Utilization of light by fucoxanthin-chlorophyll-binding protein in a marine centric diatom, Chaetoceros gracilis

Biodiversity risk assessment of genetically modified Chaetoceros gracilis for outdoor cultivation

A genetically modified (GM) strain of the diatom Chaetoceros gracilis expressing the phosphite dehydrogenase gene (ptxD), which is a useful gene both for the biological containment and the avoidance of microbial contamination, was characterized to estimate the risk against the biodiversity by laboratory experiments. GM strain could grow in the medium containing phosphite as a sole source of phosphorus, while its general characteristics such as growth, salt tolerance, heat and dehydration resistance in the normal phosphate-containing medium were equivalent to those of wild type (WT) strain. The increase in potential toxicity of GM strain against plant, crustacean, fish and mammal was also disproved. The dispersal ability of WT strain cultured in an outdoor raceway pond was investigated for 28 days by detecting the psb31 gene in vessels, settled at variable distances (between 5 and 60 m) from the pond. The diatom was detected only in one vessel placed 5 m apart. To estimate the influence on the environment, WT and GM strains were inoculated into freshwater, seawater and soil. The influence on the microbiome in those samples was assessed by 16S rRNA gene amplicon sequencing, in addition to the analysis of the survivability of those strains in the freshwater and the seawater. The results indicated that the effect to the microbiome and the survivability were comparable between WT and GM strains. All results showed that the introduction of the ptxD gene into the diatom had a low risk on biodiversity.

A review on the progress, challenges and prospects in commercializing microalgal fucoxanthin

Fucoxanthin, the most abundant but nearly untapped carotenoid resource, is in the spotlight in the last decade from various perspectives due to a wide range of bioactivities and healthy benefits. The exploitation of fucoxanthin for nutraceutical and pharmaceutical purposes encompasses enormous scientific and economic potentials. Traditional production of fucoxanthin from brown algae (macroalgae) is constrained by limited yield and prohibitively high cost. Microalgae, as the most diverse photoautotrophs, hold the promises as sustainable sources and ideal cell factories for commercial fucoxanthin production, owing to their rich fucoxanthin content and excellent biomass productivity. In this work, the recent progress in upstream (microalgae selection, optimization of culture conditions, trophic modes, cultivation strategies and biosynthesis pathway) as well as downstream processes (extraction) of fucoxanthin production has been comprehensively and critically reviewed. The major bottlenecks, such as screening of fucoxanthin-producers, conflict between biomass and fucoxanthin accumulation under high light condition, unclear steps in biosynthesis pathway and limited evaluation of outdoor scale-up cultivation and extraction, have been pinpointed. Most importantly, the applications of emerging and conventional techniques facilitating commercialization of microalgal fucoxanthin are highlighted. The reviewed and evaluated include breeding and high-throughput screening methods of elite strains; flashing light effect inducing concurrent biomass and fucoxanthin accumulation; fucoxanthin biosynthesis and the regulatory mechanisms associating with its accumulation elucidated with the development of genetic engineering and omics techniques; and photobioreactors, harvesting and extraction techniques suitable for scaling up fucoxanthin production. In conclusion, the prospects of microalgal fucoxanthin commercialization can be expected with the joint development of fundamental phycology and biotechnology.

Adaptation of light-harvesting and energy-transfer processes of a diatom Chaetoceros gracilis to different light qualities

Diatoms are a major group of microalgae in marine and freshwater environments. To utilize the light energy in blue to green region, diatoms possess unique antenna pigment-protein complexes, fucoxanthin chlorophyll a/c-binding proteins (FCPs). Depending on light qualities and quantities, diatoms form FCPs with different energies: normal-type and red-shifted FCPs. In the present study, we examined changes in light-harvesting and energy-transfer processes of a diatom Chaetoceros gracilis cells grown using white- and single-colored light-emitting diodes (LEDs), by means of time-resolved fluorescence spectroscopy. The blue LED, which is harvested by FCPs, modified energy transfer involving CP47, and suppressed energy transfer to PSI. Under the red-LED conditions, which is absorbed by both FCPs and PSs, energy transfer to PSI was enhanced, and the red-shifted FCP appeared. The red-shifted FCP was also recognized under the green- and yellow-LEDs, suggesting that lack of the shorter-wavelength light induces the red-shifted FCP. Functions of the red-shifted FCPs are discussed.

Adaptation of light-harvesting and energy-transfer processes of a diatom Phaeodactylum tricornutum to different light qualities

Fucoxanthin-chlorophyll (Chl) a/c-binding proteins (FCPs) are light-harvesting pigment-protein complexes found in diatoms and brown algae. Due to the characteristic pigments, such as fucoxanthin and Chl c, FCPs can capture light energy in blue-to green regions. A pennate diatom Phaeodactylum tricornutum synthesizes a red-shifted form of FCP under weak or red light, extending a light-absorption ability to longer wavelengths. In the present study, we examined changes in light-harvesting and energy-transfer processes of P. tricornutum cells grown under white- and single-colored light-emitting diodes (LEDs). The red-shifted FCP appears in the cells grown under the green, yellow, and red LEDs, and exhibited a fluorescence peak around 714 nm. Additional energy-transfer pathways are established in the red-shifted FCP; two forms (F713 and F718) of low-energy Chl a work as energy traps at 77 K. Averaged fluorescence lifetimes are prolonged in the cells grown under the yellow and red LEDs, whereas they are shortened in the blue-LED-grown cells. Based on these results, we discussed the light-adaptation machinery of P. tricornutum cells involved in the red-shifted FCP.

Spectral Properties and Excitation Relaxation of Novel Fucoxanthin Chlorophyll a/c-Binding Protein Complexes

Fucoxanthin chlorophyll a/c-binding proteins (FCPs) are unique light harvesters for some photosynthetic organisms. There were several reports for the alterations of FCPs in response to light conditions. Here, we present the spectral profiles and excitation dynamics of novel FCP complexes isolated from the diatom Chaetoceros gracilis. Under a red-light condition, C. gracilis cells expressed three types of FCP complexes, two of which are very similar to FCP complexes found in the white-light grown cells, and one of which is the novel FCP complex. The combination of steady-state absorption and fluorescence spectra and time-resolved fluorescence spectra revealed that, compared to other types of FCP complexes, the novel FCP complexes exhibited red-shifted absorption and fluorescence spectra and fast decay of excitation. This finding will provide new insights into not only the light-harvesting strategies of diatoms but also the diversity of light adaptation machinery for photosynthetic organisms.

Dark metabolism: a molecular insight into how the Antarctic sea-ice diatom Fragilariopsis cylindrus survives long-term darkness

Light underneath Antarctic sea-ice is below detectable limits for up to 4 months of the year. The ability of Antarctic sea-ice diatoms to survive this prolonged darkness relies on their metabolic capability. This study is the first to examine the proteome of a prominent sea-ice diatom in response to extended darkness, focusing on the protein-level mechanisms of dark survival. The Antarctic diatom Fragilariopsis cylindrus was grown under continuous light or darkness for 120 d. The whole cell proteome was quantitatively analysed by nano-LC-MS/MS to investigate metabolic changes that occur during sustained darkness and during recovery under illumination. Enzymes of metabolic pathways, particularly those involved in respiratory processes, tricarboxylic acid cycle, glycolysis, the Entner-Doudoroff pathway, the urea cycle and the mitochondrial electron transport chain became more abundant in the dark. Within the plastid, carbon fixation halted while the upper sections of the glycolysis, gluconeogenesis and pentose phosphate pathways became less active. We have discovered how F. cylindrus utilises an ancient alternative metabolic mechanism that enables its capacity for long-term dark survival. By sustaining essential metabolic processes in the dark, F. cylindrus retains the functionality of the photosynthetic apparatus, ensuring rapid recovery upon re-illumination.

Molecular Structures and Functions of Chlorophylls-a Esterified with Geranylgeranyl, Dihydrogeranylgeranyl, and Tetrahydrogeranylgeranyl Groups at the 17-Propionate Residue in a Diatom, Chaetoceros calcitrans

The 17-propionate ester group of chlorophyll(Chl)-a in some oxygenic phototrophs was investigated using HPLC. Chls-a esterified with partially dehydrogenated forms of a phytyl group were found in fully grown cells of a diatom, Chaetoceros calcitrans: geranylgeranyl (GG), dihydrogeranylgeranyl (DHGG), and tetrahydrogeranylgeranyl (THGG). Chls-a bearing such esterifying groups were reported to be found only in greening processes of higher plants, and thus these Chls-a have been thought to be biosynthetic precursors for phytylated Chl-a. Their molecular structures were unambiguously determined using ¹H and ¹³C NMR spectroscopy and mass spectrometry. In particular, the positions of C═C double bonds in DHGG were identified at C2═C3, C6═C7, and C14═C15, and those in THGG were determined to be at C2═C3 and C14═C15. Notably, the present DHGG was different from the previously determined DHGG of bacteriochlorophyll-a in purple bacteria (C2═C3, C10═C11, and C14═C15). Moreover, thylakoid membranes as well as fucoxanthin-chlorophyll-a/c proteins called FCPs were isolated from the diatom, and their Chl-a compositions were analyzed. Chls-a esterified with GG, DHGG, and THGG were detected by HPLC, indicating that such Chls-a were not merely biosynthetic precursors, but photosynthetically active pigments.

Conversion of photosystem II dimer to monomers during photoinhibition is tightly coupled with decrease in oxygen-evolving activity in the diatom Chaetoceros gracilis

The rapid turnover of photosystem II (PSII) in diatoms is thought to be at an exceptionally high rate compared with other oxyphototrophs; however, its molecular mechanisms are largely unknown. In this study, we examined the photodamage and repair processes of PSII in the marine centric diatom Chaetoceros gracilis incubated at 30 or 300 μmol photons m^-2 s^-1 in the presence of a de novo protein-synthesis inhibitor. When de novo protein synthesis was blocked by chloramphenicol (Cm), oxygen-evolving activity gradually decreased even at 30 μmol photons m^-2 s^-1 and could not be detected at 12 h. PSII inactivation was enhanced by higher illumination. Using Cm-treated cells, the conversion of PSII dimer to monomers was observed by blue native PAGE. The rate of PSII monomerization was very similar to that of the decrease in oxygen-evolving activity under both light conditions. Immunological detection of D1 protein in the Cm-treated cells showed that the rate of D1 degradation was slower than that of the former two events, although it was more rapid than that observed in other oxyphototrophs. Thus, the three accelerated events, especially PSII monomerization, appear to cause the unusually high rate of PSII turnover in diatoms.

Advantageous characteristics of the diatom Chaetoceros gracilis as a sustainable biofuel producer

BACKGROUND

Diatoms have attracted interest as biofuel producers. Here, the contents of lipids and photosynthetic pigments were analyzed in a marine centric diatom, Chaetoceros gracilis. This diatom can be genetically engineered using our previously reported transformation technique and has a potential to produce valuable materials photosynthetically. Sustainable culture conditions for cost-effective production of biological materials under autotrophic conditions with atmospheric carbon dioxide were investigated in the laboratory. A large-scale, open-air culture was also performed.

RESULTS

Cell population doubling time was ~10 h under continuous illumination without CO₂ enrichment, and large amounts of triacylglycerols (TAG) and fucoxanthin accumulated under a wide range of salinity and nutrient conditions, reaching ~200 and 18.5 mg/L, respectively. It was also shown that C. gracilis produced high amounts of TAG without the need for nitrogen or silica deprivation, which is frequently imposed to induce lipid production in many other microalgae. Furthermore, C. gracilis was confirmed to be highly tolerant to changes in environmental conditions, such as salinity. The diatom grew well and produced abundant lipids when using sewage water or liquid fertilizer derived from cattle feces without augmented carbon dioxide. High growth rates (doubling time <20 h) were obtained in a large-scale, open-air culture, in which light irradiance and temperature fluctuated and were largely different from laboratory conditions.

CONCLUSIONS

The ability of this microalga to accumulate TAG without nutrient deprivation, which incurs added labor, high costs, and complicates scalability, is important for low-cost industrial applications. Furthermore, its high tolerance to changes in environmental conditions and high growth rates observed in large-scale, open-air culture implied scalability of this diatom for industrial applications. Therefore, C. gracilis would have great potential as a biofactory.