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
09:00 AM
EVIDENCE FOR NUTRIENT-DEPENDENT CYANOBACTERIVORY BY THE MIXOTROPHIC CHRYSOPHYTE OCHROMONAS (9280)
Primary Presenter: Sarah Princiotta, Penn State Schuylkill (sbp20@psu.edu)
Mixotrophs occupy a unique position within the microbial food web as primary producers and consumers. The ability to combine two forms of nutrition is theorized to allow mixotrophs to gain prevalence in a variety of aquatic ecosystems, especially when ingestion of particulate prey can supplement nutrient limitation. Ochromonas is a genus of mixotrophic Chrysophyte that includes species spanning the spectrum of mixotrophic strategies, and there is a long history of work dedicated to their variable capacity for phagotrophic ingestion of heterotrophic bacteria. More recently, studies have demonstrated the ability for Ochromonas to ingest potentially toxic cyanobacteria, including Microcystis, a cosmopolitan cyanobacteria with a reputation for forming harmful algal blooms. We used a combination of laboratory experiments in culture and in situ field incubations to investigate cyanobacterivory by Ochromonas with emphasis on the impact of nutrient limitation. Grazing by Ochromonas on toxic Microcystis ranged from 14 cell-1 day to 31 cell-1 day, whereas that on the non-toxic strain ranged from 5 cell-1 day to 19 cell-1 day. Our results provide evidence that co-culture of Ochromonas with toxic Microcystis with can lead to significant cyanobacterial population decay and microcystin biodegradation under P-limitation. However, in laboratory conditions, N-limitation enhanced grazing by Ochromonas on toxic Microcystis. Collectively, we suggest a role for mixotrophic nanoflagellates as intraguild predators and highlight an unresolved linkage within the microbial food web.
09:15 AM
RESPONSES OF MIXOTROPHIC PROTISTS TO RESOURCE AVAILABILITY – FROM PHENOTYPIC PLASTICITY TO POPULATION DYNAMICS (9103)
Primary Presenter: Susanne Wilken, University of Amsterdam (s.wilken@uva.nl)
Their access to alternative resources allows mixotrophic protists to thrive under conditions that are limiting the growth of specialist photoautotrophic or phago-heterotrophic competitors. The relative availability of light, nutrients and prey therefore shape their success in plankton communities. Yet nutritional strategies among mixotrophic protists are diverse, with different species profiting from different ratios of resource availability, and some adjusting their strategies strongly via phenotypic plasticity. Here, we assessed the impact of resource availability on mixotrophic protists using a range of approaches. First, the subcellular rearrangements made by the model mixotroph Ochromonas in response to varying prey availability have been assessed by transcriptome analysis. This obligate mixotroph invests into prey capture when prey abundance is low, but activates machinery for prey digestion only upon ingestion of prey. We further assessed the response of mixotrophic protists to resource availability within natural communities of the subalpine oligotrophic lake Lunz. Here we found cryptophytes to feed under nutrient limitation at high light intensities, while decreasing their ingestion rates at low light intensity or upon increased nutrient availability. In contrast dictyochophytes were found to feed at high rates regardless of nutrient and light availability. Such functional diversity is reflected in differences in their population dynamics both in response to light availability in a mesocosm experiment and over their seasonal succession.
09:30 AM
INFLUENCE OF VITAMIN B12 ON MIXOTROPHIC GROWTH: INTERACTIONS BETWEEN EUGLENA GRACILIS AND MESORHIZOBIUM LOTI (9019)
Primary Presenter: Sadikshya Ghimire, University of Eastern Finland (sadikshya.ghimire@uef.fi)
Mixotrophic protists, capable of utilizing both photosynthetic and heterotrophic pathways for energy,are particularly important in high DOC lakes, i.e., lakes with high concentration of dissolved organic carbon. We studied the growth responses and biochemical traits of a freshwater mixotrophic protist Euglena gracilis in different culture conditions. We were specifically interested in its relationship with Mesorhizobium loti, a bacterium that synthetizes among other essential vitamin B12 (cobalamin). E. gracilis is a vitamin B12 auxotroph, i.e., it requires vitamin B12 for growth albeit it cannot synthesize this vitamin . We co-cultured E. gracilis and M. loti in autotrophic conditions and adjusted the concentrations of DOC, nitrogen and B12 to study their combined impact on the growth and nutritional quality,specifically polyunsaturated fatty acids (PUFA) and proteins of E. gracilis. The addition of external carbon benefited both partners possibly due to the phagocytic behaviour of E. gracilis . Additionally, co culture with bacteria significantly increased total fatty acids content in algae, including essential PUFAs:eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Co-deprivation of nitrogen and B12 adversely affected algal growth and protein content, yet the algae was more resilient under combined stress than with B12 deprivation alone. These findings emphasize the importance of vitamin B12 availability and the relationship with v itamin B12 producing bacteria on the growth and biochemical traits of mixotrophic protists, which, in turn, have cascading effects on aquatic food webs, particularly regarding their nutritional quality.
09:45 AM
DISTINCT PHYSIOLOGICAL RESPONSES OF SOUTHERN OCEAN MIXOTROPHS TO IRON AND MANGANESE LIMITATION (8834)
Primary Presenter: Claire Zwiers Cook, Skidaway Institute of Oceanography, University of Georgia (ccz99536@uga.edu)
In the Southern Ocean (SO), the physiological mechanisms mixotrophs use to cope with iron (Fe) or manganese (Mn) deficiency are not well understood, limiting predictions of carbon cycling seasonally. Laboratory cultures of two mixotrophic prasinophytes (Pyramimonas tychotreta and Mantoniella antarctica) and a cryptophyte (Geminigera cryophilia) from the SO are being examined to determine how Fe and Mn limitation affect behavior, growth, and metabolism. Growing under identical laboratory treatments, all cultures exhibited Fe limitation as indicated by reductions in growth rates, photosynthetic efficiency, and diameter compared to nutrient replete conditions. Of the three cultures, P. tychotreta showed the greatest sensitivity to metal limitation, sustaining 30% and 75% of its maximum growth rate under low Fe and low Mn conditions, respectively. G. cryophilia was also impacted by single Mn limitation, crashing after 4-6 transfers with cells displaying altered morphology not observed under Fe and Mn co-limitation. In contrast, the smallest organism M. antarctic demonstrated a 50% reduction in growth under Fe limitation, with Mn stress only evident through a reduction in diameter. This data suggests small SO mixotrophs may better tolerate Fe and Mn limitation, and that physiology under trace metal scarcity varies even among mixotrophs of the same phylogenetic group. Efforts are ongoing to integrate these results with grazing and transcriptomic data to further aid in our understanding of carbon flow in the SO.
10:00 AM
INVESTIGATING SOUTHERN OCEAN MIXOTROPHIC BACTERIVORY RATES UNDER IRON AND MANGANESE LIMITATION (9117)
Primary Presenter: Nicole Millette, Virginia Institute of Marine Science (nmillette@vims.edu)
Mixotrophs have the capacity to utilize phototrophic and phagotrophic trophic modes, with their behavior dependent on conditions such as nutrient concentrations, prey availability, and physiochemistry. Historically, mixotrophic research has focused on quantifying how their ingestion rates vary as a function of macronutrients and irradiance levels. However, in the Southern Ocean, iron (Fe) and manganese (Mn) concentration are low enough to inhibit photosynthetic growth and may stimulate bacterivory in mixotrophs. To explicitly test this, we measured the ingestion rate of three cultures of Southern Ocean mixotrophs, Mantoniella antarctica (prasinophyte), Geminigera cryophile (cryptophyte), and Pyramimonas tychotreta (prasinophyte), on bacteria under a matrix of Fe/Mn conditions. Ingestion rates on bacteria were measured using a modified version of prey removal experiments where the growth rates of bacteria were tracked over 24-hours when grown alone and in the presence of mixotrophs. The results to be presented represent a first attempt to investigate mixotrophs grazing response under Mn and Fe limitation. Preferentially relying upon grazing during low micronutrient conditions would allow Antarctic mixotrophs to persist, and perhaps thrive, at certain times of the year compared to strict phototrophs that are dependent on external dissolved metals for growth. These findings will be valuable in understanding plankton food web dynamics in a changing Antarctic climate, which likely includes shifts in Fe and Mn bioavailability to phytoplankton.
10:15 AM
Shifts from nutrient to energy limitation determine the responses of autotrophs, bacteria and mixotrophs to browning (9316)
Primary Presenter: Sebastian Diehl, Umea University (sebastian.diehl@umu.se)
Heterotrophic, mixotrophic, and autotrophic plankton interact in a network of competitive, predatory and mutualistic interactions. “Browning”, the increase of colored dissolved organic matter (CDOM) from terrestrial catchments, can affect this network of interactions by simultaneously decreasing light availability and increasing organic carbon and nutrient supplies. We introduce a conceptual, process-based numerical model to investigate the effects of browning on a microbial food web consisting of heterotrophic bacterioplankton, bacterivorous phago-mixoplankton, autotrophic phytoplankton, and the resources light, inorganic phosphorus, and DOM. We also explore how the investment in autotrophic vs. phagotrophic resource acquisition influences mixoplankton performance. Several model predictions are in broad agreement with empirical observations along a large gradient of increasing CDOM supply, including increased bacterial biomass and inorganic phosphorous, decreased light penetration, the potential for a unimodal phytoplankton biomass response, and a local minimum in mixoplankton biomass. Our results also suggest that mixoplankton with a high investment in phototrophy perform best in many conditions, but that phosphorous acquisition via prey is crucial under high light-low nutrient conditions. Overall, our model analyses suggest that responses to altered CDOM supply are largely determined by systematic changes in the relative importance of nutrient vs. energy limitation of each plankton group.
SS30A - Taking the Pulse of Mixotrophic Protists in Aquatic Ecosystems: Baseline and Response to Anthropogenic Change
Description
Time: 9:00 AM
Date: 27/3/2025
Room: W208