Summary Each spring, North American spotted salamander (Ambystoma maculatum) females each lay hundreds of eggs in shallow pools of water. Eggs are surrounded by jelly layers and deposited as large gelatinous masses. Following deposition, masses are penetrated by a mutualistic green alga, Oophila amblystomatis, which enters individual egg capsules, proliferates, and aggregates near the salamander embryo, providing oxygen that enhances development. We examined the effects of population density of intracapsular Oophila on A. maculatum embryos, and show that larger algal populations promote faster embryonic growth and development. Also, we show that carbon fixed by Oophila is transferred to the embryos, providing the first evidence of direct translocation of photosynthate from a symbiont to a vertebrate host.

Protists are traditionally described as either phototrophic or heterotrophic, but studies have indicated that mixotrophic species, organisms that combine both strategies, can have significant impacts on prey populations in marine microbial food webs. While estimates of active mixotroph abundances in environmental samples are determined microscopically by fluorescent particle ingestion, species identification is difficult. We developed SYBR-based qPCR strategies for three Antarctic algal species that we identified as mixotrophic. This method and traditional ingestion experiments were applied to determine the total mixotroph abundance in Antarctic water samples, to ascertain the abundance of known mixotrophic species, and to identify environmental variables that impact the distribution and abundance of these species. Despite differences in sampling locations and years, mixotroph distribution was strongly influenced by season. Environmental variables that best explained variation in the individual mixotroph species abundances included temperature, oxygen, date, fluorescence, conductivity, and latitude. Phosphate was identified as an additional explanatory variable when nutrients were included in the analysis. Utilizing culture-based grazing rates and qPCR abundances, the estimated summed impact on bacterial populations by the three mixotrophs was usually < 2% of the overall mixotrophic grazing, but in one sample, Pyramimonas was estimated to contribute up to 80% of mixotrophic grazing.

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