Upregulation Effect of Citrus Species on Brain-Derived Neurotrophic Factor.

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that plays fundamental roles in neuronal survival and synaptic plasticity. Its upregulation in the brain can effectively prevent and treat central nervous system (CNS) diseases, including depression, Alzheimer's disease (AD), and Parkinson's disease (PD). BDNF is synthesized in various peripheral tissues as well as in the brain and can be transported from peripheral circulation into the brain through the blood-brain barrier. Therefore, foods that upregulate BDNF in peripheral tissues may be beneficial in preventing and treating these CNS diseases. Previously, we revealed that treatment with Chinpi (Citrus unshiu peel) and Citrus natsudaidai increased BDNF levels in the human renal adenocarcinoma cell line ACHN. Here, we evaluated the effects of 21 citrus cultivars on BDNF production in ACHN cells by measuring BDNF levels in the cell culture medium. We found that treatment with peels and pulps of 13 citrus varieties increased BDNF levels in ACHN cells. Treatment with Aurantium, Acrumen, and their hybrids citrus varieties showed a potent BDNF-upregulating effect but not with varieties belonging to Limonellus, Citrophorum, and Cephalocitrus. In addition, treatment with some of those Acrumen and its hybrid citrus species resulted in elevated levels of BDNF transcripts in ACHN cells. These results suggest that peels of many citrus cultivars contain ingredients with a potential BDNF-upregulating ability, which may be novel drug seeds for treating depression, AD, and PD. Furthermore, many citrus cultivars could be used as BDNF-upregulating foods.

Upregulation of brain-derived neurotrophic factor by Shiikuwasha (Citrus depressa Hayata).

BACKGROUND: A reduction in the brain-derived neurotrophic factor (BDNF) level in the brain causes depression, whereas an increase in its level has therapeutic benefits against depression. BDNF is synthesized in various peripheral tissues and transported to the brain via the peripheral circulation across the blood-brain barrier. Therefore, substances that upregulate peripheral BDNF level may be used to prevent and treat depression. Previously, we demonstrated that Citrus unshiu peel (Chinpi) and C. natsudaidai increased BDNF level in a human renal adenocarcinoma cell line ACHN, which has BDNF-producing ability. Here, we evaluated whether Shiikuwasha (C. depressa Hayata), a citrus species cultivated in East Asia, can upregulate BDNF level in ACHN cells. METHODS: We evaluated the effects of test samples on BDNF production by measuring BDNF level in the medium of ACHN cells after a 24 h cultivation in the presence of test samples. The BDNF mRNA level was measured by quantitative reverse transcription-polymerase chain reaction, and the phosphorylation level of cyclic adenosine monophosphate response element-binding protein (CREB), a transcription factor regulating BDNF expression, was determined using Western blotting. RESULTS: We found that methanol extracts of Shiikuwasha peel, pulp, and seed increased the BDNF level in the culture medium of ACHN cells. Shiikuwasha peel and pulp extracts also upregulated BDNF mRNA level and phosphorylation of CREB. CONCLUSIONS: These results suggest that Shiikuwasha includes the candidate antidepressant substances with peripheral BDNF-upregulation effect.

Red foxtail millet upregulates brain-derived neurotrophic factor levels in vitro and in vivo.

Upregulation of the brain-derived neurotrophic factor (BDNF) in the brain can help in the prevention and treatment of depression. BDNF is synthesized in various peripheral tissues, as well as in the brain, and can reach the brain via the blood-brain barrier. Therefore, foods that upregulate peripheral BDNF levels may aid in depression management. We previously showed the BDNF-upregulating effect of white foxtail millet (WFM) using the human renal adenocarcinoma ACHN cell line, capable of producing and secreting BDNF. However, whether other varieties of foxtail millet can also upregulate BDNF is unclear. Herein, we examined the effects of red foxtail millet (RFM) on BDNF production in vitro and in vivo. RFM methanol extracts significantly increased BDNF levels in the culture medium of ACHN cells, and the levels were higher than those with WFMtreatment. Serum BDNF concentrations in rats fed a standard diet containing 20% RFM for 5 weeks were significantly higher than those in the control. Furthermore, the butanol fraction of the RFM methanol extract significantly increased BDNF levels in the culture medium of ACHN cells and upregulated BDNF mRNA expression in ACHN cells. Our results suggest that RFM has potential as a food material with BDNF-inducing activity.

Increasing Effect of Citrus natsudaidai on Brain-Derived Neurotrophic Factor.

The increase in brain-derived neurotrophic factor (BDNF) in the brain is beneficial for the treatment of depression, Alzheimer's disease (AD), and Parkinson's disease (PD); BDNF can cross the blood-brain barrier. Therefore, foods that elevate BDNF concentration in peripheral tissues may increase BDNF in the brain and thereby induce preventive and therapeutic effects against depression, AD, and PD. In this study, we aimed to determine whether Citrus natsudaidai extracts can increase BDNF concentration using the human kidney adenocarcinoma cell line ACHN, which has BDNF-producing and -secreting abilities. As test samples, methanol extracts of C. natsudaidai peel and pulp, and their n-hexane, ethyl acetate, n-butanol, and water fractions were prepared. The BDNF concentrations in culture medium of ACHN cells were assayed after 24 h cultivation in the presence of test samples. Compared with that of control (non-treated) cells, the BDNF concentration increased in the culture medium of ACHN cells treated with the methanol extract of C. natsudaidai peel and its hexane, butanol, and water fractions, as well as the butanol and water fractions of the pulp extract. Quantitative reverse transcription-polymerase chain reaction analysis revealed that ACHN cells treated with the butanol fractions of the peel and pulp extracts showed elevated levels of BDNF mRNA compared with those of non-treated cells. C. natsudaidai may increase BDNF concentration by acting on peripheral tissues and could be a medication for the prevention and treatment of depression, AD, and PD.

Development of a Novel Positive Pressure Protective Suit for a Biosafety Level 4 Laboratory in Japan.

Biosafety level 4 (BSL-4) laboratories are necessary to study microorganisms that are highly pathogenic to humans and have no prevention or therapeutic measures. Currently, most BSL-4 facilities have suit-type laboratories to conduct experiments on highly pathogenic microorganisms. In 2021, the first Japanese suit-type BSL-4 laboratory was constructed at Nagasaki University. Positive pressure protection suit (PPPS) is a primary barrier that protects and isolates laboratory workers from pathogens and the laboratory environment. Here, we developed a novel PPPS originally designed to be used in the Nagasaki BSL-4 laboratory. We modified several parts of a domestic chemical protective suit, including its front face shield, cuff, and air supply hose, for safe handling of microbiological agents. The improved suit, PS-790BSL4-AL, showed resistance to several chemicals, including quaternary ammonium disinfectant, and did not show any permeation against blood and phages. To validate the suit's integrity, we also established an airtight test that eliminated individual differences for quantitative testing. In conclusion, our developed suit performs sufficiently as a primary barrier and allows for the safe handling of pathogens in our new BSL-4 laboratory.

Multiple plasma membrane SLC4s contribute to external HCO3- acquisition during CO2 starvation in the marine diatom Phaeodactylum tricornutum.

The availability of CO2 is one of the restrictions on aquatic photosynthesis. Solute carrier (SLC) 4-2, a plasma membrane HCO3- transporter has previously been identified in the marine diatom Phaeodactylum tricornutum. In this study, we discovered two paralogs, PtSLC4-1 and PtSLC4-4, that are both localized at the plasma membrane. Their overexpression stimulated HCO3- uptake, and this was inhibited by the anion channel blocker 4,4´-diisothiocyanostilbene-2,2´-disulfonic (DIDS). Similarly to SLC4-2, PtSLC4-1 specifically required Na+ of ~100 mM for its maximum HCO3- transport activity. Unlike PtSLC4-1 and PtSLC4-2, the HCO3- transport of PtSLC4-4 depended equally on Na+, K+, or Li+, suggesting its broad selectivity for cations. Transcript analyses indicated that PtSLC4-1 was the most abundant HCO3- transporter under CO2 concentrations below atmospheric levels, while PtSLC4-4 showed little transcript induction under atmospheric CO2 but transient induction to comparable levels to PtSLC4-1 during the initial acclimation stage from high CO2 (1%) to very low CO2 (<0.002%). Our results strongly suggest a major HCO3- transport role of PtSLC4-1 with a relatively minor role of PtSLC4-2, and that PtSLC4-4 operates under severe CO2 limitation unselectively to cations when the other SLC4s do not function to support HCO3- uptake.

Observation and simulation of atmospheric gravity waves exciting subsequent tsunami along the coastline of Japan after Tonga explosion event.

Tsunamis are commonly generated by earthquakes beneath the ocean floor, volcanic eruptions, and landslides. The tsunami following the Tonga eruption of 2022 is believed to have been excited by atmospheric pressure fluctuations generated by the explosion of the volcano. The first, fast-traveling tsunami was excited by Lamb waves; however, it has not been clarified observationally or theoretically which type of atmospheric fluctuations excited more prominent tsunami which followd. In this study, we investigate atmospheric gravity waves that possibly excited the aforementioned subsequent tsunami based on observations and atmosphere-ocean coupling simulations. The atmospheric fluctuations are classified as Lamb waves, acoustic waves, or gravity waves. The arrival time of the gravity wave and the simulation shows that the gravity wave propagated at a phase speed of 215 m/s, coinciding with the tsunami velocity in the Pacific Ocean, and suggesting that the gravity wave resonantly excited the tsunami (Proudman resonance). These observations and theoretical calculations provide an essential basis for investigations of volcano-induced meteotsunamis, including the Tonga event.

Mitochondrial phosphoenolpyruvate carboxylase contributes to carbon fixation in the diatom Phaeodactylum tricornutum at low inorganic carbon concentrations.

Photosynthetic carbon fixation is often limited by CO2 availability, which led to the evolution of CO2 concentrating mechanisms (CCMs). Some diatoms possess CCMs that employ biochemical fixation of bicarbonate, similar to C4 plants, but whether biochemical CCMs are commonly found in diatoms is a subject of debate. In the diatom Phaeodactylum tricornutum, phosphoenolpyruvate carboxylase (PEPC) is present in two isoforms, PEPC1 in the plastids and PEPC2 in the mitochondria. We used real-time quantitative polymerase chain reaction, Western blots, and enzymatic assays to examine PEPC expression and PEPC activity, under low and high concentrations of dissolved inorganic carbon (DIC). We generated and analyzed individual knockout cell lines of PEPC1 and PEPC2, as well as a PEPC1/2 double-knockout strain. While we could not detect an altered phenotype in the PEPC1 knockout strains at ambient, low or high DIC concentrations, PEPC2 and the double-knockout strains grown under ambient air or lower DIC availability conditions showed reduced growth and photosynthetic affinity for DIC while behaving similarly to wild-type (WT) cells at high DIC concentrations. These mutants furthermore exhibited significantly lower 13 C/12 C ratios compared to the WT. Our data imply that in P. tricornutum at least parts of the CCM rely on biochemical bicarbonate fixation catalyzed by the mitochondrial PEPC2.

Upregulating Effect of Wheat on Brain-Derived Neurotrophic Factor in Human Lung Adenocarcinoma A549 Cells.

The neurotrophic hypothesis of depression, that is, a deficiency in hippocampal brain-derived neurotrophic factor (BDNF) leads to depression, has gained widespread acceptance. BDNF is synthesized in various peripheral tissues such as the lung, kidney, liver, heart and testis, besides the brain. Peripheral BDNF can traverse the blood-brain barrier and reach the hippocampus; accordingly, substances that upregulate BDNF production in peripheral tissues may be useful in the treatment of depression. The Mediterranean diet, containing high amounts of whole grains including unrefined wheat, vegetables, fruits, nuts, and olive oil, reportedly reduces the risk of depression. The association between the high consumption of unrefined wheat in the Mediterranean diet and BDNF production in peripheral tissues is unclear. In this study, we investigated the BDNF production capacity of human lung adenocarcinoma cell line A549 and the effect of wheat on BDNF production in the cells. Methanol extracts of whole-wheat flour and wheat bran, which are forms of unrefined wheat, increased the BDNF level in the culture medium of A549 cells. However, methanol extract of wheat endosperm had no effect on the BDNF level in these cells. Our findings suggest that wheat bran contains ingredients that upregulate BDNF production in peripheral tissues, and unrefined wheat potentially contributes to the elevation in peripheral BDNF level.

Identification of the infrasound signals emitted by explosive eruption of Mt. Shinmoedake by three-dimensional ray tracing.

Mt. Shinmoedake, a part of the Mt. Kirishima cluster of volcanoes in Kyushu, Japan, erupted on 10 March 2018. Our infrasound sensor network located at a distance of more than 200 km from the source detected signals emitted by an explosive eruption of Mt. Shinmoedake. The arrival time of the signals is divided into three time intervals. To reveal how the observed infrasound signals propagated from the source to the sensors, we carry out three-dimensional ray tracing on the basis of the Hamilton equations including the vertical profiles of the temperature and wind around the ray path. We present formulas for calculating travel time and distance of infrasound from a source to an observation site and its turning altitude in the atmosphere. We have identified four kinds of signals, namely, the waves propagated in the troposphere undergoing multiple refraction and those refracting from the stratosphere, the mesosphere, and the lower thermosphere. Brief discussion is devoted to some of the unidentified signals.

Identification of traditional Japanese Kampo medicines and crude drugs that upregulate brain­derived neurotrophic factor in human peripheral cells.

The neurotrophic hypothesis of depression, which suggests that decreased hippocampal brain­derived neurotrophic factor (BDNF) levels cause depression, has become increasingly popular. BDNF, a member of the neurotrophin family, promotes neuronal differentiation and survival. BDNF is synthesized in various peripheral tissues, as well as in the brain. Considering that peripheral BDNF can be transported into the brain across the blood­brain barrier, substances with the ability to upregulate BDNF activity in peripheral tissues may be useful in the management of depression. Previously, we demonstrated that the human kidney adenocarcinoma cell line ACHN produces BDNF; hence, this cell line was employed for screening upregulators of peripheral BDNF. Here, we aimed to identify Kampo (traditional Japanese) medicines and their crude drug components that upregulate BDNF levels using ACHN cells. Chotosan, Hochuekkito, Kososan, and Ninjinyoeito, Kampo medicines used in treating psychiatric disorders, increased BDNF levels in the culture media of ACHN cells. Furthermore, Chinpi (Citrus unshiu peel), a crude drug contained in these four Kampo medicines, as well as Onji (Polygala tenuifolia root), and Saiko (Bupleurum falcatum root) elevated BDNF levels in ACHN cells. Chinpi, showing strong BDNF elevating effect, increased BDNF mRNA expression. Inhibitors of protein kinase B, mitogen­activated protein kinase kinase, and cAMP­dependent protein kinase, involved in the transcription of BDNF, attenuated Chinpi­induced BDNF elevation. Our results suggest that Chinpi and Kampo medicines containing Chinpi can promote the production of BDNF in peripheral tissues, potentially alleviating depression symptoms.

Brain-Derived Neurotrophic Factor Up-Regulation by the Methanol Extract of Foxtail Millet in Human Peripheral Cells.

Brain-derived neurotrophic factor (BDNF) plays important roles in synaptic plasticity and neuronal differentiation. The neurotrophic hypothesis of depression, which suggests that reduced BDNF in the hippocampus underlies depression, has attracted increasing attention. Stress, a major cause of depression, leads to decreased BDNF levels, and administration of BDNF into the hippocampus shows an antidepressant effect. BDNF is synthesized in peripheral tissues as well as in the brain. Since BDNF crosses the blood-brain barrier, intake of food ingredients that elevate BDNF in peripheral tissues may be useful for the prevention and treatment of depression. However, no screening method for BDNF up-regulators in peripheral tissues has been reported. In this study, we revealed that ACHN human kidney adenocarcinoma cells secreted BDNF. In addition, we demonstrated that the methanol extract of foxtail millet up-regulated BDNF levels in ACHN cells. Our results indicate that ACHN cells could be useful in the screening for peripheral-BDNF up-regulators, and that foxtail millet may have the potential to elevate BDNF levels in peripheral tissues.

Planetary-scale streak structure reproduced in high-resolution simulations of the Venus atmosphere with a low-stability layer.

Cloud patterns are important clues for revealing the atmospheric circulation of Venus. Recently, a planetary-scale streak structure has been discovered in middle- and lower-cloud images of Venus' night-side taken by IR2, the 2-µm camera, on board the Akatsuki orbiter. However, its formation mechanism has not been investigated. Here we succeed, for the first time, in reproducing the patterns of the observed streak structure, as regions of strong downward flows that develop in high-resolution global simulations of the Venus atmosphere. The streaks are formed in both hemispheres with equatorial symmetry, which is caused by equatorial Rossby-like and Kelvin-like waves with zonal wavenumber one. The low-stability layer that has been suggested by past observations is essential for reproducing the streak structure. The streaks of downward flow result from the interaction of the meridionally tilted phase lines of the Rossby-like waves and the characteristics of baroclinic instability produced around the low-stability layer.

Decreased Plasma Octanoylated Ghrelin Levels in Mice by Oleanolic Acid.

Ghrelin is a stomach-derived peptide hormone with an appetite-stimulating effect. Octanoylation on the serine-3 residue of ghrelin by ghrelin O-acyl transferase (GOAT) is essential for its orexigenic effect. Mature octanoylated ghrelin is generated by the C-terminal cleavage of octanoylated proghrelin via prohormone convertases (furin, PC1/3, or PC2). We previously established an AGS-GHRL8 cell line that produces octanoylated ghrelin in the presence of octanoic acid, and found that oleanolic acid suppresses octanoylated ghrelin production in AGS-GHRL8 cells. Here, we investigated the effects of oleanolic acid in C57BL/6J mice fed a standard, high-fat, or high-glucose diet. Oral administration of oleanolic acid for seven days (20 or 40 mg/kg) reduced plasma octanoylated ghrelin levels and body weight gain in the standard diet-fed mice but not in other two diet-fed mice. There were no significant differences in ghrelin, GOAT, furin, PC1/3, and PC2 gene expression levels between the vehicle- and oleanolic acid-treated mice fed a standard diet. Octanoyl-CoA is a substrate for ghrelin octanoylation by GOAT. We found that oleanolic acid did not affect octanoyl-CoA production in vitro. Hence, the inhibitory effect of oleanolic acid on octanoylated ghrelin production may not be related to the decrease in octanoyl-CoA. The results of this study may provide valuable knowledge for the development of anti-obesity agents with an inhibitory effect on octanoylated ghrelin production.

Plasma Membrane-Type Aquaporins from Marine Diatoms Function as CO2/NH3 Channels and Provide Photoprotection.

Aquaporins (AQPs) are ubiquitous water channels that facilitate the transport of many small molecules and may play multiple vital roles in aquatic environments. In particular, mechanisms to maintain transmembrane fluxes of important small molecules have yet to be studied in marine photoautotrophic organisms. Here, we report the occurrence of multiple AQPs with differential cellular localizations in marine diatoms, an important group of oceanic primary producers. The AQPs play a role in mediating the permeability of membranes to CO2 and NH3 In silico surveys revealed the presence of five AQP orthologs in the pennate diatom Phaeodactylum tricornutum and two in the centric diatom Thalassiosira pseudonana GFP fusions of putative AQPs displayed clear localization to the plasma membrane (PtAGP1 and PtAQP2), the chloroplast endoplasmic reticulum (CER; PtAGP1 and PtAQP3), and the tonoplast (PtAQP5) in P. tricornutum In T. pseudonana, GFP-AQP fusion proteins were found on the vacuole membrane (TpAQP1) and CER (TpAQP2). Transcript levels of both PtAQP1 and PtAQP2 were highly induced by ammonia, while only PtAQP2 was induced by high (1%[v/v]) CO2 Constitutive overexpression of GFP-tagged PtAQP1 and PtAQP2 significantly increased CO2 and NH3 permeability in P. tricornutum, strongly indicating that these AQPs function in regulating CO2/NH3 permeability in the plasma membrane and/or CER. Cells carrying GFP-tagged PtAQP1 and PtAQP2 had higher nonphotochemical quenching under high light relative to that of wild-type cells, suggesting that these AQPs are involved in photoprotection. These AQPs may facilitate the efflux of NH3, preventing the uncoupling effect of high intracellular ammonia concentrations.

Inhibitory Effect of (-)-Epigallocatechin-3-O-gallate on Octanoylated Ghrelin Levels in Vitro and in Vivo.

Ghrelin is an orexigenic peptide hormone produced in the stomach. The major active form is octanoylated ghrelin, which is modified with an n-octanoic acid at the serine-3 residue. Inhibition of octanoylated ghrelin production is useful for the prevention and improvement of obesity. We previously developed a cell-based assay system employing a ghrelin-expressing cell line, AGS-GHRL8, and found various compounds that decreased octanoylated ghrelin levels using this system. (-)-Epigallocatechin-3-O-gallate (EGCG) is a bioactive catechin in green tea and reportedly has an anti-obesity effect; however, it remains unclear whether EGCG inhibits octanoylated ghrelin production. Therefore, in this study, we investigated the effect of EGCG on octanoylated ghrelin levels in AGS-GHRL8 cells and C57BL/6J mice. EGCG significantly reduced the octanoylated ghrelin level in AGS-GHRL8 cells. In mice, three days of treatment with TEAVIGO®, which contains 97.69% EGCG, lowered the plasma octanoylated ghrelin level by 40% from that in control mice. In addition, TEAVIGO® reduced the mRNA expression of ghrelin and prohormone convertase 1/3, an enzyme responsible for the processing of proghrelin to mature ghrelin, in the mouse stomach, suggesting that the reduced expression of these genes may contribute to the inhibition of octanoylated ghrelin production. These results suggest a decrease in the octanoylated ghrelin level to be involved in the anti-obesity effect of EGCG, which thus has potential for the development of anti-obesity agents with ghrelin-lowering effect.

Bioactive Triterpenes from the Root of Salvia miltiorrhiza Bunge.

Danshen (Salvia miltiorrhiza) is a well-known medicinal herb in the oriental medicine. The current study on bioactive triterpenoid in the root of S. miltiorrhiza led to the isolation of a new highly hydroxylated ursane-type triterpene, urs-12-ene-2α,3ß,7ß,16α-tetraol (1) and five known ones including 2ß-hydroxypomolic acid (2), maslinic acid (3), asiatic acid (4), ursolic acid (5), and oleanolic acid (6). Their structures were elucidated on the basis of extensive spectroscopic analyses and comparison with literature data. The antiproliferative testing against HL-60 cells revealed that the new compound 1 and ursolic acid (5) showed weak and moderate activities with IC50 values of 42.2 and 11.7 µM. In addition, compounds 1-3 showed inhibitory effect on ghrelin activity. Copyright © 2017 John Wiley & Sons, Ltd.

Mechanisms of carbon dioxide acquisition and CO2 sensing in marine diatoms: a gateway to carbon metabolism.

Diatoms are one of the most successful marine eukaryotic algal groups, responsible for up to 20% of the annual global CO2 fixation. The evolution of a CO2-concentrating mechanism (CCM) allowed diatoms to overcome a number of serious constraints on photosynthesis in the marine environment, particularly low [CO2]aq in seawater relative to concentrations required by the CO2 fixing enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), which is partly due to the slow diffusion rate of CO2 in water and a limited CO2 formation rate from [Formula: see text] in seawater. Diatoms use two alternative strategies to take up dissolved inorganic carbon (DIC) from the environment: one primarily relies on the direct uptake of [Formula: see text] through plasma-membrane type solute carrier (SLC) 4 family [Formula: see text] transporters and the other is more reliant on passive diffusion of CO2 formed by an external carbonic anhydrase (CA). Bicarbonate taken up into the cytoplasm is most likely then actively transported into the chloroplast stroma by SLC4-type transporters on the chloroplast membrane system. Bicarbonate in the stroma is converted into CO2 only in close proximity to RubisCO preventing unnecessary CO2 leakage. CAs play significant roles in mobilizing DIC as it is progressively moved towards the site of fixation. However, the evolutionary types and subcellular locations of CAs are not conserved between different diatoms, strongly suggesting that this DIC mobilization strategy likely evolved multiple times with different origins. By contrast, the recent discovery of the thylakoid luminal θ-CA indicates that the strategy to supply CO2 to RubisCO in the pyrenoid may be very similar to that of green algae, and strongly suggests convergent coevolution in CCM function of the thylakoid lumen not only among diatoms but among eukaryotic algae in general. In this review, both experimental and corresponding theoretical models of the diatom CCMs are discussed.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Molecular aspects of the biophysical CO2-concentrating mechanism and its regulation in marine diatoms.

Diatoms operate a CO2-concentrating mechanism (CCM) that drives upwards of 20% of annual global primary production. Recent progress in CCM research in the marine pennate diatom Phaeodactylum tricornutum revealed that this diatom directly takes up HCO3- from seawater through low-CO2-inducible plasma membrane HCO3- transporters, which belong to the solute carrier (SLC) 4 family. Apart from this, studies of carbonic anhydrases (CAs) in diatoms have revealed considerable diversity in classes and localization among species. This strongly suggests that the CA systems, which control permeability and flux of dissolved inorganic carbon (DIC) by catalysing reversible CO2 hydration, have evolved from diverse origins. Of particular interest is the occurrence of low-CO2-inducible external CAs in the centric marine diatom Thalassiosira pseudonana, offering a strategy of CA-catalysed initial CO2 entry via passive diffusion, contrasting with active DIC transport in P. tricornutum. Molecular mechanisms to transport DIC across chloroplast envelopes are likely also through specific HCO3- transporters, although details have yet to be elucidated. Furthermore, recent discovery of a luminal θ-CA in the diatom thylakoid implied a common strategy in the mechanism to supply CO2 to RubisCO in the pyrenoid, which is conserved among green algae and some heterokontophytes. These results strongly suggest an occurrence of convergent coevolution between the pyrenoid and thylakoid membrane in aquatic photosynthesis.

Diverse strategies of O2 usage for preventing photo-oxidative damage under CO2 limitation during algal photosynthesis.

Photosynthesis produces chemical energy from photon energy in the photosynthetic electron transport and assimilates CO2 using the chemical energy. Thus, CO2 limitation causes an accumulation of excess energy, resulting in reactive oxygen species (ROS) which can cause oxidative damage to cells. O2 can be used as an alternative energy sink when oxygenic phototrophs are exposed to high light. Here, we examined the responses to CO2 limitation and O2 dependency of two secondary algae, Euglena gracilis and Phaeodactylum tricornutum. In E. gracilis, approximately half of the relative electron transport rate (ETR) of CO2-saturated photosynthesis was maintained and was uncoupled from photosynthesis under CO2 limitation. The ETR showed biphasic dependencies on O2 at high and low O2 concentrations. Conversely, in P. tricornutum, most relative ETR decreased in parallel with the photosynthetic O2 evolution rate in response to CO2 limitation. Instead, non-photochemical quenching was strongly activated under CO2 limitation in P. tricornutum. The results indicate that these secondary algae adopt different strategies to acclimatize to CO2 limitation, and that both strategies differ from those utilized by cyanobacteria and green algae. We summarize the diversity of strategies for prevention of photo-oxidative damage under CO2 limitation in cyanobacterial and algal photosynthesis.