Abstract
The unique symbiosis between a vertebrate salamander, Ambystoma maculatum, and unicellular green alga, Oophila amblystomatis, involves multiple modes of interaction. These include an ectosymbiotic interaction where the alga colonizes the egg capsule, and an intracellular interaction where the alga enters tissues and cells of the salamander. One common interaction in mutualist photosymbioses is the transfer of photosynthate from the algal symbiont to the host animal. In the A. maculatum–O. amblystomatis interaction, there is conflicting evidence regarding whether the algae in the egg capsule transfer chemical energy captured during photosynthesis to the developing salamander embryo. In experiments where we took care to separate the carbon fixation contributions of the salamander embryo and algal symbionts, we show that inorganic carbon fixed by A. maculatum embryos reaches 2% of the inorganic carbon fixed by O. amblystomatis algae within an egg capsule after 2 h in the light. After 2 h in the dark, inorganic carbon fixed by A. maculatum embryos is 800% of the carbon fixed by O. amblystomatis algae within an egg capsule. Using photosynthesis inhibitors, we show that A. maculatum embryos and O. amblystomatis algae compete for available inorganic carbon within the egg capsule environment. Our results confirm earlier studies suggesting a role of heterotrophic carbon fixation during vertebrate embryonic development. Our results also show that the considerable capacity of developing A. maculatum embryos for inorganic carbon fixation precludes our ability to distinguish any minor role of photosynthetically transferred carbon from algal symbionts to host salamanders using bicarbonate introduced to the egg system as a marker.
Highlights
During embryonic development, egg capsules of multiple amphibian species found in the Northern Hemisphere are colonized by a green alga, Oophila amblystomatis
Decapsulated Embryos incubated in the dark exhibited significantly more 14C-bicarbonate incorporation than total algae from whole eggs incubated in the dark (p < 0.01) indicating that the elevated radioactive signal in the embryos could not have come from residual algae on or near the dark incubated embryos (Figure 2A)
To explicitly test whether residual algae that persisted through wash steps could account for the fixed carbon signal from embryos, 14C-bicarbonate or pre-labeled O. amblystomatis cultures were added to live or glutaraldehyde killed A. maculatum embryos
Summary
Egg capsules of multiple amphibian species found in the Northern Hemisphere are colonized by a green alga, Oophila amblystomatis. One intriguing possibility is that the intracapsular Oophila fixes carbon from the atmosphere, uses energy from the sun to build fixed carbon into energy storage molecules like carbohydrates, and transfers that chemical energy to the salamander by exporting metabolically active compounds (Hammen and Hutchison, 1962; Goff and Stein, 1978; Graham et al, 2013, 2014) Such a mechanism is at play in other animal-alga photosymbioses, such as the coral-dinoflagellate mutualism, where the photosymbiont captures and transfers energy to the animal in the form of sugars and sugar alcohols like glucose and glycerol in nutrient-poor waters (Venn et al, 2008; Tremblay et al, 2012; Raven, 2017). None of these parallel naturally occurring animal-algal photosymbioses include a vertebrate host
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