Project: Investigating Mixotrophic Algal Contribution to Copepod Diet and Reproduction

NSF Award Abstract:
A primary topic of interest within biological oceanography is the role of algal productivity in the global carbon cycle. Over the past few decades, the traditional planktonic food web has been undergoing revision with a better understanding of the significance of microbes. Many of the single-celled organisms at the base of marine food webs are mixotrophic, meaning they are capable of both photosynthesis and ingestion of prey within a single cell. Though recognized for over a century, this trophic strategy has gone from being considered a physiologically unfavorable oddity to a diverse and widespread adaptation. Despite the prevalence of mixotrophy in marine systems, very few experimental studies have assessed how mixotrophic organisms interact with and impact the zooplankton that ingest them. This project is helping define mixotrophic contributions to higher trophic levels in marine pelagic food webs. The investigators are testing whether mixotrophs can support copepod reproduction under conditions when algal food is of poor quality and assessing mixotrophic contributions to the diet of two abundant copepod species within the Gulf of Maine. Additionally, recent modeling efforts suggest that when incorporated into biogeochemical and food web models, mixotrophy has significant implications for primary productivity and carbon cycling. To facilitate broader consideration of mixotrophy in models, the investigators are participating in a mixotrophy working group and hosting a session at the Ocean Sciences Meeting. In addition, the investigators are hosting undergraduate summer students who are gaining experience with laboratory work, including through the Woods Hole Partnership Education Program (PEP) and guest students recruited from local communities to increase diversity in marine science. The investigators are also partnering with local high school teachers to have students use their art training to contribute to a graphic novel that illustrates the Gulf of Maine food web and potential effects of climate change.

Effective transfer of carbon and nutrients to higher trophic levels in the marine environment relies on the planktonic food web. Inclusion of mixotrophy in food web models suggests that it may stabilize fluctuations in the nutritional quality of microbial prey and could be an important pathway for transfer of nutrients into zooplankton However, empirical data have been lacking to parameterize these models adequately. To date, empirical studies of mixotrophs as prey have suffered from testing a limited number of taxa, confounding effects of prey abundance, and a failure to characterize their mixotrophic status. Similarly, few field studies have assessed the diet of mesozooplankton in a way that would allow mixotrophs to be identified, and as in lab studies, the mixotrophic status of prey sources has not been identified. Thus, it has been impossible to provide realistic parameters describing the contribution of mixotrophs to nutrient transfer through the food web, and the issue of whether mixotrophically-growing algae can mitigate the effects of poor-quality photosynthetic food remains largely unresolved. This study is characterizing the role of mixotrophic algae in mitigating deficiencies in phytoplankton food quality with respect to copepod reproduction. Feeding experiments are being conducted that include consistent quantities of actively-mixotrophic prey. Analysis of the nutrient and fatty acid composition of different mixotrophic algae is illustrating a continuum of prey quality that could help inform zooplankton model predictions. The project is exploring effects of nutritional plasticity by contrasting the nutritional quality of heterotrophs reared on bacteria versus phytoplankton. The work is contributing to knowledge of the ability of mixotrophs to supplement or support copepod reproduction, which can coincide with poor phototrophic food availability. Finally, the presence and identity of active mixotrophs in the field is being assessed using labeled-prey ingestion experiments, followed by amplicon sequencing of labeled grazer DNA and environmental water samples. The work is identifying the in situ grazing preferences of copepods in the Gulf of Maine over a growing season, whether it varies with life stage, and how it impacts egg production.