Integrated organismal responses induced by projected levels of CO2 and temperature exposures in the early life stages of lake sturgeon.

Atmospheric CO2 and temperature are rising concurrently, and may have profound impacts on the transcriptional, physiological and behavioural responses of aquatic organisms. Further, spring snowmelt may cause transient increases of pCO2 in many freshwater systems. We examined the behavioural, physiological and transcriptomic responses of an ancient fish, the lake sturgeon (Acipenser fulvescens) to projected levels of warming and pCO2 during its most vulnerable period of life, the first year. Specifically, larval fish were raised in either low (16°C) or high (22°C) temperature, and/or low (1000 µatm) or high (2500 µatm) pCO2 in a crossed experimental design over approximately 8 months. Following overwintering, lake sturgeon were exposed to a transient increase in pCO2 of 10,000 µatm, simulating a spring melt based on data in freshwater systems. Transcriptional analyses revealed potential connections to otolith formation and reduced growth in fish exposed to high pCO2 and temperature in combination. Network analyses of differential gene expression revealed different biological processes among the different treatments on the edges of transcriptional networks. Na+/K+-ATPase activity increased in fish not exposed to elevated pCO2 during development, and mRNA abundance of the ß subunit was most strongly predictive of enzyme activity. Behavioural assays revealed a decrease in total activity following an acute CO2 exposure. These results demonstrate compensatory and compounding mechanisms of pCO2 and warming dependent on developmental conditions in lake sturgeon. Conserved elements of the cellular stress response across all organisms provide key information for how other freshwater organisms may respond to future climate change.

Influence of ontogenetic development, temperature, and pCO2 on otolith calcium carbonate polymorph composition in sturgeons.

Changes to calcium carbonate (CaCO3) biomineralization in aquatic organisms is among the many predicted effects of climate change. Because otolith (hearing/orientation structures in fish) CaCO3 precipitation and polymorph composition are controlled by genetic and environmental factors, climate change may be predicted to affect the phenotypic plasticity of otoliths. We examined precipitation of otolith polymorphs (aragonite, vaterite, calcite) during early life history in two species of sturgeon, Lake Sturgeon, (Acipenser fulvescens) and White Sturgeon (A. transmontanus), using quantitative X-ray microdiffraction. Both species showed similar fluctuations in otolith polymorphs with a significant shift in the proportions of vaterite and aragonite in sagittal otoliths coinciding with the transition to fully exogenous feeding. We also examined the effect of the environment on otolith morphology and polymorph composition during early life history in Lake Sturgeon larvae reared in varying temperature (16/22 °C) and pCO2 (1000/2500 µatm) environments for 5 months. Fish raised in elevated temperature had significantly increased otolith size and precipitation of large single calcite crystals. Interestingly, pCO2 had no statistically significant effect on size or polymorph composition of otoliths despite blood pH exhibiting a mild alkalosis, which is contrary to what has been observed in several studies on marine fishes. These results suggest climate change may influence otolith polymorph composition during early life history in Lake Sturgeon.

Texture Analysis of Polycrystalline Vaterite Spherulites from Lake Sturgeon Otoliths.

Fish otoliths, or ear bones, are comprised of the CaCO3 polymorphs (aragonite, calcite and vaterite), which can occur either alone or in combination. The polymorph phase abundance in an otolith depends on, as yet, unexplained genetic and environmental factors. Most fish otoliths are comprised of the densest CaCO3 polymorph, aragonite. Sturgeon otoliths, on the other hand, contain significant amounts of the rare and the structurally enigmatic polymorph, vaterite. Sturgeon otoliths are frequently comprised of agglomerations of small microcrystalline vaterite spherulites (<300 µm in diameter), that range in shape from nearly perfect spheres to oblate spheroids. These spherulites are similar to the synthetic vaterite microspheres employed in laser trapping applications. Vaterite spherulites from both hatchery-reared (juvenile) and wild (adult) Lake Sturgeon exhibit extreme crystallographic texture as evidenced by X-ray diffraction patterns and their reconstructed pole-figures determined here. The vaterite crystallites making up the spherulites have excellent registry in both the axial and equatorial directions. Whether synthesized or natural, the texture manifested in these spherulites suggests that vaterite nucleates and grows similarly in vivo otolith formation as well as from laboratory synthesis. The uniaxial optical character of the vaterite spherulites, confirmed by these diffraction experiments and combined with their large birefringence, makes them well suited for laser trapping applications.

Otoliths of sub-adult Lake Sturgeon Acipenser fulvescens contain aragonite and vaterite calcium carbonate polymorphs.

In this study we quantified the percent CaCO3 polymorph composition in otoliths of larval and juvenile Lake Sturgeon Acipenser fulvescens via X-ray microdiffraction. Sagittal otoliths of sub-adults were primarily composed of aragonite (> 90%) while the lapilli otoliths were 100% vaterite. This is the first time the presence of aragonite in otoliths has been reported in an acipenseriform and is surprising given that the ability to form aragonite otoliths was not thought to have evolved until the separation of teleost and holostean species from other Actinopterygian fishes (e.g., sturgeon, paddlefish, gar).