Physiological benefits and latent effects of an algal-salamander symbiosis

Embryos of the salamander Ambystoma maculatum (Shaw) and the uni-cellular green alga Oophila amblystomatis (Lambert ex Wille) have evolved a resource exchange mutualism. Whereas some of the benefits of the symbiosis to embryos are known, the physiological limitations of the relationship to embryos and carry over or latent effects on larvae are not. To determine the impact of the relationship across life history stages, we measured the growth, survival, and metabolic rate in response to hypoxia of salamander embryos reared under 0-h light (algae absent), 14-h light (control - algae present, fluctuating light conditions) and 24-h light (algae present, chronic light conditions) and the resulting larvae two-weeks post hatch. Embryos reared under 0-h light demonstrated decreased growth and survival compared to 14- and 24-h light, with no effect on metabolic rates or the response of metabolic rates to declining oxygen partial pressure (pO₂). Conversely, larvae from embryos reared under 0-h light exhibited compensatory growth during the two-week larval rearing period, with body sizes matching those from the 14-h light treatment. Larvae from embryos reared under 24-h light had lower wet body mass and LT₅₀ values upon starvation compared to those reared under 14-h light. Coupled with the lowest metabolic rates under normoxic pO₂ levels, this indicates the presence of negative latent effects. We discuss the findings in relation to the effect of the symbiotic relationship on hypoxia tolerance and larval fitness with respect to the presence of compensatory growth and negative latent effects.