Μmol Photons Research Articles

Effects of Temperature and Light on Microalgal Growth and Nutrient Removal in Turtle Aquaculture Wastewater

The aim of this study was to explore the effects of temperature and light on microalgal growth and nutrient removal in turtle aquaculture wastewater using a single-factor experiment method. Results showed that the growth process of Desmodesmus sp. CHX1 in turtle aquaculture wastewater exhibited three stages, namely adaptation, logarithmic, and stable periods. Temperature and light significantly influenced the growth and protein and lipid accumulation of Desmodesmus sp. CHX1. The optimal conditions for the growth and biomass accumulation of Desmodesmus sp. CHX1 included a temperature of 30 °C, a photoperiod of 24L:0D, and a light intensity of 180 μmol photon/(m2·s). Increased temperature, photoperiod, and light intensity enhanced nutrient removal efficiency. Maximum nitrogen removal was achieved at a temperature of 30 °C, a photoperiod of 24L:0D, and a light intensity of 180 μmol photon/(m2·s), with the removal efficiency of 86.53%, 97.94%, 99.57%, and 99.15% for ammonia, nitrate, nitrite, and total phosphorus (TP), respectively. Temperature did not significantly affect TP removal, but increased photoperiod and light intensity improved the removal efficiency of TP. The development of microalgae biomass as a feed rich in protein and lipids could address feed shortages and meet the nutritional needs of turtles, offering a feasible solution for large-scale production.

Distinct responses of diatom- and flagellate-dominated Antarctic phytoplankton communities to altered iron and light supply

Primary production in the Southern Ocean is strongly influenced by the availability of light and iron (Fe). To examine the response of two distinct natural Antarctic phytoplankton communities (diatom vs. flagellates) to increasing light and Fe availability, we conducted two shipboard incubation experiments during late summer and exposed each community to increasing light intensities (30, 80, and 150 µmol photons m−2 s−1) with or without Fe amendment. Our results show clearly that both communities were Fe-limited since Fe addition resulted in higher particulate organic carbon (POC) production rates. The magnitude of the Fe-dependent increase in POC production, however, varied between the two stations being higher in the diatom-dominated community relative to the flagellate-dominated community. This differential response to increasing Fe supply could be attributed to the higher Fe requirement of the flagellate-dominated assemblage relative to the diatom-dominated assemblage. Irrespective of Fe availability, light also strongly stimulated the POC production of both communities between low and medium light supply (30 versus 80 µmol photons m−2 s−1), indicating that both assemblages were light-limited in situ. However, since POC production of both communities did not increase further at the highest light intensity (150 µmol photons m−2 s−1) even under high Fe supply, this suggests that light supply was saturated or that other conditions must be fulfilled (e.g., availability of trace metals other than Fe) in order for the communities to benefit from the higher light and Fe conditions.

Acyl-turnover of acylplastoquinol enhances recovery of photodamaged PSII in Synechocystis.

Photosynthetic electron transport is carried out by the electron carrier, plastoquinone (PQ). Recently, another form of PQ, acylplastoquinol (APQ), was discovered in Synechocystis sp. PCC 6803 (Synechocystis), but its physiological function in photosynthesis is unclear. In the present study, we identified a lipase encoded in sll0482 gene in Synechocystis that deacylates APQ and releases a free fatty acid and a reduced PQ (plastoquinol, PQH2), which we named acylplastoquinol lipase (APL). Disruption of apl gene increased APQ content, and recovery of photodamaged PSII under low light (LL) after the exposure to very high light (vHL) at 2500 μmol photons m-2 sec-1 without aeration (vHL) for 60 min, was suppressed in the Δapl cells. Δapl cells also show the slow rate of de novo synthesis of D1, a reaction center of PSII under such condition. Under high light, the cellular growth of Δapl was inhibited; however, disruption of apl gene did not affect the photosynthetic activity or photoinhibition of PSII. In wild-type cells, APQ content increased under vHL condition. Also, APQ was converted to PQH2 after transfer to LL with aeration by ambient air. Such striking changes in APQ were not observed in Δapl cells. The deacylation of APQ by APL may help repair PSII when PSII cannot drive photosynthetic electron transport efficiently.

Radiative energy budgets of migrational microphytobenthic biofilms

Epipelic, biofilm-forming diatoms are key drivers of the primary production of mudflats. Such primary production is strongly affected by the vertical migration of diatoms, which is modulated by diurnal photoperiods, tidal cycles, and photoprotection mechanisms. However, the role of vertical migration in the radiative energy budget (REB) of microphytobenthic biofilms remains largely unknown. Here we used microsensor measurements of temperature and O2 in combination with reflectance spectroscopy and variable chlorophyll fluorimetry to construct the REB of intertidal, epipelic diatom-dominated biofilms for different emersion times (1, 3 and 5 h after the beginning of the in-situ emersion) and photon irradiance regimes, i.e., 400 and 800 µmol photon m−2 s−1. The effect of migration on REBs was studied by inhibiting diatom motility with Latrunculin A (Lat-A). Photosynthetic activity and light utilization efficiency decreased slightly, after adding Lat-A, while the amount of reflected light energy remained constant at ~ 23% of the incident irradiance and the majority (76–78%) of the incident light energy was dissipated as heat. Of the energy dissipated as heat, < 24% was dissipated upward in Lat-A treated samples, while an increasing downward heat dissipation was observed in Lat-A treated samples, as compared to control samples under an irradiance of 800 µmol photon m−2 s−1. However, we found no statistical significant differences in the REB and physiological parameters in the different treatments. Thus, we did not find any evidence that vertical migration of diatoms affected photosynthesis and light efficiency in the microphytobenthic biofilm over an emersion cycle, and a clear effect of non-photochemical quenching in REB and heat dissipation fluxes was not observed.

Deciphering the Mechanism of Melatonin-Induced Enhancement of Photosystem II Function in Moderate Drought-Stressed Oregano Plants.

Melatonin (MT) is considered as an antistress molecule that plays a constructive role in the acclimation of plants to both biotic and abiotic stress conditions. In the present study, we assessed the impact of 10 and 100 μM MT foliar spray, on chlorophyll content, and photosystem II (PSII) function, under moderate drought stress, on oregano (Origanum vulgare L.) plants. Our aim was to elucidate the molecular mechanism of MT action on the photosynthetic electron transport process. Foliar spray with 100 μM MT was more effective in mitigating the negative impact of moderate drought stress on PSII function, compared to 10 μM MT. MT foliar spray significantly improved the reduced efficiency of the oxygen-evolving complex (OEC), and PSII photoinhibition (Fv/Fm), which were caused by drought stress. Under moderate drought stress, foliar spray with 100 μM MT, compared with the water sprayed (WA) leaves, increased the non-photochemical quenching (NPQ) by 31%, at the growth irradiance (GI, 205 μmol photons m-2 s-1), and by 13% at a high irradiance (HI, 1000 μmol photons m-2 s-1). However, the lower NPQ increase at HI was demonstrated to be more effective in decreasing the singlet-excited oxygen (1O2) production at HI (-38%), in drought-stressed oregano plants sprayed with 100 μM MT, than the corresponding decrease in 1O2 production at the GI (-20%), both compared with the respective WA-sprayed leaves under moderate drought. The reduced 1O2 production resulted in a significant increase in the quantum yield of PSII photochemistry (ΦPSII), and the electron transport rate (ETR), in moderate drought-stressed plants sprayed with 100 μM MT, compared with WA-sprayed plants, but only at the HI (+27%). Our results suggest that the enhancement of PSII functionality, with 100 μM MT under moderate drought stress, was initiated by the NPQ mechanism, which decreased the 1O2 production and increased the fraction of open PSII reaction centers (qp), resulting in an increased ETR.

Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae

Photosynthesis is one of the most important biological processes on Earth, providing the main source of bioavailable energy, carbon, and oxygen via the use of sunlight. Despite this importance, the minimum light level sustaining photosynthesis and net growth of primary producers in the global ocean is still unknown. Here, we present measurements from the MOSAiC field campaign in the central Arctic Ocean that reveal the resumption of photosynthetic growth and algal biomass buildup under the ice pack at a daily average irradiance of not more than 0.04 ± 0.02 µmol photons m−2 s−1 in late March. This is at least one order of magnitude lower than previous estimates (0.3–5 µmol photons m−2 s−1) and near the theoretical minimum light requirement of photosynthesis (0.01 µmol photons m−2 s−1). Our findings are based on measurements of the temporal development of the under-ice light field and concurrent measurements of both chlorophyll a concentrations and potential net primary production underneath the sea ice at 86 °N. Such low light requirements suggest that euphotic zones where photosynthesis can occur in the world’s oceans may extend further in depth and time, with major implications for global productivity estimates.

Role of cultivation parameters in carbohydrate accretion for production of bioethanol and C-phycocyanin from a marine cyanobacterium Leptolyngbya valderiana BDU 41001: A sustainable approach

The investigation aimed to augment carbohydrate accumulation in the marine cyanobacterium Leptolyngbya valderiana BDU 41001 to facilitate bioethanol production. Under the standardised physiochemical condition (SPC), i.e. 90 µmol photon m−2 s−1 light intensity, initial culture pH 8.5, 35 °C temperature and mixing at 150 rpm increased the carbohydrate productivity ∼70 % than the control, while a 47 % rise in content was obtained under the nitrate (N)-starved condition. Therefore, a two-stage cultivation strategy was implemented, combining SPC at the 1st stage and N starvation at the 2nd stage, resulting in 80 % augmentation of carbohydrate yield, which enhanced the bioethanol yield by ∼86 % as compared to the control employing immobilised yeast fermentation. Moreover, biomass utilisation was maximised by extracting C-phycocyanin, where a ∼77 % rise in productivity was recorded under the SPC. This study highlights the potential of L. valderiana for pilot-scale biorefinery applications, advancing the understanding of sustainable biofuel production.

Calcification increases carbon supply, photosynthesis, and growth in a globally distributed coccolithophore

AbstractCoccolithophores fix organic carbon and produce calcite plates (coccoliths) that ballast organic matter and facilitate carbon export. Photosynthesis consumes carbon dioxide, while calcification produces it, raising questions about whether coccolithophores are a net sink or source of carbon. We characterized the physiology of calcified and noncalcified (“naked”) phenotypes of Emiliania huxleyi (CCMP374) and investigated the relationship between calcification and photosynthesis across a gradient of light (25–2000 μmol photons m−2 s−1) spanning the euphotic zone. Growth and photophysiological parameters increased with light until reaching a mid‐light (150 μmol photons m−2 s−1) maximum for both phenotypes. Calcified cells were characterized by enhanced photophysiology and less photoinhibition. Further, enhanced bicarbonate transport in calcified cells led to higher rates of particulate organic carbon fixation and growth compared to naked cells at mid‐light to high light (150–2000 μmol photons m−2 s−1). Coccolith production was similarly high at mid and high light, but the rate of coccolith shedding was &gt;3‐fold lower at high‐light (1.2 vs. 0.35 coccoliths cell−1 h−1). The cellular mechanims(s) of this differential shedding remain unknown and underly light‐related controls on coccosphere maintenance. Our data suggest coccoliths shade cells at high light and that enhanced bicarbonate transport associated with calcification increases internal carbon supplies available for organic carbon fixation.

Abiotic triggers for maximising germling numbers in Asparagopsis taxiformis (Rhodophyta, Bonnemaisoniales) via tetrasporogenesis

AbstractThere is global interest in cultivating the red alga Asparagopsis taxiformis due to its efficacy as a potent anti-methanogenic feed supplement and as a biofilter for the bioremediation of nutrient-enriched waters. However, the development of A. taxiformis cultivation is currently hindered by a lack of information about the conditions required to maximise tetraspore release and thus secure a reliable source of germlings for out-planting. In this study, we examined the effects of temperature, irradiance, and standard nutrient supplementation (F/8, potassium iodide (KI) and arsenic trioxide (As2O3)) on the number of germlings produced per tetrasporophyte, using a strain of A. taxiformis widespread within the Great Barrier Reef, Australia. Temperature, irradiance and nutrient supplementation played a pivotal role in germling numbers, which was optimised at 22 °C under 7 µmol photons m−2 s−1 and with supplementation of F/8 nutrient media, arsenic trioxide (As2O3; 98 µg L−1) and potassium iodide (KI; 166 µg L−1). Once tetrasporophytes were removed from these inducing conditions, tetrasporogenesis ceased within 12 days. In a further five-week experiment investigating the effect of separate supplementation of As2O3 and KI, germling numbers were maximised under supplementation with either As2O3 or As2O3 + KI, with the relative growth rate of tetrasporophytes maximised under supplementation with F/8 + As2O3 + KI. Under optimum conditions, an average of 3,261 ± 826 (SD) germlings were produced per tetrasporophyte over a five-week period. Our results provide a strong starting point for developing hatchery protocols for generating a reliable supply of germlings for nursery cultivation in tropical settings.

Growth characteristics and molecular identification of indigenous Limnospira strains from Ethiopian soda lakes as a protein source

Malnutrition poses a significant global challenge in regions such as Ethiopia, particularly in children with limited dietary diversity. Arthrospira, a protein-rich cyanobacterium known for its rapid growth even under extreme conditions, is a promising solution for biomass production. This study addresses knowledge gaps in the morphological characterization and taxonomic classification of Arthrospira by focusing on precise species identification and strategic strain selection for optimal biomass production. Cyanobacterial strains were cultivated in a modified Spirulina Ogawa Terui medium at 35 °C under the photosynthetically active radiation of 160 μmol photons m−2 s−1 with a 12/12 h light/dark period in laboratory conditions. One hundred Arthrospira-like strains were isolated and characterized based on their morphological attributes according to accepted criteria, from Ethiopian soda lakes. Systematic evaluation of the growth rate comparison test identified four exceptionally fast-growing strains, with LC-30 exhibiting the highest dry weight (3.27 ± 0.51 g L˗1) and specific growth rate (1.63 ± 0.05 d˗1) among them. DNA analysis confirmed that all four examined strains belong to the Limnospira genus, specifically identified as Limnospira fusiformis. LC-30 and LA-08 demonstrated protein and phycocyanin levels (582.80 ± 0.07 and 557.90 ± 0.09 mg g˗1, respectively) comparable to those reported for Arthrospira platensis, and surpassing conventional protein sources, thus suggesting their potential for nutritional supplementation. The study highlights the importance of bioprospecting indigenous strains from soda lakes, revealing promising candidates for large-scale biomass production.

Photoperiodic dependent regulation of photosynthesis in the polar diatom Fragilariopsis cylindrus

Introduction: Polar microalgae are exposed to dramatic seasonal changes in light availability, from continuous summer days to winter nights with rapid changes of the daylength in spring and fall. Under this challenging light climate, large diatoms spring blooms occur at the bottom sea-ice and underneath the icepack, accounting for a significant proportion of the annual marine primary production in the Arctic Ocean. The on-going earlier melt down of the snow and ice covers result in a stronger light penetration and consequent increase in irradiance at the bottom of the sea ice leading to earlier seasonal sea-ice diatom blooms under shorter daylengths. Therefore, elucidating the response of polar diatoms to different photoperiods will help to better understand the consequences of the changing arctic climate on their photosynthetic productivity.Methods: In this study, we characterized the response of F. cylindrus, a model polar diatom, across five different photoperiods with similar light and temperature conditions (30 μmol photons m-2 s-1 and 0°C respectively).Results: We report different photoacclimative strategies under shorter and longer daylengths, with the special case of prolonged darkness (mimicking winter polar night). We also observed a repeated daily regulation of the photochemistry and photoprotection parameters when cells were exposed to a light:darkness alternation, despite the constant and optimal light intensity during the light periods.Discussion: Our results highlight the ability of F. cylindrus to grow efficiently under a wide range of daylengths, finely adjusting the balance between photochemistry and photoprotection to make the best use of the available light, supporting sustained production and growth despite low light and temperature.

Acclimation Strategies for the Black Sea Diatom Algae Ditylum brightwellii to High Intensity of Light

In cells of a culture of the large diatom Ditylum brightwellii (T. West) Grunow acclimated to faint light (17 μmol photons/(m2 s)), numerous chloroplasts are evenly distributed throughout the cell cytoplasm. After 10 min of exposure of algae to extremely high illumination (1100 μmol photons/(m2 s)), their aggregates gradually form in the center of the cell, and their formation continues until the end of the 2-h exposure period. At light intensities of 510–935 µmol photons/(m2 s) during short-term photoacclimation, the aggregation of chloroplasts is recorded for 20–60 min, after which their reverse movement and uniform distribution in the cytoplasm are revealed by the end of the second hour. Under conditions of a longer culture stay at a light intensity of 1100 μmol photons/(m2 s), the algae retains viability for only 6 h. Long-term photoacclimation of this species, which stops by the end of the second day, is detected when the light becomes half as weak. This is manifested in an increase in cell volume and in the C/Chl a ratio, in the increased aggregation of chloroplasts in the center of the cell, and in a decrease in a number of fluorescent parameters of the efficiency of photosystem II and of culture viability.

Hormetic Response of Photosystem II Function Induced by Nontoxic Calcium Hydroxide Nanoparticles.

In recent years, inorganic nanoparticles, including calcium hydroxide nanoparticles [Ca Ca(OH)2 NPs], have attracted significant interest for their ability to impact plant photosynthesis and boost agricultural productivity. In this study, the effects of 15 and 30 mg L-1 oleylamine-coated calcium hydroxide nanoparticles [Ca(OH)2@OAm NPs] on photosystem II (PSII) photochemistry were investigated on tomato plants at their growth irradiance (GI) (580 μmol photons m-2 s-1) and at high irradiance (HI) (1000 μmol photons m-2 s-1). Ca(OH)2@OAm NPs synthesized via a microwave-assisted method revealed a crystallite size of 25 nm with 34% w/w of oleylamine coater, a hydrodynamic size of 145 nm, and a ζ-potential of 4 mV. Compared with the control plants (sprayed with distilled water), PSII efficiency in tomato plants sprayed with Ca(OH)2@OAm NPs declined as soon as 90 min after the spray, accompanied by a higher excess excitation energy at PSII. Nevertheless, after 72 h, the effective quantum yield of PSII electron transport (ΦPSII) in tomato plants sprayed with Ca(OH)2@OAm NPs enhanced due to both an increase in the fraction of open PSII reaction centers (qp) and to the enhancement in the excitation capture efficiency (Fv'/Fm') of these centers. However, the decrease at the same time in non-photochemical quenching (NPQ) resulted in an increased generation of reactive oxygen species (ROS). It can be concluded that Ca(OH)2@OAm NPs, by effectively regulating the non-photochemical quenching (NPQ) mechanism, enhanced the electron transport rate (ETR) and decreased the excess excitation energy in tomato leaves. The delay in the enhancement of PSII photochemistry by the calcium hydroxide NPs was less at the GI than at the HI. The enhancement of PSII function by calcium hydroxide NPs is suggested to be triggered by the NPQ mechanism that intensifies ROS generation, which is considered to be beneficial. Calcium hydroxide nanoparticles, in less than 72 h, activated a ROS regulatory network of light energy partitioning signaling that enhanced PSII function. Therefore, synthesized Ca(OH)2@OAm NPs could potentially be used as photosynthetic biostimulants to enhance crop yields, pending further testing on other plant species.

Microalgal lipid production: A comparative analysis of Nannochloropsis and Microchloropsis strains

AbstractThe oleaginous genera Nannochloropsis and Microchloropsis are recognized for their lipid accumulation capacity. Microalgal lipid accumulation is triggered by nitrogen starvation, negatively affecting photosynthesis and growth. Moreover, light and temperature play pivotal roles in microalgal physiology, lipid accumulation and composition. This study focuses on comparing the responses of eight microalgal strains from Nannochloropsis (N. oceanica Necton, N. oceanica IMET1, Nannochloropsis. sp. CCAP211/78, N. oculata, and N. limnetica) and Microchloropsis (M. gaditana CCFM01, M.gaditana CCMP526, and M.salina) to light, temperature, and nitrogen availability. Biomass, lipid content and productivities were monitored under different light intensities (150 (LL) and 600 μmol photons m−2 s−1 (HL)) and temperatures (15, 25, 30℃) under nitrogen (N-) starvation and replete conditions. Under N-starvation and HL, N. sp. exhibited the highest lipid content (59%) and productivity (0.069 g L-1 day-1), while N. oculata had the lowest lipid content (37.5%) and productivity (0.037 g L-1 day-1) among the eight strains. Notably, M. gaditana CCFM01 achieved the highest EPA content (4.7%), contrasting with N.oceanica IMET1 lowest EPA content (2.9%) under 150 μmol photons m−2 s−1 and N-repletion. The response to temperature fluctuations under LL was strain-dependent. Microchloropsis salina and M. gaditana CCFM01 demonstrated the highest and lowest lipid productivities (0.069 g L-1 day-1 and 0.022 g L-1 day-1, respectively) at 15℃ under N-starvation. Moreover, significant EPA accumulation across various strains was observed in N. oculata (5.7%) under N-repletion at 15°C, surpassing M. gaditana CCFM01 by 40%. Ultimately, the physiological responses to cultivation conditions vary markedly among microalgal strains, even within the same genus or species. This knowledge is essential for selecting suitable strains for the efficient microalgal lipid production industry. Graphical Abstract Optimi zing cultivation conditions for the maximal lipid production in Nannochloropsis andMicrochloropsis

Employment of light-inducible promoter in genetically engineered cyanobacteria for photosynthetic isobutanol production with simulated diurnal sunlight and CO2

Cyanobacteria are oxygen-evolving prokaryotes that can be engineered for biofuel production from solar energy, CO2, and water. Isobutanol (IB) has the potential to serve as an alternative fuel and important chemical feedstock. The research involves engineering Synechocystis sp. PCC 6803, for photosynthetic isobutanol production via the 2-keto-acid pathway and their cultivation in lab-scale photobioreactors. This synthetic pathway involves the heterologous expression of two enzymes, α-ketoisovalerate decarboxylase (Kivd) and alcohol dehydrogenase (Yqhd), under a strong light-inducible promotor, psbA2, known to show increased gene expression under high light. The use of psbA2 could be a valuable strategy for isobutanol production as economic scaling up demands the utilization of natural sunlight, which also provides very high light intensity at midday, facilitating increased production. The study reports isobutanol production from engineered strains containing both pathway genes and with only kivd. In shake flask studies, the highest isobutanol titre of 75 mg L−1 (12th day) was achieved from an engineered strain DM12 under optimized light intensity. DM12 was cultivated in a 2 L flat panel photobioreactor, resulting in a maximum isobutanol titre of 371.8 mg L−1 (10th day) with 2 % CO2 and 200 μmol photons m−2 s−1. Cultivation of DM12 in a photobioreactor under mimic diurnal sunlight demonstrated the highest productivity of 39 mg L−1 day−1 with the maximum titre of 308.5 mg L−1 (9th day). This work lays the foundation for sustainable, large-scale biobutanol production using solar energy.

Photosynthetic performance of the red algae Gracilariopsis lemaneiformis under high seawater pH: Excess reactive oxygen production due to carbon limitation.

The red algae Gracilariopsis lemaneiformis is extensively cultivated at high densities, leading to significant increases in regional seawater pH due to its photosynthetic removal of inorganic carbon. We conducted a study on G. lemaneiformis cultured under various pH conditions (normal pH, pH 9.3, and pH 9.6) and light levels (dark and 100 μmol photons m-2 s-1) to investigate how high pH seawater environments affect the metabolic processes of G. lemaneiformis. The high pH did not directly damage the photosynthetic light reactions or the Calvin cycle. Instead, the observed reduction in photosynthetic rates was primarily due to CO2 limitation. However, under illuminated conditions, a high pH environment leads to a decrease in electron transport efficiency (ETo/RC) and reaction center density (RC/CSo), while simultaneously increasing the levels of hydrogen peroxide (H2O2), malondialdehyde (MDA), and the activity of antioxidant enzymes. Under illuminated conditions, the limitation of inhibit the photosynthetic electron transport process, leading to energy imbalance and excessive production of reactive oxygen species, which in turn resulted in lipid peroxidation of the cell membrane. This might be one of the inducing factors responsible for the bleaching in sea-farmed G. lemaneiformis plants.

Kelp dissolved organic carbon release is seasonal and annually enhanced during senescence.

Macroalgae influence local and global biogeochemical cycles through their production of dissolved organic carbon (DOC). Yet, data remain scarce and annualized estimates are typically based on high growth periods without considering seasonal variability. Although the mechanisms of active exudation and passive leakage need clarifying, ecophysiological stress is known to enhance DOC release. Therefore, DOC leakage from seasonally senescent macroalgae may be overlooked. This study focuses on the annual kelp Saccharina japonica var. religiosa (class Phaeophyceae) from Oshoro Bay, Hokkaido, Japan. Three years (2020-2022) of seasonal data were collected and analyzed, with least squares mean DOC release rates established for kelp (n = 88) across 16 incubation experiments (t ≥ 4 d, DOC samples ≥1 · d-1) under different photosynthetically active radiation (PAR) treatments (200, 400, 1200, or 1500 μmol photons · m-2 · s-1). Differences in PAR, dry weight biomass (g DW), sea surface temperature, or salinity could not explain DOC release-rate variability, which was high between individual kelp. Instead, there were significant intra-annual differences, with mean DOC release rates (mg C · g-1 DW · d-1 ± standard error between n kelp) higher during the autumn "late decay" period (0.71 ± 0.10, n = 27) compared to the winter "early growth" period (0.14 ± 0.025, n = 10) and summer "early decay" period (0.25 ± 0.050, n = 24). This relationship between seasonal senescence and macroalgal DOC release is further evidence that long-term, place-based studies of DOC dynamics are essential and that global extrapolations are premature.

The interplay of temperature, light, and substrate type in driving growth and reproduction of an important tropical crustose coralline alga

Crustose coralline algae (CCA) from the genus Titanoderma are reported to induce high levels of coral larval settlement across a wide diversity of species. Consequently, Titanoderma is a promising taxon to cultivate in aquaculture facilities for application in coral reef restoration projects. However, knowledge on the optimum conditions to promote growth and reproduction in Titanoderma is limited. To investigate this, we cultured adult fragments of Titanoderma sp. at two temperatures (27.5 or 30 °C) and two light levels (mean maximum midday irradiance of 10 or 40 µmol photons m−2 s−1) on three different tile materials (CaCO3, concrete, or PVC). We found that the combination of 27.5 °C and 40 µmol photons m−2 s−1 were best for adult fragment growth. Greater number of conceptacles were formed under higher light intensities, while temperature did not have an influence. Sporeling settlement and subsequent growth into juveniles were only evident at 40 µmol photons m−2 s−1, with substantially higher recruitment on substrates made of concrete. These results provide important insights for developing optimal conditions to cultivate Titanoderma sp. in aquaculture facilities to support reef restoration projects using sexually produced corals.

Biochemical and Functional Profiling of Thioredoxin-Dependent Cytosolic GPX-like Proteins in Euglena gracilis.

Unlike plants and animals, the phytoflagellate Euglena gracilis lacks catalase and contains a non-selenocysteine glutathione peroxidase-like protein (EgGPXL), two peroxiredoxins (EgPrx1 and EgPrx4), and one ascorbate peroxidase in the cytosol to maintain reactive oxygen species (ROS) homeostasis. In the present study, the full-length cDNA of three cytosolic EgGPXLs was obtained and further characterized biochemically and functionally. These EgGPXLs used thioredoxin instead of glutathione as an electron donor to reduce the levels of H2O2 and t-BOOH. The specific peroxidase activities of these enzymes for H2O2 and t-BOOH were 1.3 to 4.9 and 0.79 to 3.5 µmol/min/mg protein, respectively. Cytosolic EgGPXLs and EgPrx1/EgPrx4 were silenced simultaneously to investigate the synergistic effects of these genes on the physiological function of E. gracilis. The suppression of cytosolic EgGPXL genes was unable to induce any critical phenomena in Euglena under normal (100 μmol photons m-2 s-1) and high-light conditions (350 μmol photons m-2 s-1) at both autotrophic and heterotrophic states. Unexpectedly, the suppression of EgGPXL genes was able to rescue the EgPrx1/EgPrx4-silenced cell line from a critical situation. This study explored the potential resilience of Euglena to ROS, even with restriction of the cytosolic antioxidant system, indicating the involvement of some compensatory mechanisms.

Identification of genes involved in cadmium-ion tolerance in evolutionary Synechocystis sp. PCC 6803 tolerant to both cadmium and high light

Photosynthetic cyanobacteria have shown great potential as "autotrophic cell factories" for the synthesis of fuels and chemicals. However, poor tolerance to various environmental stressors like high light and heavy metals is an important factor limiting their economic viability. While numerous studies have focused on the tolerance mechanism of cyanobacteria to individual stressors, their response to simultaneous stresses remains to be recovered. To investigate the mechanism of cross tolerance to heavy metal Cd2+ and high light, the model cyanobacterium Synechocystis sp. PCC 6803 tolerant to both Cd2+ and high light was obtained via about 800 days’ cross-adaptive laboratory evolution. Three evolutionary strains capable of tolerating both 5.5 μM Cd2+ and 600 μmol photons m−2s−1 high light were successfully obtained, achieving about 83% enhancement of Cd2+ tolerance compared with the parent strain. The different response of parent and evolutionary strains to Cd2+ was elucidated via metabolomics. Further, a total of 15 genes that were mutated during evolution were identified by whole-genome re-sequencing. Finally, by single gene knockout and complementation analysis, four genes including ssl2615, sll1732, ssr1480 and sll1659 involved in the improvement of Cd2+ tolerance under high light condition were successfully identified. This work explored the tolerance mechanism of Synechocystis sp. PCC 6803 to cadmium under high light condition, and provided valuable reference for deciphering multi-tolerance mechanism of cyanobacteria in the future.