Future temperature and salinity do not exert selection pressure on cyst germination of a toxic phytoplankton species

Abstract

Environmental conditions regulate the germination of phytoplankton resting stages. While some factors lead to synchronous germination, others stimulate germination of only a small fraction of the resting stages. This suggests that habitat filters may act on the germination level and thus affect selection of blooming strains. Benthic “seed banks” of the toxic dinoflagellate Alexandrium ostenfeldii from the Baltic Sea are genetically and phenotypically diverse, indicating a high potential for adaptation by selection on standing genetic variation. Here, we experimentally tested the role of climate‐related salinity and temperature as selection filters during germination and subsequent establishment of A. ostenfeldii strains. A representative resting cyst population was isolated from sediment samples, and germination and reciprocal transplantation experiments were carried out, including four treatments: Average present day germination conditions and three potential future conditions: high temperature, low salinity, and high temperature in combination with low salinity. We found that the final germination success of A. ostenfeldii resting cysts was unaffected by temperature and salinity in the range tested. A high germination success of more than 80% in all treatments indicates that strains are not selected by temperature and salinity during germination, but selection becomes more important shortly after germination, in the vegetative stage of the life cycle. Moreover, strains were not adapted to germination conditions. Instead, highly plastic responses occurred after transplantation and significantly higher growth rates were observed at higher temperature. High variability of strain‐specific responses has probably masked the overall effect of the treatments, highlighting the importance of testing the effect of environmental factors on many strains. It is likely that A. ostenfeldii populations can persist in the future, because suitable strains, which are able to germinate and grow well at potential future climate conditions, are part of the highly diverse cyst population. OPEN RESEARCH BADGES This article has earned an Open Data Badge for making publicly available the digitally‐shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.c8c83nr .

Generalist Life Cycle Aids Persistence of Alexandrium ostenfeldii (Dinophyceae) in Seasonal Coastal Habitats of the Baltic Sea

“…A broad temperature range permitting germination is in line with earlier findings, reporting that temperature does not affect germination success between 16°C and 20°C (Jerney et al. ). Lower temperatures reduced germination of A. tamarense from Irish coastal waters, whereas excystment of A. minutum from the same habitat showed no temperature effect with almost equal success between 5°C and 25°C (Ní Rathaille and Raine ).…”

“…, Jerney et al. ). Due to global warming, an overall reduction of sea surface salinity and increase of mean sea surface temperature are expected in the future (e.g., Meier et al.…”

“…These storage conditions were chosen to imitate winter conditions and have been applied successfully earlier (Jerney et al. ). Prior to the experiments ~ 5 mL sub‐samples of the sediment were diluted to 10 mL with sterile local sea water with a salinity of 6 and sonicated for 30 s on constant duty cycle with a frequency of 20 kHz (Brandelin Sonoplus sonicator HD 2200) to detach resting cysts from sediment particles.…”

“…Most viable cysts usually germinate within 2 weeks (Jerney et al. ), but to ascertain that all capable cysts had germinated, the well plates were re‐inspected after 4 weeks.…”

“…Cultured clonal isolates of A. ostenfeldii used in these experiments were established in June to August 2015 (Jerney et al. ). Cultures were grown in 50 mL tissue culture flasks, filled with 40 mL f/2‐Si enriched local sea water at salinity of 6, 16°C–20°C, 100 μmol photons · m −2 · s −1 and a 14:10 hours light:dark cycle until the beginning of the experiment.…”