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

Fatty acid production and associated gene pathways are altered by increased salinity and dimethyl sulfoxide treatments during cryopreservation of Symbiodinium pilosum (Symbiodiniaceae)

The Symbiodinium genus is ancestral among other Symbiodiniaceae lineages with species that are both symbiotic and free living. Changes in marine ecosystems threaten their existence and crucial ecological roles. Cryopreservation offers an avenue for their long-term storage for future habitat restoration after coral bleaching. In our previous study we demonstrated that high salinity treatments of Symbiodiniaceae isolates led to changes in their fatty acid (FA) profiles and higher cell viabilities after cryopreservation. In this study, we investigated the role of increased salinity on FA production and the genes involved in FA biosynthesis and degradation pathways during the cryopreservation of Symbiodinium pilosum. Overall, there was a twofold increase in mass of FAs produced by S. pilosum after being cultured in medium with increased salinity (54 parts per thousand; ppt). Dimethyl sulfoxide (Me2SO) led to a ninefold increase of FAs in standard salinity (SS) treatment, compared to a fivefold increase in increased salinity (IS) treatments. The mass of the FA classes returned to baseline during recovery. Transcriptomic analyses showed an acyl carrier protein gene was significantly upregulated after Me2SO treatment in the SS cultures. Cytochrome P450 reductase genes were significantly down regulated after Me2SO addition in SS treatment preventing FA degradation. These changes in the expression of FA biosynthesis and degradation genes contributed to more FAs in SS treated isolates. Understanding how increased salinity changes FA production and the roles of specific genes in regulating FA pathways will help improve current freezing protocols for Symbiodiniaceae and other marine microalgae.

Cryopreservation of the Microalgae Scenedesmus sp

Each phytoplankton species presents a different behavior and tolerance to the cryopreservation process. Therefore, in a species-specific protocol, it is essential to ensure both growth and post-thawing cell viability. In this study, we explored the effect of cryopreservation of Scenedesmus sp. with two cryoprotectants, dimethyl sulfoxide (DMSO) and methanol (MET), at 5% and 10% inclusion for each. In the control treatment, the microalgae were not exposed to cryoprotective agents (Control). Three post-thawing cell viability criteria were used: no cell damage (NCD), cell damage (CD), and marked lesions (LM), and mitochondrial and cell membrane damage was evaluated by flow cytometry. The study was a 2 × 2 factorial design, with five replications by treatments, population growth, and cell damage evaluated from the fifth day after thawing. On the fifth day, the highest percentage of NCD was observed when the microalgae were cryopreserved with DMSO 5% (50%); Regarding the control group, it showed 0% NCD. Flow cytometry analysis reveals minor damage at the membrane and mitochondria (9-10.7%) when DMSO is used at both inclusion percentages (5-10%) after thawing. In the exponential phase, the highest growth rates, doubling time, and yield was observed in cryopreserved cells with MET 5%. The results suggest that DMSO 5% is an ideal treatment for cryopreserving microalgae Scenedesmus 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.