Mixotrophic protists, capable of both autotrophy and phagotrophy, are pivotal yet understudied components of aquatic microbial food webs. Through this unique trophic behavior mixotrophs are able to optimize nutrient and food uptake and likely enhance primary production and energy transfer. Furthermore, mixotrophs significantly contribute to biogeochemical cycles and stabilize food webs, supporting higher trophic levels. Their distribution and diversity are primarily driven by light, nutrient and prey availability but also respond to a multitude of factors such as temperature, salinity, alkalinity or turbulence. They exhibit a remarkable range of strategies and functions, allowing them to thrive in various marine and freshwater environments. However, metabolic trade-offs triggered by environmental drivers are not well understood, making it difficult to predict their diversity, distribution and role in aquatic systems.

As we strive to take the pulse of our aquatic environments, understanding the dynamics and functions of mixotrophs and their contributions to marine food webs, becomes vital. Anthropogenic pressure on aquatic environments and climate change may trigger changes in mixotroph metabolism and their prevalence in the plankton community, which would alter their role in the food web and biogeochemical processes. However, mixotrophs metabolic activity and phototrophy/phagotrophy resource acquisition balance still remains methodologically challenging to study in situ. For example, ocean acidification is suspected to shift mixotroph behavior toward photoautotrophy and help capture anthropogenic CO₂ on the one hand, while global warming is suspected to favor phagotrophy, thus releasing more CO₂ in the atmosphere on the other hand. Yet none of these responses are well understood. Resolving the tight interplay between metabolic processes, environmental drivers, and food web interactions of mixotrophs will be crucial to assess the future health of our aquatic systems.

This session aims to shed light on the complex roles of mixotrophs in food webs, under both current and future conditions. We particularly foster contributions at the cutting-edge of research on mixotroph metabolism, diversity, and ecological role in the food web that can help to better assess the future of aquatic ecosystems.

Lead Organizer: Thomas Trombetta, University of Amsterdam (t.trombetta@uva.nl)

Co-organizers:

Susanne Wilken, University of Amsterdam (s.wilken@uva.nl)

Shai Slomka, University of Amsterdam (s.slomkadeoliveira@uva.nl)

Nicole Millette, Virginia Institute of Marine Science (nmillette@vims.edu)

Sarah Princiotta, Penn State Schuylkill (spb20@psu.edu)

Presentations

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06:00 PM

THE EFFECTS OF NUTRIENTS AND LIGHT ON MIXOTROPHS IN NARRAGANSETT BAY: A MESOCOSM AND FLOW CYTOMETRY STUDY (8953)

Primary Presenter: Deyanneira Colon Maldonado, University of Rhode Island - Graduate School of Oceanography (colondeyanneira@gmail.com)

Mixotrophy, a form of nutrition involving autotrophy and heterotrophy within one planktonic cell, changes what is known about the transfer of energy to higher trophic levels and carbon cycling on the marine microbial level. However, little is known about mixotrophic activity under environmental stressors including nutrient limitation in situ because the field is relatively new with few methods found to measure data. Understanding the effects of nutrients and light on mixotrophs is important because these organisms affect higher trophic levels and productivity in an environment. Narragansett Bay (NB) is a highly productive estuarine ecosystem with primary production typically limited by nutrients in the summer providing an ideal place to study the effects of nutrients on mixotrophs. This study examined mixotroph abundance in plankton samples collected from NB in tandem with the Narragansett Bay Long Term Plankton Time Series, and determined the effects of nutrient availability and light limitation on mixotrophs by comparing against samples with added nutrients and incubated in low light. Lysotracker Green, a common method to characterize mixotroph abundance, was applied to samples for analysis on a flow cytometer. The study found that mixotroph abundance decreased in low light treatments indicating NB’s mixotrophs may not turn to heterotrophy in time of light stress. Mixotroph abundance did increase when there were lower nutrients available indicating that mixotrophy is an effective form of nourishment in times of nutrient stress. Results from this study provide meaningful data regarding the relationship between mixotrophs and nutrients/light in NB, which has been part of a long-standing discussion due to sewage disposal, water treatment upgrades, and runoff from agriculture or storms.

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06:00 PM

MICROPLANKTON COMMUNITY STRUCTURE AND CARBON BIOMASS IN RELATION TO ENVIRONMENTAL GRADIENTS IN THE GULF OF MEXICO (SUMMER-FALL, 2021). (9472)

Primary Presenter: Lucy Roussa, North Carolina State University (lroussa@ncsu.edu)

The Gulf of Mexico (GOM) experiences multiple anthropogenic stressors including ocean acidification, eutrophication, harmful algal blooms (HABs), and large-scale hypoxia. Sea surface temperatures across the GOM are increasing at a rate twice that of the global ocean. Climate drivers have generally been associated with loss of ocean biodiversity and declines in ecological resilience, but less research is available on the effect of climate change on lower trophic level organisms. Protists are an ecologically diverse group of single-celled eukaryotes that play central roles as primary producers, consumers, decomposers, and that serve as the link for energy transfer to higher trophic levels. Changes in protistan assemblage patterns across the GOM regions can be used as an indication of ecosystem health. HAB taxa are of particular concern in areas where nutrient-rich freshwater input is intensified (e.g., Mississippi river, upwelling regions). This study explores the protistan community across gradients of pCO₂, temperature, and nutrients in the GOM. Biological samples were collected during summer/fall of 2021 from locations throughout the GOM as part of the fourth Gulf of Mexico Ecosystems and Carbon Cruise (GOMECC-4). Seawater samples were used for analyzing microplankton community composition via automated-imaging microscopy (FlowCam) and identified by functional groups (phototroph, mixotroph, and heterotroph). The goal of this research is to characterize spatial patterns in protistan microplankton (15 – 200 µm) and estimate carbon stocks and diversity across the GOM.

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