Heterotrophic Carbon Fixation in a Salamander-Alga Symbiosis

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 hours in the light. After 2 hours 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.

Diversity and substrate-specificity of green algae and other micro-eukaryotes colonizing amphibian clutches in Germany, revealed by DNA metabarcoding

“…Oophila within the capsular chamber provide an increase in the partial pressure of oxygen during the day, potentially remove nitrogenous waste, and have been reported to transfer photosynthate to the amphibian embryos (Bachmann et al 1986;Goff and Stein 1978;Graham et al 2013;Kerney 2011;Pinder and Friet 1994), which may lead to acceleration of embryonal development, larger sized embryos, increased viability and hatching success (Gilbert 1942(Gilbert , 1944. However, other studies have reported no measurable exchange of photosynthate from algae to amphibian embryos (Burns et al 2020). Additional effects by the algae, such as a reduction of micro-organisms that are potentially harmful to the host, have been hypothesized (Kim et al 2014) but not yet tested.…”

“…Isolates of this clade have been consistently considered as O. amblystomatis, which is regarded as the numerically most abundant alga in A. maculatum (Jurga et al 2020) and A. gracile (Kerney et al 2019;Marco and Blaustein 2000) egg capsule chambers. These O. amblystomatis isolates have been used in studies of gene expression (Burns et al 2017;Kerney et al 2019), carbon fixation (Burns et al 2020), and host-symbiont fidelity (Kerney et al 2019). As an exception, a few isolates from A. maculatum egg masses, along with environmental samples, were assigned to Chlamydomonas gloeophila; however, C. gloeophila was suggested to represent low abundance green algae occurring in these egg masses that outcompeted Oophila under agar media growth conditions in culture (Kim et al 2014).…”