Marine diatom algae cultivation in simulated dairy wastewater and biomass valorization

Liquid byproducts and organic wastes generated from dairy processing units contribute as the largest source of industrial food wastewater. Though bacteria-mediated treatment strategies are largely implemented, a more effective and innovative management system is needed of the hour. Thus, the current study involves the cultivation of centric diatoms, Chaetoceros gracilis, and Thalassiosira weissflogii in simulated dairy wastewater (SDWW) formulated using varying amounts of milk powder with artificial seawater f/2 media (ASW). The results revealed that cell density and biomass productivity were highest in the 2.5% SDWW treatment cultures of both the strains, the maximum being in C. gracilis (7.5 × 106 cells mL - 1; 21.1 mg L-1 day-1). Conversely, the total carotenoid, chrysolaminarin, and phenol content were negatively impacted by SDWW. However, a considerable enhancement in the total lipid content was reported in the 2.5% SDWW culture of both species. Furthermore, the fatty acid profiling revealed that though the total polyunsaturated fatty acid (PUFA) content was highest in the control setups, the total mono polyunsaturated fatty acid (MUFA) content was higher in the 5% SDWW setups (30.66% in C. gracilis and 33.21% in T. weissflogii). In addition to it, in the cultures utilizing energy from external carbon sources provided by SDWW, the biodiesel produced was also enhanced owing to the heightened cetane number. Thus, the current study evidently highlights the organic carbon acquisition potential of marine diatoms with the scope of providing sustainable biorefinery.

Envisaging the role of pharmaceutical contaminant 17-β estradiol on growth and lipid productivity of marine diatom Chaetoceros gracilis

In the recent past, the presence of steroid hormones in marine bodies has led to the eruption of endocrine - disrupting molecules which have detrimental effects on aquatic life. However, the resilience and robustness of diatoms to adsorb and grow under the multitude of nutrient stress allow them to utilize the plethora of such compounds. Hence, in this study, we have implemented this unique ability of diatoms to sustain on simulated steroidal wastewater made of estradiol pills and analyze their corresponding impact on growth, biomass production, and lipid synthesis. We hereby report that with an increasing concentration of estradiol (0.5-2.0 mg L^-1) there was an increment in cell numbers, and a 1.5-fold increase in the dry cell weight and lipid content (up to 29.5% DW). Thus, culturing Chaetoceros gracilis in the optimized media had a significant impact on biomass productivity which could further promote the untapped potential of diatoms.

Characterization of chloroplast genome of the marine diatom Chaetoceros gracilis

In the present study, the chloroplast genome of Chaetoceros gracilis was sequenced using the PacBio sequencing platform and phylogenetic analysis was conducted using 38 other complete chloroplast genomes of the Bacillariophyta. The chloroplast genome of C. gracilis was 116,421 bp in length with the typical quadripartite structure, including a large single copy (LSC) region of 61,904 bp, a small single copy (SSC) region of 39,367 bp, and a pair of inverted repeats (IR) regions of 7575 bp. The overall GC content of C. gracilis chloroplast genome was 30.79%. This genome encoded 131 genes incuding 93 protein-coding genes, 30 transfer RNA (tRNA) genes and 8 ribosomal RNA (rRNA) genes. Phylogenetic results exhibited that three Chaetoceros species were clustered together. Chaetoceros gracilis was closely related with Chaetoceros muelleri, and then formed a clade with Chaetoceros simplex with 100% bootstrap value This study will facilitate species identification and study of evolutionary in the family Chaetoceroceae.

Evaluation of co-culturing a diatom and a coccolithophore using different silicate concentrations

Globally, the demand for sustainable energy production and high-value biological compounds have become intertwined in an attempt to improve the feasibility of sustainable algal cultivation. Marine microalgae, especially diatoms and coccolithophores, represent viable cultures that can produce biofuels and high-value compounds. Growing them in co-culture offers the potential to produce lipids and pigments, while also generating CaCO₃ for C sequestration. The main objective of this work was to investigate competition or co-existence of the diatom Chaetoceros gracilis and the coccolithophore Pleurochrysis Carterae. The focus was on the effects of silicate and co-culturing on the growth rate, productivity, pigment production, and ash production for C. gracilis and P. carterae in laboratory conditions. The results showed that, in monoculture, 2-mM Si enhanced the specific growth rate of C. gracilis, but did not affect P. carterae. Regardless of silicate concentration, C. gracilis was more productive than P. carterae. In co-culture, P. carterae had a slower growth rate, indicating an inhibitory effect of C. gracilis on P. carterae. Neither silicate concentration nor co-culturing had an impact on the contents of pigments fucoxanthin, chlorophyll-a, and chlorophyll-c, which means that pigment productivity was proportional to biomass productivity. Finally, the ash content increased in all cultures with the lower silicate concentration (0.2 mM) in the medium. With one exception, the ash content was dominated by SiO₂ regardless of silicate amount, and CaCO₃ was a major part of the ash only when P. carterae was grown separately with the higher silicate level. These results highlight that co-culturing did not provide an advantage for improving biomass, pigments, or CaCO₃ productivity.

High Productivity of Eicosapentaenoic Acid and Fucoxanthin by a Marine Diatom Chaetoceros gracilis in a Semi-Continuous Culture

Significantly high eicosapentaenoic acid (EPA) and fucoxanthin contents with high production rate were achieved in semi continuous culture of marine diatom. Effects of dilution rate on the production of biomass and high value biocompounds such as EPA and fucoxanthin were evaluated in semi-continuous cultures of Chaetoceros gracilis under high light condition. Cellular dry weight increased at lower dilution rate and higher light intensity conditions, and cell size strongly affected EPA and fucoxanthin contents. The smaller microalgae cells showed significantly higher (p < 0.05) value of 17.1 mg g-dw^-1 fucoxanthin and 41.5% EPA content per total fatty acid compared to those observed in the larger cells. Chaetoceros gracilis can accumulate relatively higher EPA and fucoxanthin than those reported previously. In addition, maintenance of small cell size by supplying sufficient nutrients and light energy can be the key for the increase production of valuable biocompounds in C. gracilis.

Efficient Transformation of the Diatoms Phaeodactylum tricornutum by Multipulse Electroporation

An efficient nuclear transformation method has been established for the pennate marine diatom Phaeodactylum tricornutum using an electroporation system that drives multisequence pulses to introduce foreign DNAs into the cells. By removing excess salts in the culture medium and optimizing pulse conditions, diatom cells can be transformed with high transformation efficiency. This method is also applicable to other marine diatoms, such as the centric diatom Chaetoceros gracilis. This efficient and stable transformation system will be useful for both functional analysis of diatom-specific genes and for further biotechnological applications.

Biochemical characterization of photosystem I complexes having different subunit compositions of fucoxanthin chlorophyll a/c-binding proteins in the diatom Chaetoceros gracilis

Diatoms are dominant phytoplankton in aquatic environments and have unique light-harvesting apparatus, fucoxanthin chlorophyll a/c-binding protein (FCP). Diatom photosystem I (PSI) interacts with specific FCPs (FCPI); however, it remains unclear how PSI cores receive excitation energy from FCPI. To analyze the energy transfer dynamics, it is necessary to isolate both PSI cores and PSI-FCPI complexes. In this study, we prepared three PSI complexes, which are PSI-FCPI membrane fragments, detergent-solubilized PSI-FCPI supercomplexes and PSI core-like complexes, from the marine centric diatom, Chaetoceros gracilis, and examined their biochemical properties. Both the PSI-FCPI membrane fragments and supercomplexes showed similar subunit compositions including FCPI, whereas the PSI complexes were devoid of most FCPI subunits. The purity and homogeneity of the two detergent-solubilized PSI preparations were verified by clear-native PAGE and electron microscopy. The difference of pigment contents among the three PSI samples was shown by absorption spectra at 77 K. The intensity in the whole spectrum of PSI-FCPI membranes was much higher than those of the other two complexes, while the spectral shape of PSI complexes was similar to that of cyanobacterial PSI core complexes. 77-K fluorescence spectra of the three PSI preparations exhibited different spectral shapes, especially peak positions and band widths. Based on these observations, we discuss the merits of three PSI preparations for evaluating excitation energy dynamics in diatom PSI-FCPI complexes.

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.

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

The major light-harvesting pigment protein complex (fucoxanthin-chlorophyll-binding protein complex; FCP) was purified from a marine centric diatom, Chaetoceros gracilis, by mild solubilization followed by sucrose density gradient centrifugation, and then characterized. The dynamic light scattering measurement showed unimodality, indicating that the complex was highly purified. The amount of chlorophyll a (Chl a) bound to the purified FCP accounted for more than 60 % of total cellular Chl a. The complex was composed of three abundant polypeptides, although there are nearly 30 FCP-related genes. The two major components were identified as Fcp3 (Lhcf3)- and Fcp4 (Lhcf4)-equivalent proteins based on their internal amino acid sequences and a two-dimensional isoelectric focusing electrophoresis analysis developed in this work. Compared with the thylakoids, the FCP complex showed higher contents of fucoxanthin and chlorophyll c but lower contents of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Fluorescence excitation spectra analyses indicated that light harvesting, rather than photosystem protection, is the major function of the purified FCP complex, which is associated with more than 60 % of total cellular Chl a. These findings suggest that the huge amount of Chl bound to the FCP complex composed of Lhcf3, Lhcf4, and an unidentified minor protein has a light-harvesting function to allow efficient photosynthesis under the dim-light conditions in the ocean.