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
02:30 PM
INDIVIDUAL AND INTERACTIVE EFFECTS OF OCEAN ACIDIFICATION AND WARMING ON A MIXOTROPHIC PROTIST (8707)
Primary Presenter: Shai Slomka, University of Amsterdam (s.slomkadeoliveira@uva.nl)
Mixotrophic protists, capable of both photosynthesis and phagotrophy of prey, constitute a significant proportion of primary producers and bacterivores in aquatic food webs. They are especially dominant in oligotrophic marine systems, which are only expected to expand in a future climate. Mixotrophs are able to modulate both photosynthesis and phagotrophy in response to changes in the environment, which in turn affects their function in the food web. However, we know very little on how mixotrophic species respond to interactive effects of environmental changes. We studied the response of a constitutive mixotrophic marine protist of the Ochromonas genus to ocean acidification and warming conditions. We acclimatized cultures of the Ochromonas CCMP 2951 strain to four treatments including high and low CO2 concentrations and temperature in a factorial design. We show that the strain benefited from higher growth rates under higher temperature and ocean acidification conditions. In addition to individual effects on physiology, we also observed interactions between the two environmental factors on this strain’s mixotrophic balance. For example, a positive effect of ocean acidification conditions on photosynthetic carbon fixation rates was temperature dependent. Physiological responses will be linked to cellular composition measurements to assess the potential impact of observed responses on higher trophic levels. Finally, we will incorporate results from a differential gene expression analysis to identify some of the regulatory mechanisms of the observed physiological responses.
02:45 PM
EFFECT OF INCREASING WATER TEMPERATURE ON NON-TOXIN PRODUCING AND TOXIN PRODUCING MIXOTROPHS’ INGESTION RATES (9236)
Primary Presenter: Zabdiel Roldan Ayala, Virginia Institute of Marine Sciences, College of William & Mary (zaroldanayala@vims.edu)
Anthropogenic activity has caused large-scale changes to marine environments such as increasing water temperatures. Previous research has observed that some mixotrophic species increase their heterotrophic activity as temperature increases. However, it is not known if a similar response in heterotrophic activity to temperatures will be observed in toxin producing harmful algal bloom (HAB) mixotrophs. We hypothesized that toxin producing HAB mixotrophs will decrease their toxin production as their ingestion rate increase because previous research has suggested that photosynthesis can be important to toxin production. We measured the ingestion rates of two non-toxin producing mixotrophs (Prorocentrum micans and Heterocapsa steinii) and two toxins producing mixotrophs (Alexandrium monilatum and Karenia brevis) on bacteria under different temperatures using a modified prey removal. Cultures were exposed to three different temperatures for five days and on the sixth day the prey removal experiments were set up and ran for 24 hours. Samples were taken for dinoflagellate abundance, cellular chlorophyll a content, toxin content, and bacteria abundances. Ingestion rates for all species are expected to increase in the higher temperature treatments. Results from this study can help inform managers whether increasing ingestion rates in mixotrophic HABs will decrease the toxicity of the bloom event. Therefore, it can potentially aid in improving our knowledge regarding HAB dynamics in future climate change scenarios.
03:00 PM
Spatial and Temporal Distribution and Correlates of Small Potentially Phagotrophic Phytoplankton in the North East US Shelf (9123)
Primary Presenter: Erica Ewton, University of Rhode Island (eewton@gmail.com)
Mixoplankton, phytoplankton capable of performing phagotrophy in complement with phototrophy, alter energy and carbon flow through marine food webs. However, much remains to be learned about drivers of mixoplankton abundance and trophic mode tendencies within whole communities in situ. Temperate coasts of the North East US Shelf (NES) are inherently dynamic with respect to temperature, nutrient concentrations, and plankton community composition and thus are an ideal location to study mixoplankton abundance. Samples measuring the abundance of potentially mixoplanktonic nanoeukaryotes (‘mixoplankton’) via LysoTracker and associated biological and environmental conditions were collected during six cruises spanning two years as part of the NES Long Term Ecological Research (LTER) project. We quantify the dependence of mixoplankton abundance on temperature, light, salinity, depth, nutrients, community composition, and find both non-linear and linear dependencies, including a strong inverse association with salinity. The empirical estimates obtained within this study fill knowledge gaps in analyses that quantify coastal ecosystem energy transfer and carbon biogeochemical cycling, as well as help better parameterize models that predict the changing climate.
03:15 PM
The importance of mixotrophy for estuarine primary production in the Chesapeake Bay (8876)
Primary Presenter: Dante Horemans, Virginia Institute of Marine Science (dmlhoremans@pm.me)
Eutrophication is often associated with nutrient pollution, reduced biodiversity, hypoxia, and increases in algal biomass and primary production (PP). Although insights into PP are crucial to understand eutrophication, the main processes affecting PP are not always known. More specifically, the importance of mixotrophy (i.e., combining autotrophy and phagotrophy) to PP has been understudied. Here we identify some of the main processes affecting PP (e.g., mixotrophy, nutrient limitation) and their variation in time and space along a large-scale gradient of environmental conditions (e.g., salinity, temperature, and nutrients). We apply an empirical-model integrated approach; we calibrate various PP models including the effects of mixotrophy to multi-decadal PP observations from the Chesapeake Bay (U.S.A.), focusing on specific regions and seasons. When mixotrophy is not included, the optimal model fit to the data suggests an absence of nitrogen limitation in the Bay, which contradicts the observations and results in autotrophic PP to be overestimated by ∼ 40 % in fall and ~ 75 % in spring in the polyhaline region. Mixotrophy is required to realistically capture nitrogen limitation. The relative contribution of mixotrophy to total PP strongly varies depending on season and region (∼ 10 - 75 %). Our results are one of the first estimates of the relative contribution of mixotrophy along a large-scale gradient of environmental conditions and may be important for both experimental scientists and modelers given that they often focus on autotrophy only.
03:30 PM
Assessment of Prymnesyophyta (Haptophyta) Abundance by CARD-FISH in the Sargasso Sea (9536)
Primary Presenter: Josué Millán, Indiana State University (josue.g.millan@gmail.com)
Phytoplankton are central to oceanic primary productivity and the marine food web, playing a critical role in the biogeochemical cycling of carbon (C) through their photosynthetic uptake of atmospheric CO2. The phytoplankton group, prymnesiophytes, include coccolithophores that are easily identifiable in their calcifying state. However, increasing evidence suggests that many prymnesiophytes undergo complex life cycles, alternating between haploid and diploid stages that may exclude calcification. This complicates their identification using traditional light microscopy or scanning electron microscopy (SEM) during their non-calcifying phases. To overcome these limitations, fluorescence in situ hybridization (FISH) coupled with catalyzed reporter deposition (CARD-FISH), was used to target Eukaryotes, Chlorophytes, non-Chlorophytes and Prymnesiophytes regardless of calcification status. Surface, Deep Chlorophyll Maximum, and Lower Photic Zone samples were collected from the Sargasso Sea during the Fall of 2020. Prymnesiophyte abundance was compared to the total eukaryotic phytoplankton community and concurrent SEM analyses. While CARD-FISH produced results consistent with SEM at the surface, it underestimated Prymnesiophyte abundance—by ~50%—in deeper waters. These regions are characterized by a high diversity of rare and uncultured species with sequences unavailable during probe design. This underestimation suggests that the molecular diversity of Prymnesiophytes in the Sargasso Sea, and likely the broader phytoplankton community, is higher than previously anticipated.
SS30B - Taking the Pulse of Mixotrophic Protists in Aquatic Ecosystems: Baseline and Response to Anthropogenic Change
Description
Time: 2:30 PM
Date: 27/3/2025
Room: W208