You are what you eat (or produce): Using stable carbon isotope analysis to quantify mixotrophic feeding in three freshwater Chrysophyte algae

Background/Question/Methods

Mixotrophic phytoplankton play a unique role in aquatic food webs, functioning both as photoautotrophic producers and consumers of bacteria, exogenous DOC compounds, and even other phytoplankton. Given the complexity of these relationships, it has thus far been difficult to quantify directly the degree to which mixotrophs function as either producers or consumers in natural systems. We report on a new technique using stable carbon isotopes (δ¹³C) as tracers to quantify autotrophic vs. heterotrophic growth of three mixotrophic Chrysophyte phytoplankton representing three distinct mixotrophic strategies: first, a primarily heterotrophic species with low levels of photosynthesis (Poterioochromonas malhamensis), second, a strong phototroph, which can substitute high rates of predation when light is limited (Ochromonas danica), and third, a primarily phototrophic “nutrient strategist” mixotroph, which obtains nutrients from its prey, but carbon and energy mainly through photosynthesis (Dinobryon divergens). Each species was grown under a variety of light and nutrient conditions, and offered the bacterium Pseudomonas fluorescens as a food source. By growing P. fluorescens with a δ¹³C value distinct from that of carbon fixed photosynthetically by the algae, the resulting δ¹³C value of the algae was used to differentiate and quantify heterotrophic and autotrophic biomass production within the algae. 

Results/Conclusions

Pseudomonas fluorescens was grown on glucose media (δ¹³C =-11‰) and the resulting bacterial biomass had a similar isotopic composition (-11.6‰). This was substantially higher than the δ¹³C value for strict autotrophic growth of any of the three Chrysophytes used in these experiments.  When grown without bacterial supplementation, D. divergens had a δ¹³C value of -29‰. When offered P. fluorescens bacterial prey under similar growth conditions, the δ¹³C value of D. divergens was enriched to -27‰, indicative of the contribution of bacterial carbon to its newly produced biomass.

Using δ¹³C values of each algae grown under strictly autotrophic and heterotrophic conditions, a mixing model was developed, allowing for the estimation of the degree to which heterotrophic consumption and autotrophic production differentially contributed to newly created algal biomass under a range of conditions. As mixotrophy is well documented in a wide range of algal taxa, but is in most cases poorly quantified, these resulting models provide a powerful tool to connect mixotrophic growth patterns with specific conditions of light, nutrient, and prey availability. This technique is applicable to a variety of species, contributing to a more definitive understanding of the significance of mixotrophy in aquatic trophic dynamics.