Cryopreservation of economically valuable marine micro-algae in the classes Bacillariophyceae, Chlorophyceae, Cyanophyceae, Dinophyceae, Haptophyceae, Prasinophyceae, and Rhodophyceae

A comprehensive review on versatile microalga Tetraselmis: Potentials applications in wastewater remediation and bulk chemical production

Microalgae are considered sustainable resources for the production of biofuel, feed, and bioactive compounds. Among various microalgal genera, the Tetraselmis genus, containing predominantly marine microalgal species with wide tolerance to salinity and temperature, has a high potential for large-scale commercialization. Until now, Tetraselmis sp. are exploited at smaller levels for aquaculture hatcheries and bivalve production. However, its prolific growth rate leads to promising areal productivity and energy-dense biomass, so it is considered a viable source of third-generation biofuel. Also, microbial pathogens and contaminants are not generally associated with Tetraselmis sp. in outdoor conditions due to faster growth as well as dominance in the culture. Numerous studies revealed that the metabolite compositions of Tetraselmis could be altered favorably by changing the growth conditions, taking advantage of its acclimatization or adaptation ability in different conditions. Furthermore, the biorefinery approach produces multiple fractions that can be successfully upgraded into various value-added products along with biofuel. Overall, Tetraselmis sp. could be considered a potential strain for further algal biorefinery development under the circular bioeconomy framework. In this aspect, this review discusses the recent advancements in the cultivation and harvesting of Tetraselmis sp. for wider application in different sectors. Furthermore, this review highlights the key challenges associated with large-scale cultivation, biomass harvesting, and commercial applications for Tetraselmis sp.

Assessment of the recovery and photosynthetic efficiency of Breviolum psygmophilum and Effrenium voratum (Symbiodiniaceae) following cryopreservation

Many strains of Symbiodiniaceae have been isolated and their genetics, taxonomy, and metabolite production studied. Maintaining these cultures requires careful and regular sub-culturing that is costly with a high risk of species contamination or loss. Cryopreservation is a viable alternative for their long-term storage; however, there is uncertainty as to whether cryopreservation impacts the photosynthetic performance of Symbiodiniaceae. We investigated the growth rates and photosynthetic efficiency of two species, Breviolum psygmophilum and Effrenium voratum before and after cryopreservation. Rapid light curves (RLCs) produced using Pulse Amplitude Modulated (PAM) fluorometry were used to generate detailed information on the characteristics of photosystem II (PSII). The maximum electron transport rate (ETRmax) and the quantum yield (F_v/F_m) of the control (non-cryopreserved) and cryopreserved culture isolates were assessed across the growth cycle. The non-cryopreserved isolate of B. psygmophilum had a higher quantum yield than the cryopreserved isolate from day 12 to day 24, whereas there were no differences from day 28 to the late stationary phase. There were no significant differences in ETRmax. No significant differences were observed in quantum yield or ETRmax between the control and cryopreserved E. voratum isolates. The ability of cryopreserved strains to recover and regain their photosynthetic efficiency after freezing demonstrates the utility of this method for the long-term storage of these and other Symbiodiniaceae species.

Cryopreservation of six Symbiodiniaceae genera and assessment of fatty acid profiles in response to increased salinity treatments

Symbiodiniaceae are a diverse group of dinoflagellates, the majority of which are free-living and/or associated with a variety of protists and other invertebrate hosts. Maintenance of isolated cultures is labour-intensive and expensive, and cryopreservation provides an excellent avenue for their long-term storage. We aimed to cryopreserve 15 cultured isolates from six Symbiodiniaceae genera using dimethyl sulfoxide (DMSO) as the cryoprotectant agent (CPA). Under 15% DMSO, 10 isolates were successfully cryopreserved using either rapid freezing or controlled-rate freezing. Cultures that failed or had low survival, were subjected to (1) a reduction of CPA to 10%, or (2) increased salinity treatment before freezing. At 10% DMSO, three further isolates were successfully cryopreserved. At 15% DMSO there were high cell viabilities in Symbiodinium pilosum treated with 44 parts per thousand (ppt) and 54 ppt culture medium. An isolate of Fugacium sp. successfully cryopreserved after salinity treatments of 54 ppt and 64 ppt. Fatty acid (FA) analyses of S. pilosum after 54 ppt salinity treatment showed increased saturated FA levels, whereas Fugacium sp. had low poly-unsaturated FAs compared to normal salinity (34 ppt). Understanding the effects of salinity and roles of FAs in cryopreservation will help in developing protocols for these ecologically important taxa.

Cryoprotectant treatment tests on three morphologically diverse marine dinoflagellates and the cryopreservation of Breviolum sp. (Symbiodiniaceae)

Dinoflagellates are among the most diverse group of microalgae. Many dinoflagellate species have been isolated and cultured, and these are used for scientific, industrial, pharmaceutical, and agricultural applications. Maintaining cultures is time-consuming, expensive, and there is a risk of contamination or genetic drift. Cryopreservation offers an efficient means for their long-term preservation. Cryopreservation of larger dinoflagellate species is challenging and to date there has been only limited success. In this study, we explored the effect of cryoprotectant agents (CPAs) and freezing methods on three species: Vulcanodinium rugosum, Alexandrium pacificum and Breviolum sp. A total of 12 CPAs were assessed at concentrations between 5 and 15%, as well as in combination with dimethyl sulfoxide (DMSO) and other non-penetrating CPAs. Two freezing techniques were employed: rapid freezing and controlled-rate freezing. Breviolum sp. was successfully cryopreserved using 15% DMSO. Despite exploring different CPAs and optimizing the freezing techniques, we were unable to successfully cryopreserve V. rugosum and A. pacificum. For Breviolum sp. there was higher cell viability (45.4 ± 2.2%) when using the controlled-rate freezing compared to the rapid freezing technique (10.0 ± 2.8%). This optimized cryopreservation protocol will be of benefit for the cryopreservation of other species from the family Symbiodiniaceae.

Effect of Cryopreservation on Proteins from the Ubiquitous Marine Dinoflagellate Breviolum sp. (Family Symbiodiniaceae)

Coral reefs around the world are exposed to thermal stress from climate change, disrupting the delicate symbiosis between the coral host and its symbionts. Cryopreservation is an indispensable tool for the preservation of species, as well as the establishment of a gene bank. However, the development of cryopreservation techniques for application to symbiotic algae is limited, in addition to the scarceness of related studies on the molecular level impacts post-thawing. Hence, it is essential to set up a suitable freezing protocol for coral symbionts, as well as to analyze its cryo-injury at the molecular level. The objective of this study was to develop a suitable protocol for the coral symbiont Breviolum subjected to two-step freezing. The thawed Breviolum were then cultured for 3, 7, 14, and 28 days before they were analyzed by Western blot for protein expression, light-harvesting protein (LHP), and red fluorescent protein (RFP) and tested by adenosine triphosphate bioassay for cell viability. The results showed the highest cell viability for thawed Breviolum that was treated with 2 M propylene glycol (PG) and 2 M methanol (MeOH) and equilibrated with both cryoprotectants for 30 min and 20 min. Both treatment groups demonstrated a significant increase in cell population after 28 days of culture post-thawing, especially for the MeOH treatment group, whose growth rate was twice of the PG treatment group. Regarding protein expression, the total amounts of each type of protein were significantly affected by cryopreservation. After 28 days of culture, the protein expression for the MeOH treatment group showed no significant difference to that of the control group, whereas the protein expression for the PG treatment group showed a significant difference. Breviolum that were frozen with MeOH recovered faster upon thawing than those frozen with PG. LHP was positively and RFP was negatively correlated with Symbiodiniaceae viability and so could serve as health-informing biomarkers. This work represents the first time to document it in Symbiodiniaceae, and this study established a suitable protocol for the cryopreservation of Breviolum and further refined the current understanding of the impact of low temperature on its protein expression. By gaining further understanding of the use of cryopreservation as a way to conserve Symbiodiniaceae, we hope to make an effort in the remediation and conservation of the coral reef ecosystem and provide additional methods to rescue coral reefs.

Novel method enabling a rapid vitality determination of cyanobacteria

Cyanobacteria represent a large group of bacteria with underestimated scientific potential. Recent studies indicate them as a great reservoir of secondary metabolites with antifungal, antiviral or antibacterial activity. However, common, well established research techniques cannot be easily adapted to these organisms. Slow growth rates and irregular cell aggregates constitute challenges for researchers dealing with cyanobacteria. In this work, we present an innovative new method enabling a quick, easy and economical vitality determination of cyanobacterial strains, as, e.g. required for the finding of optimal cryopreservation conditions. We were able to measure the vitality of previously cryopreserved and defrosted Trichocoleus sociatus samples within 45 min by means of their O2-production. For each run, a cell wet mass of only 0.5 g was required. By application of this method, we could find DMSO (5% v/v) and glycerin (15% v/v) to be the most promising cryoprotectants for the conservation of T. sociatus cells. DMSO and glycerin guaranteed a vitality rate of 80-90% and 60-70% after up to four weeks of cryopreservation, compared to fresh cell material.

Assessing the impacts of cryopreservation on the mitochondria of a thermotolerant Symbiodinium lineage: Implications for reef coral conservation

Earth's coral reefs are threatened by a barrage of anthropogenic insults, and cryopreservation-based conservation measures are warranted. Successfully cryopreserved corals could then thawed and out-planted on reefs when ocean temperatures stabilize. In such efforts, it will be necessary to also cryopreserve the photosynthetic dinoflagellates (genus Symbiodinium) that reside within the corals' gastrodermal cells. Given this need, Symbiodinium (clade D) cells were cryopreserved in 2 M propylene glycol by a two-step freezing protocol herein and then cultured for 42 days post-thaw. To gauge the effect of cryopreservation, mitochondrial DNA content and intracellular ATP concentration were assessed, and the former parameter was nearly 2-fold higher in freeze-thawed cells compared to controls after 14 days of post-thaw culture. In contrast, intracellular ATP concentration was relatively lower in freeze-thawed cells after seven days of post-thaw culture, though returned to control levels in samples cultured for 42 days post-thaw.

Cryopreservation of Paramecium bursaria Chlorella Virus-1 during an active infection cycle of its host

Best practices in laboratory culture management often include cryopreservation of microbiota, but this can be challenging with some virus particles. By preserving viral isolates researchers can mitigate genetic drift and laboratory-induced selection, thereby maintaining genetically consistent strains between experiments. To this end, we developed a method to cryopreserve the model, green-alga infecting virus, Paramecium bursaria Chlorella virus 1 (PBCV-1). We explored cryotolerance of the infectivity of this virus particle, whereby freezing without cryoprotectants was found to maintain the highest infectivity (~2.5%). We then assessed the cryopreservation potential of PBCV-1 during an active infection cycle in its Chlorella variabilis NC64A host, and found that virus survivorship was highest (69.5 ± 16.5%) when the infected host is cryopreserved during mid-late stages of infection (i.e., coinciding with virion assembly). The most optimal condition for cryopreservation was observed at 240 minutes post-infection. Overall, utilizing the cell as a vehicle for viral cryopreservation resulted in 24.9–30.1 fold increases in PBCV-1 survival based on 95% confidence intervals of frozen virus particles and virus cryopreserved at 240 minutes post-infection. Given that cryoprotectants are often naturally produced by psychrophilic organisms, we suspect that cryopreservation of infected hosts may be a reliable mechanism for virus persistence in non-growth permitting circumstances in the environment, such as ancient permafrosts.

Expanding the toolbox for cryopreservation of marine and freshwater diatoms

Diatoms constitute the most diverse group of microalgae and have long been recognised for their large biotechnological potential. In the wake of growing research interest in new model species and development of commercial applications, there is a pressing need for long-term preservation of diatom strains. While cryopreservation using dimethylsulfoxide (DMSO) as a cryoprotective agent is the preferred method for long-term strain preservation, many diatom species cannot be successfully cryopreserved using DMSO. Therefore, in this study, we studied cryopreservation success in six different diatom species, representing the major morphological and ecological diatom groups, using a range of DMSO concentrations and Plant Vitrification Solution 2 (PVS2) as an alternative cryoprotectant to DMSO. In addition, we tested whether suppressing bacterial growth by antibiotics accelerates the post-thaw recovery process. Our results show that the effects of cryoprotectant choice, its concentration and the addition of antibiotics are highly species specific. In addition, we showed that PVS2 and antibiotics are useful agents to optimize cryopreservation of algae that cannot survive the traditional cryopreservation protocol using DMSO. We conclude that a species-specific approach will remain necessary to develop protocols for diatom cryopreservation and to increase their representation in public culture collections.

Effect of Marine-Derived Ice-Binding Proteins on the Cryopreservation of Marine Microalgae

Ice-binding protein (IBPs) protect cells from cryo-injury during cryopreservation by inhibiting ice recrystallization (IR), which is a main cause of cell death. In the present study, we employed two IBPs, one, designated LeIBP from Arctic yeast, and the other, designated FfIBP from Antarctic sea ice bacterium, in the cryopreservation of three economically valuable marine microalgae, Isochrysis galbana, Pavlova viridis, and Chlamydomonas coccoides. Both of the IBPs showed IR inhibition in f/2 medium containing 10% DMSO, indicating that they retain their function in freezing media. Microalgal cells were frozen in 10% DMSO with or without IBP. Post-thaw viability exhibited that the supplementation of IBPs increased the viability of all cryopreserved cells. LeIBP was effective in P. viridis and C. coccoides, while FfIBP was in I. galbana. The cryopreservative effect was more drastic with P. viridis when 0.05 mg/mL LeIBP was used. These results clearly demonstrate that IBPs could improve the viability of cryopreserved microalgal cells.

Cryobanking of aquatic species

This review is focused on the applications of genome cryobanking of aquatic species including freshwater and marine fish, as well as invertebrates. It also reviews the latest advances in cryobanking of model species, widely used by the scientific community worldwide, because of their applications in several fields. The state of the art of cryopreservation of different cellular types (sperm, oocytes, embryos, somatic cells and primordial germ cells or early spermatogonia) is discussed focusing on the advantages and disadvantages of each procedure according to different applications. A special review on the need of standardization of protocols has also been carried out. In summary, this comprehensive review provides information on the practical details of applications of genome cryobanking in a range of aquatic species worldwide, including the cryobanks established in Europe, USA, Brazil, Australia and New Zealand, the species and type of cells that constitute these banks and the utilization of the samples preserved.

STATEMENT OF RELEVANCE

This review compiles the last advances on germplasm cryobanking of freshwater and marine fish species and invertebrates, with high value for commercial aquaculture or conservation. It is reviewed the most promising cryopreservation protocols for different cell types, embryos and larvae that could be applied in programs for genetic improvement, broodstock management or conservation of stocks to guarantee culture production.

Bioremediation efficacy-comparison of nutrient removal from an anaerobic digest waste-based medium by an algal consortium before and after cryopreservation

An algal consortium was isolated from an integrated steelmaking site at TATA Steel Strip Products Ltd. in Port Talbot, UK, and its bioremediation capacity tested. Excellent "bioremediation" was observed when the mixed culture was "applied" to diluted effluent from an enhanced anaerobic digestion plant at Dŵr Cymru Welsh Water at Port Talbot, UK. After 5 days of cultivation in a 600-L photobioreactor, 99% of the total nitrogen (initial level, 4500 μmol L-1) and total phosphorus (initial level, 690.4 μmol L-1) were removed from the waste stream. The consortium was deposited in the Culture Collection of Algae and Protozoa (CCAP), an international depository authority for microalgal patents, as CCAP 293/1. This material has been successfully cryopreserved using a two-step cryopreservation protocol with dimethyl sulphoxide (5% v/v) used as a cryoprotectant. On recovery of samples after 3 months storage at -196 °C, the specific bioremediation activity of the revived consortium was tested. The capacity of the revived culture to bioremediate effluent was not significantly different (p < 0.05) from a non-cryopreserved control, with 99% of total nitrogen and phosphorus remediated by day 4. Although non-axenic algal cultures have previously been cryopreserved, this is the first report of the successful cryopreservation of mixed algal consortium, with validation of its ability to bioremediate after thawing comparing non-cryopreserved cultures with a revived post-thaw algal consortium. The study also highlights the need to ensure the long-term security and the requirement to validate the functionality of conserved inocula with biotechnological/bioremediation potential.

Challenges for the Maintenance and Cryopreservation of Multiple Isolates of Model Microorganisms: An Example Using the Marine Diatom Skeletonema marinoi

Modern genomic and metabolomic tools have provided the possibility of generating and interrogating large datasets that can provide answers to previously imponderable taxonomic, evolutionary, ecological, and physiological questions. However, the curatorial tools needed to provide and maintain the relevant biological resources on which new knowledge can be built have not kept pace with this meteoric rise in scientific capacity, its associated activity, or the huge increase in published science. The availability of biological material of guaranteed identity and quality in Biological Resource Centers is fundamental for scientific research, but it crucially depends on there being adequate preservation/maintenance methods that are capable of ensuring phenotypic, genotypic, and functional security of the biological material(s). This article highlights the challenges to the long-term maintenance of genetic resources in general, focusing specifically on the issues associated with the maintenance of a large collection of strains of the ecologically significant diatom Skeletonema marinoi. This research collection, held at the Department of Marine Sciences, University of Gothenburg, has been systematically tested for its capacity to survive cryopreservation. A method, involving incubation in the dark for 20-24 hours before cryopreservation, followed by cryoprotection employing 10% dimethysulphoxide (DMSO) and conventional cooling in a passive cooler, before plunging into liquid nitrogen was successfully applied to ∼80% of the strains tested. In addition, the growth characteristics of exemplar strains were confirmed after storage.

Cryobiology: principles, species conservation and benefits for coral reefs

Coral reefs are some of the oldest, most diverse and valuable ecosystems on Earth because they can support one-quarter of all marine life in our oceans. Despite their importance, the world’s coral reefs continue to be degraded at unprecedented rates by local and global threats that are warming and creating a more acidic ocean. This paper explores the reproductive challenges of coral for ex situ conservation, using IVF and cryopreservation, and our practical biobanking methods. Coral present challenges for cryopreservation because their reproductive period is often limited to a few nights yearly, they are mostly hermaphrodites with diverse modes of reproduction, including asexual reproduction (i.e. fragmentation and parthenogenesis) and sexual reproduction (i.e. self- and cross-fertilisation) and they express physiological toxins that can inhibit cryopreservation. We have banked spermatozoa from 12 coral species using the same field-hardy methods and have created new coral with thawed spermatozoa. In addition, we describe the cryopreservation of coral symbionts, whose physiology only permits the highest success seasonally. As part of a multidisciplinary conservation strategy, these collections may provide a major hedge against extinction for corals facing the damaging effects of climate change and loss of genetic diversity, and promise to help offset threats to our reefs worldwide.

Seasonal Preservation Success of the Marine Dinoflagellate Coral Symbiont, Symbiodinium sp

Coral reefs are some of the most diverse and productive ecosystems on the planet, but are threatened by global and local stressors, mandating the need for incorporating ex situ conservation practices. One approach that is highly protective is the development of genome resource banks that preserve the species and its genetic diversity. A critical component of the reef are the endosymbiotic algae, Symbiodinium sp., living within most coral that transfer energy-rich sugars to their hosts. Although Symbiodinium are maintained alive in culture collections around the world, the cryopreservation of these algae to prevent loss and genetic drift is not well-defined. This study examined the quantum yield physiology and freezing protocols that resulted in survival of Symbiodinium at 24 h post-thawing. Only the ultra-rapid procedure called vitrification resulted in success whereas conventional slow freezing protocols did not. We determined that success also depended on using a thin film of agar with embedded Symbiodinium on Cryotops, a process that yielded a post-thaw viability of >50% in extracted and vitrified Symbiodinium from Fungia scutaria, Pocillopora damicornis and Porites compressa. Additionally, there also was a seasonal influence on vitrification success as the best post-thaw survival of F. scutaria occurred in winter and spring compared to summer and fall (P < 0.05). These findings lay the foundation for developing a viable genome resource bank for the world’s Symbiodinium that, in turn, will not only protect this critical element of coral functionality but serve as a resource for understanding the complexities of symbiosis, support selective breeding experiments to develop more thermally resilient strains of coral, and provide a ‘gold-standard’ genomics collection, allowing for full genomic sequencing of unique Symbiodinium strains.

Comparison of cryopreservation methods for the long term storage of the marine diatom Haslea ostrearia (simonsen)

Long term maintenance of microalgal strains by serial subculturing is often expensive and time-consuming. Alternative methods, such as cryopreservation, present several benefits and thus seem more relevant. Our study aimed at comparing two cryopreservation procedures applied to the marine diatom Haslea ostrearia (Simonsen): (1) a two-step freezing method in liquid media using 5%, 10% and 20% MeOH, Me₂SO or Glycerol, and (2) an immobilization-dehydration method consisting in an algal cell entrapped in 0.7 M sucrose dehydrated and air-flow desiccated calcium alginate beads before "direct" or "two-step" freezing. Our results showed that the cryopreservation of H. ostrearia was feasible. With the two-step freezing protocol only Me₂SO maintained cell viability without contamination but the low percentage of viability (<10%) prevents its use. Conversely, the immobilization-dehydration methods tested in this study were effective. Average viability of 57% and 77% were obtained with the "direct" and the "two step" cooling assays respectively, ensuring preservation of the genetic traits of H. ostrearia.

Bioprospecting for hyper-lipid producing microalgal strains for sustainable biofuel production

Global petroleum reserves are shrinking at a fast pace, increasing the demand for alternate fuels. Microalgae have the ability to grow rapidly, and synthesize and accumulate large amounts (approximately 20-50% of dry weight) of neutral lipid stored in cytosolic lipid bodies. A successful and economically viable algae based biofuel industry mainly depends on the selection of appropriate algal strains. The main focus of bioprospecting for microalgae is to identify unique high lipid producing microalgae from different habitats. Indigenous species of microalgae with high lipid yields are especially valuable in the biofuel industry. Isolation, purification and identification of natural microalgal assemblages using conventional techniques is generally time consuming. However, the recent use of micromanipulation as a rapid isolating tool allows for a higher screening throughput. The appropriate media and growth conditions are also important for successful microalgal proliferation. Environmental parameters recorded at the sampling site are necessary to optimize in vitro growth. Identification of species generally requires a combination of morphological and genetic characterization. The selected microalgal strains are grown in upscale systems such as raceway ponds or photobireactors for biomass and lipid production. This paper reviews the recent methodologies adopted for site selection, sampling, strain selection and identification, optimization of cultural conditions for superior lipid yield for biofuel production. Energy generation routes of microalgal lipids and biomass are discussed in detail.

Physiology and cryosensitivity of coral endosymbiotic algae (Symbiodinium)

Coral throughout the world are under threat. To save coral via cryopreservation methods, the Symbiodinium algae that live within many coral cells must also be considered. Coral juvenile must often take up these important cells from their surrounding water and when adult coral bleach, they lose their endosymbiotic algae and will die if they are not regained. The focus of this paper was to understand some of the cryo-physiology of the endosymbiotic algae, Symbiodinium, living within three species of Hawaiian coral, Fungia scutaria, Porites compressa and Pocillopora damicornis in Kaneohe Bay, Hawaii. Although cryopreservation of algae is common, the successful cryopreservation of these important coral endosymbionts is not common, and these species are often maintained in live serial cultures within stock centers worldwide. Freshly-extracted Symbiodinium were exposed to cryobiologically appropriate physiological stresses and their viability assessed with a Pulse Amplitude Fluorometer. Stresses included sensitivity to chilling temperatures, osmotic stress, and toxic effects of various concentrations and types of cryoprotectants (i.e., dimethyl sulfoxide, propylene glycol, glycerol and methanol). To determine the water and cryoprotectant permeabilities of Symbiodinium, uptake of radio-labeled glycerol and heavy water (D(2)O) were measured. The three different Symbiodinium subtypes studied demonstrated remarkable similarities in their morphology, sensitivity to cryoprotectants and permeability characteristics; however, they differed greatly in their sensitivity to hypo- and hyposmotic challenges and sensitivity to chilling, suggesting that standard slow freezing cryopreservation may not work well for all Symbiodinium. An appendix describes our H(2)O:D(2)O water exchange experiments and compares the diffusionally determined permeability with the two parameter model osmotic permeability.