Photosystem II protein clearance and FtsH function in the diatom Thalassiosira pseudonana

Elevated CO2 modulates the physiological responses of Thalassiosira pseudonana to ultraviolet radiation

Diatoms account for a large proportion of marine primary productivity, they tend to be the predominant species in the phytoplankton communities in the surface ocean with frequent and large light fluctuations. To understand the impacts of increased CO2 on diatoms' capacity in exploitation of variable solar radiation, we cultured a model diatom Thalassiosira pseudonana with 400 or 1000ppmv CO2 and exposed it to high photosynthetically active radiation (PAR) alone or PAR plus ultraviolet radiation (UVR) to examine its physiological performances. The results showed that the maximum photochemical efficiency (Fv/fm) was significantly reduced by high PAR and PAR + UVR in T. pseudonana, UVR-induced inhibition on PSII activity was exacerbated by high CO2. PSII activity drops coincide approximately with PsbA content in the cells exposed to high PAR or PAR + UVR, which was pronounced at high CO2. The removal of PsbD in T. pseudonana cells declined under high CO2 during UVR exposure, limiting the repair capacity of PSII. In addition, high CO2 reversed the induction of energy-dependent form of NPQ by UVR to the increase of Y(No), indicating the severe damage of the photoprotective reactions. Our findings suggest that the adverse impacts of UVR on PSII function of T. pseudonana were aggravated by the elevated CO2 through modulating its capacity in repair and protection, which thereby would influence its abundance and competitiveness in phytoplankton communities.

Expression of nuclear-encoded, haptophyte-derived ftsH genes support extremely rapid PSII repair and high-light photoacclimation in Karenia brevis (Dinophyceae)

Karenia brevis, a neurotoxic dinoflagellate that produces brevetoxins, is endemic to the Gulf of Mexico and can grow at high irradiances typical of surface waters found there. To build upon a growing number of studies addressing high-light tolerance in K. brevis, specific photobiology and molecular mechanisms underlying this capacity were evaluated in culture. Since photosystem II (PSII) repair cycle activity can be crucial to high light tolerance in plants and algae, the present study assessed this capacity in K. brevis and characterized the ftsH-like genes which are fundamental to this process. Compared with cultures grown in low-light, cultures grown in high-light showed a 65-fold increase in PSII photoinactivation, a ∼50-fold increase in PSII repair, enhanced nonphotochemical quenching (NPQ), and depressed F_v/F_m. Repair rates were among the fastest reported in phytoplankton. Publicly available K. brevis transcriptomes (MMETSP) were queried for ftsH-like sequences and refined with additional sequencing from two K. brevis strains. The genes were phylogenetically related to haptophyte orthologs, implicating acquisition during tertiary endosymbiosis. RT-qPCR of three of the four ftsH-like homologs revealed that poly-A tails predominated in all homologs, and that the most highly expressed homolog had a 5' splice leader and amino-acid motifs characteristic of chloroplast targeting, indicating nuclear encoding for this plastid-targeted gene. High-light cultures showed a ∼1.5-fold upregulation in mRNA expression of the thylakoid-associated genes. Overall, in conjunction with NPQ mechanisms, rapid PSII repair mediated by a haptophyte-derived ftsH prevents chronic photoinhibition in K. brevis. Our findings continue to build the case that high-light photobiology-supported by the acquisition and maintenance of tertiary endosymbiotic genes-is critical to the success of K. brevis in the Gulf of Mexico.

The inhibitory effects of the antifouling compound Irgarol 1051 on the marine diatom Skeletonema sp. across a broad range of photosynthetically active radiation

The release of anthropogenic organic pollutants has resulted in extensive environmental risks to coastal waters. Among pollutants released, the most common antifoulant, Irgarol 1051, is an effective inhibitor of photosystem II of photoautotrophs; thus, the continuous release of this compound into surrounding seawater would potentially threaten marine algae. To investigate this, we grew the model marine diatom Skeletonema sp. at different concentrations of Irgarol 1051 and levels of photosynthetically active radiation (PAR). Irgarol did not affect the photochemical capacity when cells were incubated in the dark, but photochemical yields all significantly decreased, and relative inhibition by Irgarol increased once cells were exposed to even the lowest PAR, with lower photochemical yields observed under increased level of Irgarol. In addition, the rate of decrease in yield increased with Irgarol concentration but was unchanged among PAR treatments. The growth rates showed a similar pattern to photochemical yields, with lower values under higher Irgarol concentrations, but with no significant differences in the effect of Irgarol observed between the light levels employed. The ratio of repair to damage rates of PSII clearly shows that this ratio decreased with light intensity, largely due to increases in damage rates and that the PAR level at which repair balanced damage decreased under a high level of Irgarol. Our results suggest that the inhibitory effects of Irgarol become obvious after PAR exposure even at a relatively low light level, suggesting that Irgarol would affect phytoplankton throughout the daytime, and may therefore have a broad environmental risk, potentially limiting coastal primary production.

Restoration, conservation and phytoplankton hysteresis

Phytoplankton growth depends not only upon external factors that are not strongly altered by the presence of phytoplankton, such as temperature, but also upon factors that are strongly influenced by activity of phytoplankton, including photosynthetically active radiation, and the availability of the macronutrients carbon, nitrogen, phosphorus and, for some, silicate. Since phytoplankton therefore modify, and to an extent create, their own habitats, established phytoplankton communities can show resistance and resilience to change, including managed changes in nutrient regimes. Phytoplankton blooms and community structures can be predicted from the overall biogeochemical setting and inputs, but restorations may be influenced by the physiological responses of established phytoplankton taxa to nutrient inputs, temperature, second-order changes in illumination and nutrient recycling. In this review we discuss the contributions of phytoplankton ecophysiology to biogeochemical hysteresis and possible effects on community composition in the face of management, conservation or remediation plans.

Genome-scale metabolic model of the diatom Thalassiosira pseudonana highlights the importance of nitrogen and sulfur metabolism in redox balance

Diatoms are unicellular photosynthetic algae known to secrete organic matter that fuels secondary production in the ocean, though our knowledge of how their physiology impacts the composition of dissolved organic matter remains limited. Like all photosynthetic organisms, their use of light for energy and reducing power creates the challenge of avoiding cellular damage. To better understand the interplay between redox balance and organic matter secretion, we reconstructed a genome-scale metabolic model of Thalassiosira pseudonana strain CCMP 1335, a model for diatom molecular biology and physiology, with a 60-year history of studies. The model simulates the metabolic activities of 1,432 genes via a network of 2,792 metabolites produced through 6,079 reactions distributed across six subcellular compartments. Growth was simulated under different steady-state light conditions (5–200 μmol photons m^-2 s^-1) and in a batch culture progressing from exponential growth to nitrate-limitation and nitrogen-starvation. We used the model to examine the dissipation of reductants generated through light-dependent processes and found that when available, nitrate assimilation is an important means of dissipating reductants in the plastid; under nitrate-limiting conditions, sulfate assimilation plays a similar role. The use of either nitrate or sulfate uptake to balance redox reactions leads to the secretion of distinct organic nitrogen and sulfur compounds. Such compounds can be accessed by bacteria in the surface ocean. The model of the diatom Thalassiosira pseudonana provides a mechanistic explanation for the production of ecologically and climatologically relevant compounds that may serve as the basis for intricate, cross-kingdom microbial networks. Diatom metabolism has an important influence on global biogeochemistry; metabolic models of marine microorganisms link genes to ecosystems and may be key to integrating molecular data with models of ocean biogeochemistry.

Divergence of photosynthetic strategies amongst marine diatoms

Marine phytoplankton, and in particular diatoms, are responsible for almost half of all primary production on Earth. Diatom species thrive from polar to tropical waters and across light environments that are highly complex to relatively benign, and so have evolved highly divergent strategies for regulating light capture and utilization. It is increasingly well established that diatoms have achieved such successful ecosystem dominance by regulating excitation energy available for generating photosynthetic energy via highly flexible light harvesting strategies. However, how different light harvesting strategies and downstream pathways for oxygen production and consumption interact to balance excitation pressure remains unknown. We therefore examined the responses of three diatom taxa adapted to inherently different light climates (estuarine Thalassioisira weissflogii, coastal Thalassiosira pseudonana and oceanic Thalassiosira oceanica) during transient shifts from a moderate to high growth irradiance (85 to 1200 μmol photons m^-2 s^-1). Transient high light exposure caused T. weissflogii to rapidly downregulate PSII with substantial nonphotochemical quenching, protecting PSII from inactivation or damage, and obviating the need for induction of O₂ consuming (light-dependent respiration, LDR) pathways. In contrast, T. oceanica retained high excitation pressure on PSII, but with little change in RCII photochemical turnover, thereby requiring moderate repair activity and greater reliance on LDR. T. pseudonana exhibited an intermediate response compared to the other two diatom species, exhibiting some downregulation and inactivation of PSII, but high repair of PSII and induction of reversible PSII nonphotochemical quenching, with some LDR. Together, these data demonstrate a range of strategies for balancing light harvesting and utilization across diatom species, which reflect their adaptation to sustain photosynthesis under environments with inherently different light regimes.

Rising atmospheric CO2 levels result in an earlier cyanobacterial bloom-maintenance phase with higher algal biomass

The effect of rising atmospheric CO₂ on freshwater lakes is a subject of considerable debate. However, it is not clear how rising CO₂ concentration affects cyanobacterial bloom development under potential nutrient limitation conditions and if CO₂ should be taken into account in making nutrient reduction strategy. To fill the knowledge gaps, this study investigated the spatiotemporal variability in aquatic CO₂ concentration (pCO₂) from 2006 to 2016 in Lake Taihu, where cyanobacterial blooms often occurred from late spring to the early fall. Lake Taihu is an atmospheric CO₂ source in May and November, with only 18% and 11% pCO₂-undersaturated areas, respectively. During cyanobacterial bloom in August, 81% of the lake areas are pCO₂-undersaturated, absorbing ~ 0.53 t C/h of atmospheric CO₂. The results demonstrated that CO₂ transfer across air-water interface was important in supporting cyanobacterial bloom development. Besides, Field investigation showed that the chlorophyll a level is significantly positively correlated with supersaturated pCO₂ (>13.56 µmol/L) in May, but pCO₂ decreases with high chlorophyll a levels in August, suggesting that cyanobacterial growth would be promoted by high pCO₂ over a threshold. These observations suggested that the effect of rising atmospheric CO₂ on freshwater lakes and cyanobacterial blooms should be paid attention to. Further, when the N- and P-levels are >0.3 mg/L and >0.02 mg/L, respectively, high-pCO₂ conditions allow a more rapid growth rate of cyanobacteria via improved nutrient-use efficiency. Moreover, cyanobacteria afford maximum N- or P-use efficiency at lower N- or P-concentrations with high CO₂ concentration. This improvement would result in an earlier bloom-maintenance phase and higher cyanobacterial biomass. In this case, nutrient reduction is more imperative under future high CO₂ conditions.

Transcriptomic reprogramming of the oceanic diatom Skeletonema dohrnii under warming ocean and acidification

Under ocean warming and acidification, diatoms use a unique acclimation and adaptation strategy by saving energy and utilizing it for other cellular processes. However, the molecular mechanisms that underlie this reprogramming of energy utilization are currently unknown. Here, we investigate the metabolic reprogramming of the ecologically important diatom Skeletonema dohrnii grown under two different temperature (21°C and 25°C) and pCO₂ (400 and 1000 ppm) levels, utilizing global transcriptomic analysis. We find that evolutionary changes in the baseline gene expression, which we termed transcriptional up- and downregulation, is the primary mechanism used by diatoms to acclimate to the combined conditions of ocean warming and acidification. This transcriptional regulation shows that under higher temperature and pCO₂ conditions, photosynthesis, electron transport and carboxylation were modified with increasing abundances of genes encoding ATP, NADPH and carbon gaining for the carbon-dioxide-concentrating mechanisms (CCMs). Our results also indicate that changes in the transcriptional regulation of CCMs led to a decrease in the metabolic cost to save energy by promoting amino acid synthesis and nitrogen assimilation for the active protein processing machinery to adapt to warming and ocean acidification. This study generated unique metabolic insights into diatoms and suggests that future climate change conditions will cause evolutionary changes in oceanic diatoms that will facilitate their acclimation strategy.

Strain specific differences in rates of Photosystem II repair in picocyanobacteria correlate to differences in FtsH protein levels and isoform expression patterns

Picocyanobacteria are the numerically dominant photoautotrophs of the oligotrophic regions of Earth’s oceans. These organisms are characterized by their small size and highly reduced genomes. Strains partition to different light intensity and nutrient level niches, with differing photosynthetic apparatus stoichiometry, light harvesting machinery and susceptibility to photoinactivation. In this study, we grew three strains of picocyanobacteria: the low light, high nutrient strain Prochlorococcus marinus MIT 9313; the high light, low nutrient Prochlorococcus marinus MED 4; and the high light, high nutrient marine Synechococcus strain WH 8102; under low and high growth light levels. We then performed matched photophysiology, protein and transcript analyses. The strains differ significantly in their rates of Photosystem II repair under high light and in their capacity to remove the PsbA protein as the first step in the Photosystem II repair process. Notably, all strains remove the PsbD subunit at the same rate that they remove PsbA. When grown under low light, MIT 9313 loses active Photosystem II quickly when shifted to high light, but has no measurable capacity to remove PsbA. MED 4 and WH 8102 show less rapid loss of Photosystem II and considerable capacity to remove PsbA. MIT 9313 has less of the FtsH protease thought to be responsible for the removal of PsbA in other cyanobacteria. Furthermore, by transcript analysis the predominant FtsH isoform expressed in MIT 9313 is homologous to the FtsH 4 isoform characterized in the model strain Synechocystis PCC 6803, rather than the FtsH 2 and 3 isoforms thought to be responsible for PsbA degradation. MED 4 on the other hand shows high light inducible expression of the isoforms homologous to FtsH 2 and 3, consistent with its faster rate of PsbA removal. MIT 9313 has adapted to its low light environment by diverting resources away from Photosystem II content and repair.

Sexual ancestors generated an obligate asexual and globally dispersed clone within the model diatom species Thalassiosira pseudonana

Sexual reproduction roots the eukaryotic tree of life, although its loss occurs across diverse taxa. Asexual reproduction and clonal lineages persist in these taxa despite theoretical arguments suggesting that individual clones should be evolutionarily short-lived due to limited phenotypic diversity. Here, we present quantitative evidence that an obligate asexual lineage emerged from a sexual population of the marine diatom Thalassiosira pseudonana and rapidly expanded throughout the world’s oceans. Whole genome comparisons identified two lineages with characteristics expected of sexually reproducing strains in Hardy-Weinberg equilibrium. A third lineage displays genomic signatures for the functional loss of sexual reproduction followed by a recent global colonization by a single ancestral genotype. Extant members of this lineage are genetically differentiated and phenotypically plastic, potentially allowing for rapid adaptation when they are challenged by natural selection. Such mechanisms may be expected to generate new clones within marginal populations of additional unicellular species, facilitating the exploration and colonization of novel environments, aided by exponential growth and ease of dispersal.

Changes in the fucoxanthin production and protein profiles in Cylindrotheca closterium in response to blue light-emitting diode light

BACKGROUND

Marine diatoms have a higher fucoxanthin content in comparison to macroalgae. Fucoxanthin features many potent bioactive properties, particularly anti-obesity properties. Despite the great potential for harvesting larger amounts of fucoxanthin, the impacts of light quality (light source, intensity, and photoperiod) on fucoxanthin production and the essential proteins involved in fucoxanthin biosynthesis in marine diatoms remain unclear.

RESULTS

In the present study, Cylindrotheca closterium was selected from four different species of diatoms based on its high fucoxanthin content and productivity. Optimal light conditions (light source, intensity, and regime) were determined by a "Design of Experiment" approach (software MODDE Pro 11 was used). The model indicated that an 18/6 light/darkness regime increased fucoxanthin productivity remarkably as opposed to a 12/12 or 24/0 regime. Eventually, blue light-emitting diode light, as an alternative to fluorescent light, at 100 μmol/m2/s and 18/6 light/darkness regime yielded maximum fucoxanthin productivity and minimal energy consumption. The fucoxanthin production of C. closterium under the predicted optimal light conditions was assessed both in bottle and bag photobioreactors (PBRs). The high fucoxanthin content (25.5 mg/g) obtained from bag PBRs demonstrated the feasibility of large-scale production. The proteomes of C. closterium under the most favorable and unfavorable fucoxanthin biosynthesis light/darkness regimes (18/6 and 24/0, respectively) were compared to identify the essential proteins associated with fucoxanthin accumulation by matrix-assisted laser desorption/ionization-time of flight-mass spectrometry. Six proteins that were up-regulated in the 18/6 regime but down-regulated in the 24/0 were identified as important chloroplastic proteins involved in photosynthesis, energy metabolism, and cellular processes.

CONCLUSIONS

Blue light-emitting diode light at 100 μmol/m2/s and 18/6 light/darkness regime induced maximum fucoxanthin productivity in C. closterium and minimized energy consumption. The high fucoxanthin production in the bag photobioreactor under optimal light conditions demonstrated the possibility of commercialization. Proteomics suggests that fucoxanthin biosynthesis is intimately associated with the photosynthetic efficiency of the diatom, providing another technical and bioengineering outlook on fucoxanthin enhancement.

Combined effects of ocean acidification and warming on physiological response of the diatom Thalassiosira pseudonana to light challenges

Diatoms are one of the most important groups of phytoplankton in terms of abundance and ecological functionality in the ocean. They usually dominate the phytoplankton communities in coastal waters and experience frequent and large fluctuations in light. In order to evaluate the combined effects of ocean warming and acidification on the diatom's exploitation of variable light environments, we grew a globally abundant diatom Thalassiosira pseudonana under two levels of temperature (18, 24 °C) and pCO₂ (400, 1000 μatm) to examine its physiological performance after light challenge. It showed that the higher temperature increased the photoinactivation rate in T. pseudonana at 400 μatm pCO₂, while the higher pCO₂ alleviated the negative effect of the higher temperature on PSII photoinactivation. Higher pCO₂ stimulated much faster PsbA removal, but it still lagged behind the photoinactivation of PSII under high light. Although the sustained phase of nonphotochemical quenching (NPQs) and activity of superoxide dismutase (SOD) were provoked during the high light exposure in T. pseudonana under the combined pCO₂ and temperature conditions, it could not offset the damage caused by these multiple environmental changes, leading to decreased maximum photochemical yield.

Diatom growth responses to photoperiod and light are predictable from diel reductant generation

Light drives phytoplankton productivity, so phytoplankton must exploit variable intensities and durations of light exposure, depending upon season, latitude, and depth. We analyzed the growth, photophysiology and composition of small, Thalassiosira pseudonana, and large, Thalassiosira punctigera, centric diatoms from temperate, coastal marine habitats, responding to a matrix of photoperiods and growth light intensities. T. pseudonana showed fastest growth rates under long photoperiods and low to moderate light intensities, while the larger T. punctigera showed fastest growth rates under short photoperiods and higher light intensities. Photosystem II function and content responded primarily to instantaneous growth light intensities during the photoperiod, while diel carbon fixation and RUBISCO content responded more to photoperiod duration than to instantaneous light intensity. Changing photoperiods caused species-specific changes in the responses of photochemical yield (e^- /photon) to growth light intensity. These photophysiological variables showed complex responses to photoperiod and to growth light intensity. Growth rate also showed complex responses to photoperiod and growth light intensity. But these complex responses resolved into a close relation between growth rate and the cumulative daily generation of reductant, across the matrix of photoperiods and light intensities.

Arctic Micromonas uses protein pools and non-photochemical quenching to cope with temperature restrictions on Photosystem II protein turnover

Micromonas strains of small prasinophyte green algae are found throughout the world's oceans, exploiting widely different niches. We grew arctic and temperate strains of Micromonas and compared their susceptibilities to photoinactivation of Photosystem II, their counteracting Photosystem II repair capacities, their Photosystem II content, and their induction and relaxation of non-photochemical quenching. In the arctic strain Micromonas NCMA 2099, the cellular content of active Photosystem II represents only about 50 % of total Photosystem II protein, as a slow rate constant for clearance of PsbA protein limits instantaneous repair. In contrast, the temperate strain NCMA 1646 shows a faster clearance of PsbA protein which allows it to maintain active Photosystem II content equivalent to total Photosystem II protein. Under growth at 2 °C, the arctic Micromonas maintains a constitutive induction of xanthophyll deepoxidation, shown by second-derivative whole-cell spectra, which supports strong induction of non-photochemical quenching under low to moderate light, even if xanthophyll cycling is blocked. This non-photochemical quenching, however, relaxes during subsequent darkness with kinetics nearly comparable to the temperate Micromonas NCMA 1646, thereby limiting the opportunity cost of sustained downregulation of PSII function after a decrease in light.

Photoinactivation of Photosystem II in Prochlorococcus and Synechococcus

The marine picocyanobacteria Synechococcus and Prochlorococcus numerically dominate open ocean phytoplankton. Although evolutionarily related they are ecologically distinct, with different strategies to harvest, manage and exploit light. We grew representative strains of Synechococcus and Prochlorococcus and tracked their susceptibility to photoinactivation of Photosystem II under a range of light levels. As expected blue light provoked more rapid photoinactivation than did an equivalent level of red light. The previous growth light level altered the susceptibility of Synechococcus, but not Prochlorococcus, to this photoinactivation. We resolved a simple linear pattern when we expressed the yield of photoinactivation on the basis of photons delivered to Photosystem II photochemistry, plotted versus excitation pressure upon Photosystem II, the balance between excitation and downstream metabolism. A high excitation pressure increases the generation of reactive oxygen species, and thus increases the yield of photoinactivation of Photosystem II. Blue photons, however, retained a higher baseline photoinactivation across a wide range of excitation pressures. Our experiments thus uncovered the relative influences of the direct photoinactivation of Photosystem II by blue photons which dominates under low to moderate blue light, and photoinactivation as a side effect of reactive oxygen species which dominates under higher excitation pressure. Synechococcus enjoyed a positive metabolic return upon the repair or the synthesis of a Photosystem II, across the range of light levels we tested. In contrast Prochlorococcus only enjoyed a positive return upon synthesis of a Photosystem II up to 400 μmol photons m-2 s-1. These differential cost-benefits probably underlie the distinct photoacclimation strategies of the species.

Multiple roles of oxygen in the photoinactivation and dynamic repair of Photosystem II in spinach leaves

Oxygen effects have long been ambiguous: exacerbating, being indifferent to, or ameliorating the net photoinactivation of Photosystem II (PS II). We scrutinized the time course of PS II photoinactivation (characterized by rate coefficient k i) in the absence of repair, or when recovery (characterized by k r) occurred simultaneously in CO2 ± O2. Oxygen exacerbated photoinactivation per se, but alleviated it by mediating the utilization of electrons. With repair permitted, the gradual net loss of functional PS II during illumination of leaves was better described phenomenologically by introducing τ, the time for an initial k r to decrease by half. At 1500 μmol photons m(-2) s(-1), oxygen decreased the initial k r but increased τ. Similarly, at even higher irradiance in air, there was a further decrease in the initial k r and increase in τ. These observations are consistent with an empirical model that (1) oxygen increased k i via oxidative stress but decreased it by mediating the utilization of electrons; and (2) reactive oxygen species stimulated the degradation of photodamaged D1 protein in PS II (characterized by k d), but inhibited the de novo synthesis of D1 (characterized by k s), and that the balance between these effects determines the net effect of O2 on PS II functionality.

Photosystem II repair in marine diatoms with contrasting photophysiologies

Skeletonema costatum and Phaeodactylum tricornutum are model marine diatoms with differing strategies for non-photochemical dissipation of excess excitation energy within photosystem II (PSII). We showed that S. costatum, with connectivity across the pigment bed serving PSII, and limited capacity for induction of sustained non-photochemical quenching (NPQ), maintained a large ratio of [PSII(Total)]/[PSII(Active)] to buffer against fluctuations in light intensity. In contrast, P. tricornutum, with a larger capacity to induce sustained NPQ, could maintain a lower [PSII(Total)]/[PSII(Active)]. Induction of NPQ was correlated with an active PSII repair cycle in both species, and inhibition of chloroplastic protein synthesis with lincomycin leads to run away over-excitation of remaining PSII(Active), particularly in S. costatum. We discuss these distinctions in relation to the differing capacities, induction and relaxation rates for NPQ, and as strain adaptations to the differential light regimes of their originating habitats. The present work further confirms the important role for the light-dependent fast regulation of photochemistry by NPQ interacting with PSII repair cycle capacity in the ecophysiology of both pennate and centric diatoms.

On the use of the serial dilution culture method to enumerate viable phytoplankton in natural communities of plankton subjected to ballast water treatment

Discharge standards for ballast water treatment (BWT) systems are based on concentrations of living cells, for example, as determined with vital stains. Ultraviolet radiation (UV) stops the reproduction of microorganisms without killing them outright; they are living, but not viable, and ecologically as good as dead. Consequently, UV-treated discharge can be compliant with the intent of regulation while failing a live/dead test. An alternative evaluation of BWT can be proposed based on the assessment of viable, rather than living, cells in discharge water. In principle, the serial dilution culture-most probable number (SDC-MPN) method provides the appropriate measure for phytoplankton. But, the method has been criticized, particularly because it is thought that many phytoplankton species cannot be cultured. A review of the literature shows that although SDC-MPN has been used for more than 50 years-generally to identify and count phytoplankton species that cannot be preserved-its application to enumerate total viable phytoplankton seems to be new, putting past criticisms of the method in a different light. Importantly, viable cells need to grow only enough to be detected, not to be brought into sustained culture, and competition between species in a dilution tube is irrelevant as long as the winner is detectable. Thorough consideration of sources of error leads to recommendations for minimizing and quantifying uncertainties by optimizing growth conditions and conducting systematic comparisons. We conclude that with careful evaluation, SDC-MPN is potentially an effective method for assessing the viability of phytoplankton after BWT.

The nitrogen costs of photosynthesis in a diatom under current and future pCO2

With each cellular generation, oxygenic photoautotrophs must accumulate abundant protein complexes that mediate light capture, photosynthetic electron transport and carbon fixation. In addition to this net synthesis, oxygenic photoautotrophs must counter the light-dependent photoinactivation of Photosystem II (PSII), using metabolically expensive proteolysis, disassembly, resynthesis and re-assembly of protein subunits. We used growth rates, elemental analyses and protein quantitations to estimate the nitrogen (N) metabolism costs to both accumulate the photosynthetic system and to maintain PSII function in the diatom Thalassiosira pseudonana, growing at two pCO2 levels across a range of light levels. The photosynthetic system contains c. 15-25% of total cellular N. Under low growth light, N (re)cycling through PSII repair is only c. 1% of the cellular N assimilation rate. As growth light increases to inhibitory levels, N metabolite cycling through PSII repair increases to c. 14% of the cellular N assimilation rate. Cells growing under the assumed future 750 ppmv pCO2 show higher growth rates under optimal light, coinciding with a lowered N metabolic cost to maintain photosynthesis, but then suffer greater photoinhibition of growth under excess light, coincident with rising costs to maintain photosynthesis. We predict this quantitative trait response to light will vary across taxa.

System responses to equal doses of photosynthetically usable radiation of blue, green, and red light in the marine diatom Phaeodactylum tricornutum

Due to the selective attenuation of solar light and the absorption properties of seawater and seawater constituents, free-floating photosynthetic organisms have to cope with rapid and unpredictable changes in both intensity and spectral quality. We have studied the transcriptional, metabolic and photo-physiological responses to light of different spectral quality in the marine diatom Phaeodactylum tricornutum through time-series studies of cultures exposed to equal doses of photosynthetically usable radiation of blue, green and red light. The experiments showed that short-term differences in gene expression and profiles are mainly light quality-dependent. Transcription of photosynthesis-associated nuclear genes was activated mainly through a light quality-independent mechanism likely to rely on chloroplast-to-nucleus signaling. In contrast, genes encoding proteins important for photoprotection and PSII repair were highly dependent on a blue light receptor-mediated signal. Changes in energy transfer efficiency by light-harvesting pigments were spectrally dependent; furthermore, a declining trend in photosynthetic efficiency was observed in red light. The combined results suggest that diatoms possess a light quality-dependent ability to activate photoprotection and efficient repair of photodamaged PSII. In spite of approximately equal numbers of PSII-absorbed quanta in blue, green and red light, the spectral quality of light is important for diatom responses to ambient light conditions.

Photophysiological responses of marine diatoms to elevated CO2 and decreased pH: a review

Diatoms dominate nearly half of current oceanic productivity, so their responses to ocean acidification are of general concern regarding future oceanic carbon sequestration. Community, mesocosm and laboratory studies show a range of diatom growth and photophysiological responses to increasing pCO2. Nearly 20 studies on effects of elevated pCO2 on diatoms have shown stimulations, no effects or inhibitions of growth rates. These differential responses could result from differences in experimental setups, cell densities, levels of light and temperature, but also from taxon-specific physiology. Generally, ocean acidification treatments of lowered pH with elevated CO2 stimulate diatom growth under low to moderate levels of light, but lead to growth inhibition when combined with excess light. Additionally, diatom cell sizes and their co-varying metabolic rates can influence responses to increasing pCO2 and decreasing pH, although cell size effects are confounded with taxonomic specificities in cell structures and metabolism. Here we summarise known diatom growth and photophysiological responses to increasing pCO2 and decreasing pH, and discuss some reasons for the diverse responses observed across studies.

Plasticity in the proteome of Emiliania huxleyi CCMP 1516 to extremes of light is highly targeted

Optimality principles are often applied in theoretical studies of microalgal ecophysiology to predict changes in allocation of resources to different metabolic pathways, and optimal acclimation is likely to involve changes in the proteome, which typically accounts for > 50% of cellular nitrogen (N). We tested the hypothesis that acclimation of the microalga Emiliania huxleyi CCMP 1516 to suboptimal vs supraoptimal light involves large changes in the proteome as cells rebalance the capacities to absorb light, fix CO2 , perform biosynthesis and resist photooxidative stress. Emiliania huxleyi was grown in nutrient-replete continuous culture at 30 (LL) and 1000 μmol photons m(-2) s(-1) (HL), and changes in the proteome were assessed by LC-MS/MS shotgun proteomics. Changes were most evident in proteins involved in the light reactions of photosynthesis; the relative abundance of photosystem I (PSI) and PSII proteins was 70% greater in LL, light-harvesting fucoxanthin-chlorophyll proteins (Lhcfs) were up to 500% greater in LL and photoprotective LI818 proteins were 300% greater in HL. The marked changes in the abundances of Lhcfs and LI818s, together with the limited plasticity in the bulk of the E. huxleyi proteome, probably reflect evolutionary pressures to provide energy to maintain metabolic capabilities in stochastic light environments encountered by this species in nature.