Life cycle strategies in free-living unicellular eukaryotes: Diversity, evolution, and current molecular tools to unravel the private life of microorganisms.

An astonishing range of morphologies and life strategies has arisen across the vast diversity of protists, allowing them to thrive in most environments. In model protists, like Tetrahymena, Dictyostelium, or Trypanosoma, life cycles involving multiple life stages with different morphologies have been well characterized. In contrast, knowledge of the life cycles of free-living protists, which primarily consist of uncultivated environmental lineages, remains largely fragmentary. Various life stages and lineage-specific cellular innovations have been observed in the field for uncultivated protists, but such innovations generally lack functional characterization and have unknown physiological and ecological roles. In the actual state of knowledge, evidence of sexual processes is confirmed for 20% of free-living protist lineages. Nevertheless, at the onset of eukaryotic diversification, common molecular trends emerged to promote genetic recombination, establishing sex as an inherent feature of protists. Here, we review protist life cycles from the viewpoint of life cycle transitions and genetics across major eukaryotic lineages. We focus on the scarcely observed sexual cycle of free-living protists, summarizing evidence for its existence and describing key genes governing its progression, as well as, current methods for studying the genetics of sexual cycles in both cultivable and uncultivated protist groups.

Exploring the phycosphere of Emiliania huxleyi: From bloom dynamics to microbiome assembly experiments.

Coccolithophores have global ecological and biogeochemical significance as the most important calcifying marine phytoplankton group. The structure and selection of prokaryotic communities associated with the most abundant coccolithophore and bloom-forming species, Emiliania huxleyi, are still poorly known. In this study, we assessed the diversity of bacterial communities associated with an E. huxleyi bloom in the Celtic Sea (Eastern North Atlantic), exposed axenic E. huxleyi cultures to prokaryotic communities derived from bloom and non-bloom conditions, and followed the dynamics of their microbiome composition over one year. Bloom-associated prokaryotic communities were dominated by SAR11, Marine group II Euryarchaeota and Rhodobacterales and contained substantial proportions of known indicators of phytoplankton bloom demises such as Flavobacteriaceae and Pseudoalteromonadaceae. The taxonomic richness of bacteria derived from natural communities associated with axenic E. huxleyi rapidly shifted and then stabilized over time. The succession of microorganisms recruited from the environment was consistently dependent on the composition of the initial bacterioplankton community. Phycosphere-associated communities derived from the E. huxleyi bloom were highly similar to one another, suggesting deterministic processes, whereas cultures from non-bloom conditions show an effect of stochasticity. Overall, this work sheds new light on the importance of the initial inoculum composition in microbiome recruitment and elucidates the temporal dynamics of its composition and long-term stability.

Divergent fate of coccolithophores in a warming tropical ecosystem.

Rising ocean temperatures will alter the diversity of marine phytoplankton communities, likely leading to modifications in food-web and biogeochemical dynamics. Here we focus on coccolithophores, a prominent group of calcifying phytoplankton that plays a central role in the global carbon cycle. Using both new (2017-2020) and historical (1975-1976) data from the northern Red Sea, we found that during 'mild summers', the most common coccolithophores - Emiliania huxleyi and Gephyrocapsa ericsonii - co-exist at similar densities. Both species then particularly flourish during subsequent winter periods where nutrient availability is higher due to convective mixing. However, during 'hot summers', which have become progressively the norm over the last decades with average surface temperatures exceeding 27°C for long time-periods, G. ericsonii density markedly declined. Moreover, G. ericsonii remains at low background levels even during winter mixing periods, while E. huxleyi succession and development during winter appears unchanged. Further incubation assays using native assemblages confirmed that G. ericsonii's growth over 27°C is significantly reduced relative to E. huxleyi. Additional factors likely contribute to impair G. ericsonii populations at sea, but temperature is a key factor. Our results illustrate the divergent impact of ongoing ocean warming in tropical phytoplankton species.

Unmasking cellular response of a bloom-forming alga to viral infection by resolving expression profiles at a single-cell level.

Infection by large dsDNA viruses can lead to a profound alteration of host transcriptome and metabolome in order to provide essential building blocks to support the high metabolic demand for viral assembly and egress. Host response to viral infection can typically lead to diverse phenotypic outcome that include shift in host life cycle and activation of anti-viral defense response. Nevertheless, there is a major bottleneck to discern between viral hijacking strategies and host defense responses when averaging bulk population response. Here we study the interaction between Emiliania huxleyi, a bloom-forming alga, and its specific virus (EhV), an ecologically important host-virus model system in the ocean. We quantified host and virus gene expression on a single-cell resolution during the course of infection, using automatic microfluidic setup that captures individual algal cells and multiplex quantitate PCR. We revealed high heterogeneity in viral gene expression among individual cells. Simultaneous measurements of expression profiles of host and virus genes at a single-cell level allowed mapping of infected cells into newly defined infection states and allowed detection specific host response in a subpopulation of infected cell which otherwise masked by the majority of the infected population. Intriguingly, resistant cells emerged during viral infection, showed unique expression profiles of metabolic genes which can provide the basis for discerning between viral resistant and susceptible cells within heterogeneous populations in the marine environment. We propose that resolving host-virus arms race at a single-cell level will provide important mechanistic insights into viral life cycles and will uncover host defense strategies.

Detection of Phagotrophy in the Marine Phytoplankton Group of the Coccolithophores (Calcihaptophycidae, Haptophyta) During Nutrient-replete and Phosphate-limited Growth.

Mixotrophic algae that combine photoautotrophy with phagotrophy in a single cell are prevalent in marine ecosystems. Here, we assessed the ability of food ingestion in coccolithophores, an important group of calcifying haptophytes inhabiting the oceans. We tested four species from different coccolithophore lineages (Emiliania huxleyi, Calcidiscus leptoporus, Coccolithus braarudii, and Calyptrosphaera sphaeroidea). For both E. huxleyi and C. leptoporus we included different life phases (haploid and diploid). For C. braarudii we only tested diploid heterococcolithophore cells, while for C. sphaeroidea we only tested haploid holococcolithophore cells. Phagotrophy was assessed using fluorescently labeled bacteria (FLB) as model prey item, under nutrient-replete and phosphate-limited conditions. We detected by microscopy ingestion of FLB by all species, except the diploid C. braarudii strain. However, a previous study detected ingestion by haploid cells of C. braarudii. These overall results indicate that mixotrophy and the ability to ingest prey is widespread in coccolithophores. Yet, in all tested species the ingestion of FLB was low (<1% of the population contained prey at all time points over 2 days), namely for E. huxleyi and the diploid cells from C. leptoporus where detection of ingestion was sporadic. Moreover, no clear differences were detected between life phases in E. huxleyi and C. leptoporus under equal circumstances, or between replete and limited growth conditions.

Morphological switch to a resistant subpopulation in response to viral infection in the bloom-forming coccolithophore Emiliania huxleyi.

Recognizing the life cycle of an organism is key to understanding its biology and ecological impact. Emiliania huxleyi is a cosmopolitan marine microalga, which displays a poorly understood biphasic sexual life cycle comprised of a calcified diploid phase and a morphologically distinct biflagellate haploid phase. Diploid cells (2N) form large-scale blooms in the oceans, which are routinely terminated by specific lytic viruses (EhV). In contrast, haploid cells (1N) are resistant to EhV. Further evidence indicates that 1N cells may be produced during viral infection. A shift in morphology, driven by meiosis, could therefore constitute a mechanism for E. huxleyi cells to escape from EhV during blooms. This process has been metaphorically coined the 'Cheshire Cat' (CC) strategy. We tested this model in two E. huxleyi strains using a detailed assessment of morphological and ploidy-level variations as well as expression of gene markers for meiosis and the flagellate phenotype. We showed that following the CC model, production of resistant cells was triggered during infection. This led to the rise of a new subpopulation of cells in the two strains that morphologically resembled haploid cells and were resistant to EhV. However, ploidy-level analyses indicated that the new resistant cells were diploid or aneuploid. Thus, the CC strategy in E. huxleyi appears to be a life-phase switch mechanism involving morphological remodeling that is decoupled from meiosis. Our results highlight the adaptive significance of morphological plasticity mediating complex host-virus interactions in marine phytoplankton.

Death-specific protein in a marine diatom regulates photosynthetic responses to iron and light availability.

Diatoms, unicellular phytoplankton that account for ∼40% of marine primary productivity, often dominate coastal and open-ocean upwelling zones. Limitation of growth and productivity by iron at low light is attributed to an elevated cellular Fe requirement for the synthesis of Fe-rich photosynthetic proteins. In the dynamic coastal environment, Fe concentrations and daily surface irradiance levels can vary by two to three orders of magnitude on short spatial and temporal scales. Although genome-wide studies are beginning to provide insight into the molecular mechanisms used by diatoms to rapidly respond to such fluxes, their functional role in mediating the Fe stress response remains uncharacterized. Here, we show, using reverse genetics, that a death-specific protein (DSP; previously named for its apparent association with cell death) in the coastal diatom Thalassiosira pseudonana (TpDSP1) localizes to the plastid and enhances growth during acute Fe limitation at subsaturating light by increasing the photosynthetic efficiency of carbon fixation. Clone lines overexpressing TpDSP1 had a lower quantum requirement for growth, increased levels of photosynthetic and carbon fixation proteins, and increased cyclic electron flow around photosystem I. Cyclic electron flow is an ATP-producing pathway essential in higher plants and chlorophytes with a heretofore unappreciated role in diatoms. However, cells under replete conditions were characterized as having markedly reduced growth and photosynthetic rates at saturating light, thereby constraining the benefits afforded by overexpression. Widespread distribution of DSP-like sequences in environmental metagenomic and metatranscriptomic datasets highlights the presence and relevance of this protein in natural phytoplankton populations in diverse oceanic regimes.

In situ survey of life cycle phases of the coccolithophore Emiliania huxleyi (Haptophyta).

The cosmopolitan coccolithophore Emiliania huxleyi is characterized by a strongly differentiated haplodiplontic life cycle consisting of a diploid phase, generally bearing coccoliths (calcified) but that can be also non-calcified, and a non-calcified biflagellated haploid phase. Given most studies have focused on the bloom-producing calcified phase, there is little-to-no information about non-calcified cells in nature. Using field mesocoms as experimental platforms, we quantitatively surveyed calcified and non-calcified cells using the combined calcareous detection fluorescent in situ hybridization (COD-FISH) method and qualitatively screened for haploid specific transcripts using reverse transcription-PCR during E. huxleyi bloom successions. Diploid, calcified cells formed dense blooms that were followed by the massive proliferation of E. huxleyi viruses (EhVs), which caused bloom demise. Non-calcified cells were also detected throughout the experiment, accounting for a minor fraction of the population but becoming progressively more abundant during mid-late bloom periods concomitant with EhV burst. Non-calcified cell growth also paralleled a distinct window of haploid-specific transcripts and the appearance of autotrophic flagellates morphologically similar to haploid cells, both of which are suggestive of meiosis and sexual life cycling during natural blooms of this prominent marine phytoplankton species.

Extreme diversity in noncalcifying haptophytes explains a major pigment paradox in open oceans.

The current paradigm holds that cyanobacteria, which evolved oxygenic photosynthesis more than 2 billion years ago, are still the major light harvesters driving primary productivity in open oceans. Here we show that tiny unicellular eukaryotes belonging to the photosynthetic lineage of the Haptophyta are dramatically diverse and ecologically dominant in the planktonic photic realm. The use of Haptophyta-specific primers and PCR conditions adapted for GC-rich genomes circumvented biases inherent in classical genetic approaches to exploring environmental eukaryotic biodiversity and led to the discovery of hundreds of unique haptophyte taxa in 5 clone libraries from subpolar and subtropical oceanic waters. Phylogenetic analyses suggest that this diversity emerged in Paleozoic oceans, thrived and diversified in the permanently oxygenated Mesozoic Panthalassa, and currently comprises thousands of ribotypic species, belonging primarily to low-abundance and ancient lineages of the "rare biosphere." This extreme biodiversity coincides with the pervasive presence in the photic zone of the world ocean of 19'-hexanoyloxyfucoxanthin (19-Hex), an accessory photosynthetic pigment found exclusively in chloroplasts of haptophyte origin. Our new estimates of depth-integrated relative abundance of 19-Hex indicate that haptophytes dominate the chlorophyll a-normalized phytoplankton standing stock in modern oceans. Their ecologic and evolutionary success, arguably based on mixotrophy, may have significantly impacted the oceanic carbon pump. These results add to the growing evidence that the evolution of complex microbial eukaryotic cells is a critical force in the functioning of the biosphere.

Phycobilisome light-harvesting efficiency in natural populations of the marine cyanobacteria Synechococcus increases with depth.

Cyanobacteria of the genus Synechococcus play a key role as primary producers and drivers of the global carbon cycle in temperate and tropical oceans. Synechococcus use phycobilisomes as photosynthetic light-harvesting antennas. These contain phycoerythrin, a pigment-protein complex specialized for absorption of blue light, which penetrates deep into open ocean water. As light declines with depth, Synechococcus photo-acclimate by increasing both the density of photosynthetic membranes and the size of the phycobilisomes. This is achieved with the addition of phycoerythrin units, as demonstrated in laboratory studies. In this study, we probed Synechococcus populations in an oligotrophic water column habitat at increasing depths. We observed morphological changes and indications for an increase in phycobilin content with increasing depth, in summer stratified Synechococcus populations. Such an increase in antenna size is expected to come at the expense of decreased energy transfer efficiency through the antenna, since energy has a longer distance to travel. However, using fluorescence lifetime depth profile measurement approach, which is applied here for the first time, we found that light-harvesting quantum efficiency increased with depth in stratified water column. Calculated phycobilisome fluorescence quantum yields were 3.5% at 70 m and 0.7% at 130 m. Under these conditions, where heat dissipation is expected to be constant, lower fluorescence yields correspond to higher photochemical yields. During winter-mixing conditions, Synechococcus present an intermediate state of light harvesting, suggesting an acclimation of cells to the average light regime through the mixing depth (quantum yield of ~2%). Given this photo-acclimation strategy, the primary productivity attributed to marine Synechococcus should be reconsidered.

The "Cheshire Cat" escape strategy of the coccolithophore Emiliania huxleyi in response to viral infection.

The coccolithophore Emiliania huxleyi is one of the most successful eukaryotes in modern oceans. The two phases in its haplodiploid life cycle exhibit radically different phenotypes. The diploid calcified phase forms extensive blooms, which profoundly impact global biogeochemical equilibria. By contrast, the ecological role of the noncalcified haploid phase has been completely overlooked. Giant phycodnaviruses (Emiliania huxleyi viruses, EhVs) have been shown to infect and lyse diploid-phase cells and to be heavily implicated in the regulation of populations and the termination of blooms. Here, we demonstrate that the haploid phase of E. huxleyi is unrecognizable and therefore resistant to EhVs that kill the diploid phase. We further show that exposure of diploid E. huxleyi to EhVs induces transition to the haploid phase. Thus we have clearly demonstrated a drastic difference in viral susceptibility between life cycle stages with different ploidy levels in a unicellular eukaryote. Resistance of the haploid phase of E. huxleyi provides an escape mechanism that involves separation of meiosis from sexual fusion in time, thus ensuring that genes of dominant diploid clones are passed on to the next generation in a virus-free environment. These "Cheshire Cat" ecological dynamics release host evolution from pathogen pressure and thus can be seen as an opposite force to a classic "Red Queen" coevolutionary arms race. In E. huxleyi, this phenomenon can account for the fact that the selective balance is tilted toward the boom-and-bust scenario of optimization of both growth rates of calcifying E. huxleyi cells and infectivity of EhVs.

Dimethyl sulfide mediates microbial predator-prey interactions between zooplankton and algae in the ocean.

Phytoplankton are key components of the oceanic carbon and sulfur cycles1. During bloom events, some species can emit large amounts of the organosulfur volatile dimethyl sulfide (DMS) into the ocean and consequently the atmosphere, where it can modulate aerosol formation and affect climate2,3. In aquatic environments, DMS plays an important role as a chemical signal mediating diverse trophic interactions. Yet, its role in microbial predator-prey interactions remains elusive with contradicting evidence for its role in either algal chemical defence or in the chemo-attraction of grazers to prey cells4,5. Here we investigated the signalling role of DMS during zooplankton-algae interactions by genetic and biochemical manipulation of the algal DMS-generating enzyme dimethylsulfoniopropionate lyase (DL) in the bloom-forming alga Emiliania huxleyi6. We inhibited DL activity in E. huxleyi cells in vivo using the selective DL-inhibitor 2-bromo-3-(dimethylsulfonio)-propionate7 and overexpressed the DL-encoding gene in the model diatom Thalassiosira pseudonana. We showed that algal DL activity did not serve as an anti-grazing chemical defence but paradoxically enhanced predation by the grazer Oxyrrhis marina and other microzooplankton and mesozooplankton, including ciliates and copepods. Consumption of algal prey with induced DL activity also promoted O. marina growth. Overall, our results demonstrate that DMS-mediated grazing may be ecologically important and prevalent during prey-predator dynamics in aquatic ecosystems. The role of algal DMS revealed here, acting as an eat-me signal for grazers, raises fundamental questions regarding the retention of its biosynthetic enzyme through the evolution of dominant bloom-forming phytoplankton in the ocean.

Infection Dynamics of a Bloom-Forming Alga and Its Virus Determine Airborne Coccolith Emission from Seawater.

Sea spray aerosols (SSA), have a profound effect on the climate; however, the contribution of oceanic microbial activity to SSA is not fully established. We assessed aerosolization of the calcite units (coccoliths) that compose the exoskeleton of the cosmopolitan bloom-forming coccolithophore, Emiliania huxleyi. Airborne coccolith emission occurs in steady-state conditions and increases by an order of magnitude during E. huxleyi infection by E. huxleyi virus (EhV). Airborne to seawater coccolith ratio is 1:108, providing estimation of airborne concentrations from seawater concentrations. The coccoliths' unique aerodynamic structure yields a characteristic settling velocity of ∼0.01 cm s-1, ∼25 times slower than average sea salt particles, resulting in coccolith fraction enrichment in the air. The calculated enrichment was established experimentally, indicating that coccoliths may be key contributors to coarse mode SSA surface area, comparable with sea salt aerosols. This study suggests a coupling between key oceanic microbial interactions and fundamental atmospheric processes like SSA formation.

Bacterial virulence against an oceanic bloom-forming phytoplankter is mediated by algal DMSP.

Emiliania huxleyi is a bloom-forming microalga that affects the global sulfur cycle by producing large amounts of dimethylsulfoniopropionate (DMSP) and its volatile metabolic product dimethyl sulfide. Top-down regulation of E. huxleyi blooms has been attributed to viruses and grazers; however, the possible involvement of algicidal bacteria in bloom demise has remained elusive. We demonstrate that a Roseobacter strain, Sulfitobacter D7, that we isolated from a North Atlantic E. huxleyi bloom, exhibited algicidal effects against E. huxleyi upon coculturing. Both the alga and the bacterium were found to co-occur during a natural E. huxleyi bloom, therefore establishing this host-pathogen system as an attractive, ecologically relevant model for studying algal-bacterial interactions in the oceans. During interaction, Sulfitobacter D7 consumed and metabolized algal DMSP to produce high amounts of methanethiol, an alternative product of DMSP catabolism. We revealed a unique strain-specific response, in which E. huxleyi strains that exuded higher amounts of DMSP were more susceptible to Sulfitobacter D7 infection. Intriguingly, exogenous application of DMSP enhanced bacterial virulence and induced susceptibility in an algal strain typically resistant to the bacterial pathogen. This enhanced virulence was highly specific to DMSP compared to addition of propionate and glycerol which had no effect on bacterial virulence. We propose a novel function for DMSP, in addition to its central role in mutualistic interactions among marine organisms, as a mediator of bacterial virulence that may regulate E. huxleyi blooms.

Coccolithovirus facilitation of carbon export in the North Atlantic.

Marine phytoplankton account for approximately half of global primary productivity 1 , making their fate an important driver of the marine carbon cycle. Viruses are thought to recycle more than one-quarter of oceanic photosynthetically fixed organic carbon 2 , which can stimulate nutrient regeneration, primary production and upper ocean respiration 2 via lytic infection and the 'virus shunt'. Ultimately, this limits the trophic transfer of carbon and energy to both higher food webs and the deep ocean 2 . Using imagery taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Aqua satellite, along with a suite of diagnostic lipid- and gene-based molecular biomarkers, in situ optical sensors and sediment traps, we show that Coccolithovirus infections of mesoscale (~100 km) Emiliania huxleyi blooms in the North Atlantic are coupled with particle aggregation, high zooplankton grazing and greater downward vertical fluxes of both particulate organic and particulate inorganic carbon from the upper mixed layer. Our analyses captured blooms in different phases of infection (early, late and post) and revealed the highest export flux in 'early-infected blooms' with sinking particles being disproportionately enriched with infected cells and subsequently remineralized at depth in the mesopelagic. Our findings reveal viral infection as a previously unrecognized ecosystem process enhancing biological pump efficiency.

Algal viruses hitchhiking on zooplankton across phytoplankton blooms.

Viruses infecting marine phytoplankton are key biogeochemical 'engines' of the oceans, regulating the dynamics of algal populations and the fate of their extensive blooms. In addition they are important ecological and evolutionary drivers of microbial diversification. Yet, little is known about mechanisms influencing viral dispersal in aquatic systems, enabling the rapid infection and demise of vast phytoplankton blooms. In a recent study we showed that migrating zooplankton as copepods that graze on marine phytoplankton can act as transmission vectors for algal viruses. We demonstrated that these grazers can concentrate virions through topical adsorption and by ingesting infected cells and then releasing back to the medium, via detachment or defecation, high viral titers that readily infect host populations. We proposed that this zooplankton-driven process can potentially boost viral dispersal over wide oceanic scales and enhance bloom termination. Here, we highlight key results and further discuss the ecological and evolutionary consequences of our findings.

Zooplankton may serve as transmission vectors for viruses infecting algal blooms in the ocean.

Marine viruses are recognized as a major driving force regulating phytoplankton community composition and nutrient cycling in the oceans. Yet, little is known about mechanisms that influence viral dispersal in aquatic systems, other than physical processes, and that lead to the rapid demise of large-scale algal blooms in the oceans. Here, we show that copepods, abundant migrating crustaceans that graze on phytoplankton, as well as other zooplankton can accumulate and mediate the transmission of viruses infecting Emiliania huxleyi, a bloom-forming coccolithophore that plays an important role in the carbon cycle. We detected by PCR that >80% of copepods collected during a North Atlantic E. huxleyi bloom carried E. huxleyi virus (EhV) DNA. We demonstrated by isolating a new infectious EhV strain from a copepod microbiome that these viruses are infectious. We further showed that EhVs can accumulate in high titers within zooplankton guts during feeding or can be adsorbed to their surface. Subsequently, EhV can be dispersed by detachment or via viral-dense fecal pellets over a period of 1 day postfeeding on EhV-infected algal cells, readily infecting new host populations. Intriguingly, the passage through zooplankton guts prolonged EhV's half-life of infectivity by 35%, relative to free virions in seawater, potentially enhancing viral transmission. We propose that zooplankton, swimming through topographically adjacent phytoplankton micropatches and migrating daily over large areas across physically separated water masses, can serve as viral vectors, boosting host-virus contact rates and potentially accelerating the demise of large-scale phytoplankton blooms.

Decoupling physical from biological processes to assess the impact of viruses on a mesoscale algal bloom.

Phytoplankton blooms are ephemeral events of exceptionally high primary productivity that regulate the flux of carbon across marine food webs [1-3]. Quantification of bloom turnover [4] is limited by a fundamental difficulty to decouple between physical and biological processes as observed by ocean color satellite data. This limitation hinders the quantification of bloom demise and its regulation by biological processes [5, 6], which has important consequences on the efficiency of the biological pump of carbon to the deep ocean [7-9]. Here, we address this challenge and quantify algal blooms' turnover using a combination of satellite and in situ data, which allows identification of a relatively stable oceanic patch that is subject to little mixing with its surroundings. Using a newly developed multisatellite Lagrangian diagnostic, we decouple the contributions of physical and biological processes, allowing quantification of a complete life cycle of a mesoscale (∼10-100 km) bloom of coccolithophores in the North Atlantic, from exponential growth to its rapid demise. We estimate the amount of organic carbon produced during the bloom to be in the order of 24,000 tons, of which two-thirds were turned over within 1 week. Complimentary in situ measurements of the same patch area revealed high levels of specific viruses infecting coccolithophore cells, therefore pointing at the importance of viral infection as a possible mortality agent. Application of the newly developed satellite-based approaches opens the way for large-scale quantification of the impact of diverse environmental stresses on the fate of phytoplankton blooms and derived carbon in the ocean.

Quantum requirements for growth and fatty acid biosynthesis in the marine diatom Phaeodactylum tricornutum (Bacillariophyceae) in nitrogen replete and limited conditions.

We determined the quantum requirements for growth (1/ϕµ ) and fatty acid (FA) biosynthesis (1/ϕFA ) in the marine diatom, Phaeodactylum tricornutum, grown in nutrient replete conditions with nitrate or ammonium as nitrogen sources, and under nitrogen limitation, achieved by transferring cells into nitrogen free medium or by inhibiting nitrate assimilation with tungstate. A treatment in which tungstate was supplemented to cells grown with ammonium was also included. In nutrient replete conditions, cells grew exponentially and possessed virtually identical 1/ϕµ of 40-44 mol photons · mol C(-1) . In parallel, 1/ϕFA varied between 380 and 409 mol photons · mol C(-1) in the presence of nitrate, but declined to 348 mol photons · mol C(-1) with ammonium and to 250 mol photons · mol C(-1) with ammonium plus tungstate, indicating an increase in the efficiency of FA biosynthesis relative to cells grown on nitrate of 8% and 35%, respectively. While the molecular mechanism for this effect remains poorly understood, the results unambiguously reveal that cells grown on ammonium are able to direct more reductant to lipids. This analysis suggests that when cells are grown with a reduced nitrogen source, fatty acid biosynthesis can effectively become a sink for excess absorbed light, compensating for the absence of energetically demanding nitrate assimilation reactions. Our data further suggest that optimal lipid production efficiency is achieved when cells are in exponential growth, when nitrate assimilation is inhibited, and ammonium is the sole nitrogen source.