activation, as a signal for the induction of programmed cell death.

Bacterial Quorum-Sensing Signal Arrests Phytoplankton Cell Division and Impacts Virus-Induced Mortality

Bacteria and phytoplankton form close associations in the ocean that are driven by the exchange of chemical compounds. The bacterial signal 2-heptyl-4-quinolone (HHQ) slows phytoplankton growth; however, the mechanism responsible remains unknown.

Interactions between phytoplankton and heterotrophic bacteria fundamentally shape marine ecosystems by controlling primary production, structuring marine food webs, mediating carbon export, and influencing global climate. Phytoplankton-bacterium interactions are facilitated by secreted compounds; however, linking these chemical signals, their mechanisms of action, and their resultant ecological consequences remains a fundamental challenge. The bacterial quorum-sensing signal 2-heptyl-4-quinolone (HHQ) induces immediate, yet reversible, cellular stasis (no cell division or mortality) in the coccolithophore Emiliania huxleyi; however, the mechanism responsible remains unknown. Using transcriptomic and proteomic approaches in combination with diagnostic biochemical and fluorescent cell-based assays, we show that HHQ exposure leads to prolonged S-phase arrest in phytoplankton coincident with the accumulation of DNA damage and a lack of repair despite the induction of the DNA damage response (DDR). While this effect is reversible, HHQ-exposed phytoplankton were also protected from viral mortality, ascribing a new role of quorum-sensing signals in regulating multitrophic interactions. Furthermore, our data demonstrate that in situ measurements of HHQ coincide with areas of enhanced micro- and nanoplankton biomass. Our results suggest bacterial communication signals as emerging players that may be one of the contributing factors that help structure complex microbial communities throughout the ocean.

IMPORTANCE Bacteria and phytoplankton form close associations in the ocean that are driven by the exchange of chemical compounds. The bacterial signal 2-heptyl-4-quinolone (HHQ) slows phytoplankton growth; however, the mechanism responsible remains unknown. Here, we show that HHQ exposure leads to the accumulation of DNA damage in phytoplankton and prevents its repair. While this effect is reversible, HHQ-exposed phytoplankton are also relieved of viral mortality, elevating the ecological consequences of this complex interaction. Further results indicate that HHQ may target phytoplankton proteins involved in nucleotide biosynthesis and DNA repair, both of which are crucial targets for viral success. Our results support microbial cues as emerging players in marine ecosystems, providing a new mechanistic framework for how bacterial communication signals mediate interspecies and interkingdom behaviors.

Examining the Evidence for Regulated and Programmed Cell Death in Cyanobacteria. How Significant Are Different Forms of Cell Death in Cyanobacteria Population Dynamics?

Cyanobacteria are ancient and versatile members of almost all aquatic food webs. In freshwater ecosystems some cyanobacteria form “bloom” populations containing potent toxins and such blooms are therefore a key focus of study. Bloom populations can be ephemeral, with rapid population declines possible, though the factors causing such declines are generally poorly understood. Cell death could be a significant factor linked to population decline. Broadly, three forms of cell death are currently recognized – accidental, regulated and programmed – and efforts are underway to identify these and standardize the use of cell death terminology, guided by work on better-studied cells. For cyanobacteria, the study of such differing forms of cell death has received little attention, and classifying cell death across the group, and within complex natural populations, is therefore hard and experimentally difficult. The population dynamics of photosynthetic microbes have, in the past, been principally explained through reference to abiotic (“bottom-up”) factors. However, it has become clearer that in general, only a partial linkage exists between abiotic conditions and cyanobacteria population fluctuations in many situations. Instead, a range of biotic interactions both within and between cyanobacteria, and their competitors, pathogens and consumers, can be seen as the major drivers of the observed population fluctuations. Whilst some evolutionary processes may theoretically account for the existence of an intrinsic form of cell death in cyanobacteria, a range of biotic interactions are also likely to frequently cause the ecological incidence of cell death. New theoretical models and single-cell techniques are being developed to illuminate this area. The importance of such work is underlined by both (a) predictions of increasing cyanobacteria dominance due to anthropogenic factors and (b) the realization that influential ecosystem modeling work includes mortality terms with scant foundation, even though such terms can have a very large impact on model predictions. These ideas are explored and a prioritization of research needs is proposed.

Cell Death in Cyanobacteria: Current Understanding and Recommendations for a Consensus on Its Nomenclature

Cyanobacteria are globally widespread photosynthetic prokaryotes and are major contributors to global biogeochemical cycles. One of the most critical processes determining cyanobacterial eco-physiology is cellular death. Evidence supports the existence of controlled cellular demise in cyanobacteria, and various forms of cell death have been described as a response to biotic and abiotic stresses. However, cell death research in this phylogenetic group is a relatively young field and understanding of the underlying mechanisms and molecular machinery underpinning this fundamental process remains largely elusive. Furthermore, no systematic classification of modes of cell death has yet been established for cyanobacteria. In this work, we analyzed the state of knowledge in the field of cyanobacterial cell death. Based on that, we propose unified criterion for the definition of accidental, regulated, and programmed forms of cell death in cyanobacteria based on molecular, biochemical, and morphologic aspects following the directions of the Nomenclature Committee on Cell Death (NCCD). With this, we aim to provide a guide to standardize the nomenclature related to this topic in a precise and consistent manner, which will facilitate further ecological, evolutionary, and applied research in the field of cyanobacterial cell death.

The Role of Pseudo-Orthocaspase (SyOC) of Synechocystis sp. PCC 6803 in Attenuating the Effect of Oxidative Stress

Caspases are proteases, best known for their involvement in the execution of apoptosis-a subtype of programmed cell death, which occurs only in animals. These proteases are composed of two structural building blocks: a proteolytically active p20 domain and a regulatory p10 domain. Although structural homologs appear in representatives of all other organisms, their functional homology, i.e., cell death depending on their proteolytical activity, is still much disputed. Additionally, pseudo-caspases and pseudo-metacaspases, in which the catalytic histidine-cysteine dyad is substituted with non-proteolytic amino acid residues, were shown to be involved in cell death programs. Here, we present the involvement of a pseudo-orthocaspase (SyOC), a prokaryotic caspase-homolog lacking the p10 domain, in oxidative stress in the model cyanobacterium Synechocystis sp. PCC 6803. To study the in vivo impact of this pseudo-protease during oxidative stress its gene expression during exposure to H₂O₂ was monitored by RT-qPCR. Furthermore, a knock-out mutant lacking the pseudo-orthocaspase gene was designed, and its survival and growth rates were compared to wild type cells as well as its proteome. Deletion of SyOC led to cells with a higher tolerance toward oxidative stress, suggesting that this protein may be involved in a pro-death pathway.

Identification of living and dead microalgae cells with digital holography and verified in the East China Sea

The death of microalgae plays an important role in ocean energy flow and material circulation. The existing methods for the identification of dead and living microalgae cells all have defects such as the need for staining and pre-treatment. In this work, a label-free method to identify living and dead algae cells based on digital holography microscopy and machine learning was designed. At the stage of model training, ten feature vectors were extracted from the holograms, and twelve classification models of machine learning algorithm were trained. Compared with the staining method results, the accuracy of this method can reach 94.8%. At the stage of field verification, the death rate calculated by this method was also consistent with staining method. The method proposed in this paper provides a new method for the study of marine microalgae death which has the advantages of label-free, non-invasive, high accuracy and potential for in-situ application.

Virus-induced spore formation as a defense mechanism in marine diatoms

Algal viruses are important contributors to carbon cycling, recycling nutrients and organic material through host lysis. Although viral infection has been described as a primary mechanism of phytoplankton mortality, little is known about host defense responses. We show that viral infection of the bloom-forming, planktonic diatom Chaetoceros socialis induces the mass formation of resting spores, a heavily silicified life cycle stage associated with carbon export due to rapid sinking. Although viral RNA was detected within spores, mature virions were not observed. 'Infected' spores were capable of germinating, but did not propagate or transmit infectious viruses. These results demonstrate that diatom spore formation is an effective defense strategy against viral-mediated mortality. They provide a possible mechanistic link between viral infection, bloom termination, and mass carbon export events and highlight an unappreciated role of viruses in regulating diatom life cycle transitions and ecological success.

Programmed cell death as a black queen in microbial communities

Programmed cell death (PCD) in unicellular organisms is in some instances an altruistic trait. When the beneficiaries are clones or close kin, kin selection theory may be used to explain the evolution of the trait, and when the trait evolves in groups of distantly related individuals, group or multilevel selection theory is invoked. In mixed microbial communities, the benefits are also available to unrelated taxa. But the evolutionary ecology of PCD in communities is poorly understood. Few hypotheses have been offered concerning the community role of PCD despite its far-reaching effects. The hypothesis we consider here is that PCD is a black queen. The Black Queen Hypothesis (BQH) outlines how public goods arising from a leaky function are exploited by other taxa in the community. Black Queen (BQ) traits are essential for community survival, but only some members bear the cost of possessing them, while others lose the trait In addition, BQ traits have been defined in terms of adaptive gene loss, and it is unknown whether this has occurred for PCD. Our conclusion is that PCD fulfils the two most important criteria of a BQ (leakiness and costliness), but that more empirical data are needed for assessing the remaining two criteria. In addition, we hold that for viewing PCD as a BQ, the original BQH needs to include social traits. Thus, despite some empirical and conceptual shortcomings, the BQH provides a helpful avenue for investigating PCD in microbial communities.

The molecular response mechanisms of a diatom Thalassiosira pseudonana to the toxicity of BDE-47 based on whole transcriptome analysis

Polybrominated diphenyl ethers (PBDEs) are ubiquitously distributed persistent organic pollutants (POPs) in marine environments. Phytoplankton are the entrance of PBDEs entering to biotic environments from abiotic environments, while the responding mechanisms of phytoplankton to PBDEs have not been full established. Therefore, we chose the model diatom Thalassiosira pseudonana in this study, by integrating whole transcriptome analysis with physiological-biochemical data, to reveal the molecular responding mechanisms of T. pseudonana to the toxicity of BDE-47. Our results indicated the changes of genes expressions correlated to the physiological-biochemical changes, and there were multiple molecular mechanisms of T. pseudonana responding to the toxicity of BDE-47: Gene expressions evidence explained the suppression of light reaction and proved the occurrence of cellular oxidative stress; In the meanwhile, up-regulations of genes in pathways involving carbon metabolisms happened, including the Calvin cycle, glycolysis, TCA cycle, fatty acid synthesis, and triacylglycerol synthesis; Lastly, DNA damage was found and three outcome including DNA repair, cell cycle arrest and programmed cell death (PCD) happened, which could finally inhibit the cell division and population growth of T. pseudonana. This study presented the most complete molecular responding mechanisms of phytoplankton cells to PBDEs, and provided valuable information of various PBDEs-sensitive genes with multiple functions for further research involving organic pollutants and phytoplankton.

Resource partitioning of phytoplankton metabolites that support bacterial heterotrophy

The communities of bacteria that assemble around marine microphytoplankton are predictably dominated by Rhodobacterales, Flavobacteriales, and families within the Gammaproteobacteria. Yet whether this consistent ecological pattern reflects the result of resource-based niche partitioning or resource competition requires better knowledge of the metabolites linking microbial autotrophs and heterotrophs in the surface ocean. We characterized molecules targeted for uptake by three heterotrophic bacteria individually co-cultured with a marine diatom using two strategies that vetted the exometabolite pool for biological relevance by means of bacterial activity assays: expression of diagnostic genes and net drawdown of exometabolites, the latter detected with mass spectrometry and nuclear magnetic resonance using novel sample preparation approaches. Of the more than 36 organic molecules with evidence of bacterial uptake, 53% contained nitrogen (including nucleosides and amino acids), 11% were organic sulfur compounds (including dihydroxypropanesulfonate and dimethysulfoniopropionate), and 28% were components of polysaccharides (including chrysolaminarin, chitin, and alginate). Overlap in phytoplankton-derived metabolite use by bacteria in the absence of competition was low, and only guanosine, proline, and N-acetyl-D-glucosamine were predicted to be used by all three. Exometabolite uptake pattern points to a key role for ecological resource partitioning in the assembly marine bacterial communities transforming recent photosynthate.

Prochlorococcus Cells Rely on Microbial Interactions Rather than on Chlorotic Resting Stages To Survive Long-Term Nutrient Starvation

The ability of microorganisms to withstand long periods of nutrient starvation is key to their survival and success under highly fluctuating conditions that are common in nature. Therefore, one would expect this trait to be prevalent among organisms in the nutrient-poor open ocean. Here, we show that this is not the case for Prochlorococcus, a globally abundant and ecologically important marine cyanobacterium. Instead, Prochlorococcus relies on co-occurring heterotrophic bacteria to survive extended phases of nutrient and light starvation. Our results highlight the power of microbial interactions to drive major biogeochemical cycles in the ocean and elsewhere with consequences at the global scale.

Many microorganisms produce resting cells with very low metabolic activity that allow them to survive phases of prolonged nutrient or energy stress. In cyanobacteria and some eukaryotic phytoplankton, the production of resting stages is accompanied by a loss of photosynthetic pigments, a process termed chlorosis. Here, we show that a chlorosis-like process occurs under multiple stress conditions in axenic laboratory cultures of Prochlorococcus, the dominant phytoplankton linage in large regions of the oligotrophic ocean and a global key player in ocean biogeochemical cycles. In Prochlorococcus strain MIT9313, chlorotic cells show reduced metabolic activity, measured as C and N uptake by Nanoscale secondary ion mass spectrometry (NanoSIMS). However, unlike many other cyanobacteria, chlorotic Prochlorococcus cells are not viable and do not regrow under axenic conditions when transferred to new media. Nevertheless, cocultures with a heterotrophic bacterium, Alteromonas macleodii HOT1A3, allowed Prochlorococcus to survive nutrient starvation for months. We propose that reliance on co-occurring heterotrophic bacteria, rather than the ability to survive extended starvation as resting cells, underlies the ecological success of Prochlorococcus.

ROS-mediated programmed cell death (PCD) of Thalassiosira pseudonana under the stress of BDE-47

Polybrominated diphenyl ethers (PBDEs) are a series of highly persistent organic pollutants (POPs) ubiquitously distributed in marine environments. As key primary producers, microalgae are the start of PBDEs bioaccumulations and vulnerable to their toxicities. In order to deeply investigate the toxic mechanism of PBDEs on microalgal cells, the occurrence of programmed cell death (PCD) in a model diatom Thalassiosira pseudonana and its possible mediating mechanism were studied. The results indicated: cell death of T. pseudonana happened under the stress of BDE-47, which was proved to be PCD based on the correlations with three biochemical markers (DNA fragmentation, phosphatidylserine externalization and caspase activity) and three molecular markers [Metacaspase 2 gene (TpMC2), Death-associated protein gene (DAP3) and Death-specific protein 1 gene (TpDSP1)]; Furthermore, the changes of cellular ROS levels were correlated with the PCD markers and the dead cell rates, and the cell membrane and the chloroplast were identified as the major ROS production sites. Therefore, we concluded that PCD might be an important toxic mechanism of PBDEs on microalgal cells, and that chloroplast- and cell membrane-produced ROS was an important signaling molecule to mediate the PCD activation process. Our research firstly indicated microalgal PCD could be induced by PBDEs, and increased our knowledge of the toxic mechanisms by which POPs affect microalgal cells.

Multiple Roles of Diatom-Derived Oxylipins within Marine Environments and Their Potential Biotechnological Applications

The chemical ecology of marine diatoms has been the subject of several studies in the last decades, due to the discovery of oxylipins with multiple simultaneous functions including roles in chemical defence (antipredator, allelopathic and antibacterial compounds) and/or cell-to-cell signalling. Diatoms represent a fundamental compartment of marine ecosystems because they contribute to about 45% of global primary production even if they represent only 1% of the Earth’s photosynthetic biomass. The discovery that they produce several toxic metabolites deriving from the oxidation of polyunsaturated fatty acids, known as oxylipins, has changed our perspectives about secondary metabolites shaping plant–plant and plant–animal interactions in the oceans. More recently, their possible biotechnological potential has been evaluated, with promising results on their potential as anticancer compounds. Here, we focus on some recent findings in this field obtained in the last decade, investigating the role of diatom oxylipins in cell-to-cell communication and their negative impact on marine biota. Moreover, we also explore and discuss the possible biotechnological applications of diatom oxylipins.

Diversity and dynamics of relevant nanoplanktonic diatoms in the Western English Channel

In the ocean, Bacillariophyta are one of the most successful protistan groups. Due to their considerable biogeochemical implications, diatom diversity, development, and seasonality have been at the center of research, specifically large-sized species. In comparison, nanoplanktonic diatoms are mostly disregarded from routine monitoring and are often underrepresented in genetic reference databases. Here, we identified and investigated the temporal dynamics of relevant nanodiatoms occurring in the Western English Channel (SOMLIT-Astan station). Coupling in situ and laboratory approaches, we revealed that nano-species from the genera Minidiscus and Thalassiosira are key components of the phytoplankton community that thrive in these coastal waters, but they display different seasonal patterns. Some species formed recurrent blooms whilst others were persistent year round. These results raise questions about their regulation in the natural environment. Over a full seasonal cycle at the monitoring station, we succeeded in isolating viruses which infect these minute diatoms, suggesting that these mortality agents may contribute to their control. Overall, our study points out the importance of considering nanodiatom communities within time-series surveys to further understand their role and fate in marine systems.

Phytoplankton Growth and Productivity in the Western North Atlantic: Observations of Regional Variability From the NAAMES Field Campaigns

The ability to quantify spatio-temporal variability in phytoplankton growth and productivity is essential to improving our understanding of global carbon dynamics and trophic energy flow. Satellite-based observations offered the first opportunity to estimate depth-integrated net primary production (NPP) at a global scale, but early modeling approaches could not effectively address variability in algal physiology, particularly the effects of photoacclimation on changes in cellular chlorophyll. Here, a previously developed photoacclimation model was used to derive depth-resolved estimates of phytoplankton division rate (μ) and NPP. The new approach predicts NPP values that closely match discrete measurements of 14C-based NPP and effectively captured both spatial and temporal variability observed during the four field campaigns of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). We observed favorable growth conditions for phytoplankton throughout the annual cycle in the subtropical western North Atlantic. As a result, high rates of μ are sustained year-round resulting in a strong coupling between growth and loss processes and a more moderate spring bloom compared to the high-latitude subarctic region. Considerable light limitation was observed in the subarctic province during the winter, which resulted in divergent growth dynamics, stronger decoupling from grazing pressure and a taxonomically distinct phytoplankton community. This study demonstrates how detailed knowledge of phytoplankton division rate furthers our understanding of global carbon cycling by providing insight into the resulting influence on phytoplankton taxonomy and the loss processes that dictate the fate of fixed carbon.

Responses of Marine Diatom Skeletonema marinoi to Nutrient Deficiency: Programmed Cell Death

Diatoms are important phytoplankton and contribute greatly to the primary productivity of marine ecosystems. Despite the ecological significance of diatoms and the importance of programmed cell death (PCD) in the fluctuation of diatom populations, little is known about the molecular mechanisms of PCD triggered by different nutrient stresses. Here we describe the physiological, morphological, biochemical, and molecular changes in response to low levels of nutrients in the ubiquitous diatom Skeletonema marinoi The levels of gene expression involved in oxidation resistance and PCD strongly increased upon nitrogen (N) or phosphorus (P) starvation. The enzymatic activity of caspase 3-like protein also increased. Differences in mRNA levels and protein activities were observed between the low-N and low-P treatments, suggesting that PCD could have a differential response to different nutrient stresses. When cultures were replete with N or P, the growth inhibition stopped. Meanwhile, the enzymatic activity of caspase 3-like protein and the number of cells with damaged membranes decreased. These results suggest that PCD is an important cell fate decision mechanism in the marine diatom S. marinoi Our results provide important insight into how diatoms adjust phenotypic and genotypic features of their cell-regulated death programs when stressed by nutrient limitations. Overall, this study could allow us to better understand the molecular mechanism behind the formation and termination of diatom blooms in the marine environment.IMPORTANCE Our study showed how the ubiquitous diatom S. marinoi responded to different nutrient limitations with PCD in terms of physiological, morphological, biochemical, and molecular characteristics. Some PCD-related genes (PDCD4, GOX, and HSP90) induced by N deficiency were relatively upregulated compared to those induced by P deficiency. In contrast, the expression of the TSG101 gene in S. marinoi showed a clear and constant increase during P limitation compared to N limitation. These findings suggest that PCD is a complex mechanism involving several different proteins. The systematic mRNA level investigations provide new insight into understanding the oxidative stress- and cell death-related functional genes of diatoms involved in the response to nutrient fluctuations (N or P stress) in the marine environment.

Light-dependent single-cell heterogeneity in the chloroplast redox state regulates cell fate in a marine diatom

Diatoms are photosynthetic microorganisms of great ecological and biogeochemical importance, forming vast blooms in aquatic ecosystems. However, we are still lacking fundamental understanding of how individual cells sense and respond to diverse stress conditions, and what acclimation strategies are employed during bloom dynamics. We investigated cellular responses to environmental stress at the single-cell level using the redox sensor roGFP targeted to various organelles in the diatom Phaeodactylum tricornutum. We detected cell-to-cell variability using flow cytometry cell sorting and a microfluidics system for live imaging of oxidation dynamics. Chloroplast-targeted roGFP exhibited a light-dependent, bi-stable oxidation pattern in response to H₂O₂ and high light, revealing distinct subpopulations of sensitive oxidized cells and resilient reduced cells. Early oxidation in the chloroplast preceded commitment to cell death, and can be used for sensing stress cues and regulating cell fate. We propose that light-dependent metabolic heterogeneity regulates diatoms’ sensitivity to environmental stressors in the ocean.

Microscopic algae, such as diatoms, are widely spread throughout the oceans, and are responsible for half of the oxygen we breathe. At certain times of the year these algae grow very rapidly to form large “blooms” that can be detected by satellites in space. These blooms are generally short-lived because the algae are either eaten by other marine organisms, run out of nutrients, or die as a result of being infected by viruses or bacteria. However, some diatom cells survive the end of the bloom and go on to generate new blooms in the future, but it is still not clear how.

As the bloom collapses, diatoms experience many stressful conditions which can cause active molecules known as reactive oxygen species, or ROS for short, to accumulate inside cells. Normally growing cells also produce low amounts of ROS, which regulate various processes that are important for maintaining a cell’s health. However, high amounts of ROS can cause damage, which may lead to a cell’s death. Now, Mizrachi et al. investigated why some algae survive while others die in response to stressful conditions, focusing on the amount of ROS that accumulates within the diatom Phaeodactylum tricornutum.

Laboratory experiments showed that individual cells of P. tricornutum respond differently to environmental stress, forming two distinct groups of either sensitive or resilient cells. Sensitive cells accumulated high levels of ROS within a cell compartment known as the chloroplast and eventually died. Whereas resilient cells were able to maintain low levels of ROS in the chloroplast and survived long after the other cells perished. Populations of genetically identical diatom cells also formed distinct groups of sensitive and resilient cells, demonstrating that these two opposing reactions to stress are not caused by genetic differences between cells.

Lastly, Mizrachi et al. showed that how diatoms acclimate to stress depends on the amount of light they are exposed to. When in the dark, all cells became sensitive to oxidative stress, without forming distinct groups. But, when exposed to strong light that mimics the ocean surface, cells formed distinct groups within the population. This suggests that light regulates how susceptible these microscopic algae are to environmental stress.

The different responses within a population may serve as a “bet-hedging” strategy, enabling at least some of the cells to survive unpredicted stressful conditions. The next challenge will be to find out whether algae growing in the oceans also use the same strategy and investigate what impact this has on diatom blooms.

Quantitative Operating Principles of Yeast Metabolism during Adaptation to Heat Stress

Microorganisms evolved adaptive responses to survive stressful challenges in ever-changing environments. Understanding the relationships between the physiological/metabolic adjustments allowing cellular stress adaptation and gene expression changes being used by organisms to achieve such adjustments may significantly impact our ability to understand and/or guide evolution. Here, we studied those relationships during adaptation to various stress challenges in Saccharomyces cerevisiae, focusing on heat stress responses. We combined dozens of independent experiments measuring whole-genome gene expression changes during stress responses with a simplified kinetic model of central metabolism. We identified alternative quantitative ranges for a set of physiological variables in the model (production of ATP, trehalose, NADH, etc.) that are specific for adaptation to either heat stress or desiccation/rehydration. Our approach is scalable to other adaptive responses and could assist in developing biotechnological applications to manipulate cells for medical, biotechnological, or synthetic biology purposes.

Physiological response dynamics of the brown tide organism Aureococcus anophagefferens treated with modified clay

On the basis of experiences in mitigating harmful algal blooms (HABs) with modified clay (MC), a bloom does not continue after the dispersal of the MC, even though the density of the residual cells in the water remains as high as 20-30% of the initial cell density. This interesting phenomenon indicates that in addition to flocculation, MC has additional mechanisms of HAB control. Here, Aureococcus anophagefferens was selected as a model organism to study the physiological response dynamics of residual cells treated with MC, and RT-qPCR was used to measure the differential expression of 40 genes involved in anti-oxidation, photosynthesis, phospholipid synthesis, programmed cell death and cell proliferation at five time points. The results showed that every functional gene category exhibited a "V" shaped pattern with a turning point. It was reflected that there were two processes for MC inhibiting the growth of residual cells. One is the oxidative stress process (OSP) caused by ineffective collision with MC, whose effect weakened gradually; another is the programmed cell death process (PCDP) caused by the lysis of damaged residual cells, whose effect enhanced two days after MC treatment. In addition, the scanning electron micrographs verified that some of the residual cells were deformed or even lysed. Combined with the effects of OSP and PCDP in dynamics, the growth of residual cells was inhibited and was followed by gradual bloom disappearance. This study further elucidates the mechanism of MC controlling HABs at the molecular level and enable a more comprehensive understanding of HAB mitigation using MC.

Biochemical diversity of glycosphingolipid biosynthesis as a driver of Coccolithovirus competitive ecology

Coccolithoviruses (EhVs) are large, double-stranded DNA-containing viruses that infect the single-celled, marine coccolithophore Emiliania huxleyi. Given the cosmopolitan nature and global importance of E. huxleyi as a bloom-forming, calcifying, photoautotroph, E. huxleyi-EhV interactions play a key role in oceanic carbon biogeochemistry. Virally-encoded glycosphingolipids (vGSLs) are virulence factors that are produced by the activity of virus-encoded serine palmitoyltransferase (SPT). Here, we characterize the dynamics, diversity and catalytic production of vGSLs in an array of EhV strains in relation to their SPT sequence composition and explore the hypothesis that they are a determinant of infectivity and host demise. vGSL production and diversity was positively correlated with increased virulence, virus replication rate and lytic infection dynamics in laboratory experiments, but they do not explain the success of less-virulent EhVs in natural EhV communities. The majority of EhV-derived SPT amplicon sequences associated with infected cells in the North Atlantic derived from slower infecting, less virulent EhVs. Our lab-, field- and mathematical model-based data and simulations support ecological scenarios whereby slow-infecting, less-virulent EhVs successfully compete in North Atlantic populations of E. huxleyi, through either the preferential removal of fast-infecting, virulent EhVs during active infection or by having access to a broader host range.

Algicidal bacteria trigger contrasting responses in model diatom communities of different composition

Algicidal bacteria are important players regulating the dynamic changes of plankton assemblages. Most studies on these bacteria have focused on the effect on single algal species in simple incubation experiments. Considering the complexity of species assemblages in the natural plankton, such incubations represent an oversimplification and do not allow making further reaching conclusions on ecological interactions. Here, we describe a series of co‐incubation experiments with different level of complexity to elucidate the effect of the algicidal bacterium Kordia algicida on mixed cultures of a resistant and a susceptible diatom. The growth of the resistant diatom Chaetoceros didymus is nearly unaffected by K. algicida in monoculture, while cells of the susceptible diatom Skeletonema costatum are lysed within few hours. Growth of C. didymus is inhibited if mixed cultures of the two diatoms are infected with the bacterium. Incubations with filtrates of the infected cultures show that the effects are chemically mediated. In non‐contact co‐culturing we show that low concentrations of the lysed algae support the growth of C. didymus, while higher concentrations trigger population decline. Complex cascading effects of algicidal bacteria have thus to be taken into account if their ecological role is concerned.

Nitric oxide production and antioxidant function during viral infection of the coccolithophore Emiliania huxleyi

Emiliania huxleyi is a globally important marine phytoplankton that is routinely infected by viruses. Understanding the controls on the growth and demise of E. huxleyi blooms is essential for predicting the biogeochemical fate of their organic carbon and nutrients. In this study, we show that the production of nitric oxide (NO), a gaseous, membrane-permeable free radical, is a hallmark of early-stage lytic infection in E. huxleyi by Coccolithoviruses, both in culture and in natural populations in the North Atlantic. Enhanced NO production was detected both intra- and extra-cellularly in laboratory cultures, and treatment of cells with an NO scavenger significantly reduced viral production. Pre-treatment of exponentially growing E. huxleyi cultures with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) prior to challenge with hydrogen peroxide (H₂O₂) led to greater cell survival, suggesting that NO may have a cellular antioxidant function. Indeed, cell lysates generated from cultures treated with SNAP and undergoing infection displayed enhanced ability to detoxify H₂O₂. Lastly, we show that fluorescent indicators of cellular ROS, NO, and death, in combination with classic DNA- and lipid-based biomarkers of infection, can function as real-time diagnostic tools to identify and contextualize viral infection in natural E. huxleyi blooms.

The chemical cue tetrabromopyrrole induces rapid cellular stress and mortality in phytoplankton

Eukaryotic phytoplankton contribute to the flow of elements through marine food webs, biogeochemical cycles, and Earth's climate. Therefore, how phytoplankton die is a critical determinate of the flow and fate of nutrients. While heterotroph grazing and viral infection contribute to phytoplankton mortality, recent evidence suggests that bacteria-derived cues also control phytoplankton lysis. Here, we report exposure to nanomolar concentrations of 2,3,4,5-tetrabromopyrrole (TBP), a brominated chemical cue synthesized by marine γ-proteobacteria, resulted in mortality of seven phylogenetically-diverse phytoplankton species. A comparison of nine compounds of marine-origin containing a range of cyclic moieties and halogenation indicated that both a single pyrrole ring and increased bromination were most lethal to the coccolithophore, Emiliania huxleyi. TBP also rapidly induced the production of reactive oxygen species and the release of intracellular calcium stores, both of which can trigger the activation of cellular death pathways. Mining of the Ocean Gene Atlas indicated that TBP biosynthetic machinery is globally distributed throughout the water column in coastal areas. These findings suggest that bacterial cues play multiple functions in regulating phytoplankton communities by inducing biochemical changes associated with cellular death. Chemically-induced lysis by bacterial infochemicals is yet another variable that must be considered when modeling oceanic nutrient dynamics.

The mutual interplay between calcification and coccolithovirus infection

Two prominent characteristics of marine coccolithophores are their secretion of coccoliths and their susceptibility to infection by coccolithoviruses (EhVs), both of which display variation among cells in culture and in natural populations. We examined the impact of calcification on infection by challenging a variety of Emiliania huxleyi strains at different calcification states with EhVs of different virulence. Reduced cellular calcification was associated with increased infection and EhV production, even though calcified cells and associated coccoliths had significantly higher adsorption coefficients than non-calcified (naked) cells. Sialic acid glycosphingolipids, molecules thought to mediate EhV infection, were generally more abundant in calcified cells and enriched in purified, sorted coccoliths, suggesting a biochemical link between calcification and adsorption rates. In turn, viable EhVs impacted cellular calcification absent of lysis by inducing dramatic shifts in optical side scatter signals and a massive release of detached coccoliths in a subpopulation of cells, which could be triggered by resuspension of healthy, calcified host cells in an EhV-free, 'induced media'. Our findings show that calcification is a key component of the E. huxleyi-EhV arms race and an aspect that is critical both to the modelling of these host-virus interactions in the ocean and interpreting their impact on the global carbon cycle.

Dynamics of transparent exopolymer particle production and aggregation during viral infection of the coccolithophore, Emiliania huxleyi

Emiliania huxleyi produces calcium carbonate (CaCO₃ ) coccoliths and transparent exopolymer particles (TEP), sticky, acidic carbohydrates that facilitate aggregation. E. huxleyi's extensive oceanic blooms are often terminated by coccolithoviruses (EhVs) with the transport of cellular debris and associated particulate organic carbon (POC) to depth being facilitated by TEP-bound 'marine snow' aggregates. The dynamics of TEP production and particle aggregation in response to EhV infection are poorly understood. Using flow cytometry, spectrophotometry and FlowCam visualization of alcian blue (AB)-stained aggregates, we assessed TEP production and the size spectrum of aggregates for E. huxleyi possessing different degrees of calcification and cellular CaCO₃ :POC mass ratios, when challenged with two EhVs (EhV207 and EhV99B1). FlowCam imaging also qualitatively assessed the relative amount of AB-stainable TEP (i.e., blue:red ratio of each particle). We show significant increases in TEP during early phase EhV207-infection (∼ 24 h) of calcifying strains and a shift towards large aggregates following EhV99B1-infection. We also observed the formation of large aggregates with low blue:red ratios, suggesting that other exopolymer substances contribute towards aggregation. Our findings show the potential for virus infection and the associated response of their hosts to impact carbon flux dynamics and provide incentive to explore these dynamics in natural populations.