Temporal and spatial variability of constitutive mixotroph abundance and proportion

Abstract

Mixotrophic plankton can comprise a substantial portion of the plankton community compared to phytoplankton and zooplankton. However, there is a gap in the understanding of conditions that influence mixotroph prevalence and activity in situ because current methods often over- or under-estimate mixotroph abundance. A labeled prey tracer method was utilized to identify active mixotrophs present at two locations in a temperate estuary over a year. The tracer method was combined with light microscopy data to estimate active mixotroph abundance and proportion. This study estimated that actively grazing mixotrophic taxa were more abundant in the spring and autumn compared to summer. Dinoflagellates typically dominated the mixotrophic taxa except during autumn at the low salinity location when cryptophytes dominated. Further analysis suggested that active mixotroph abundances might not be only regulated by environmental conditions favorable to mixotrophy but, instead, environmental conditions favorable to different mixotrophs utilization of phagotrophy. By focusing on mixotrophic taxa that were identified to be actively grazing at time of sampling, this study provided a more nuanced estimation of mixotroph abundance, increasing the understanding of how mixotrophic abundance and proportion in situ are influenced by the planktonic community composition and environmental factors.

Recommendations for advancing mixoplankton research through empirical-model integration

“…Any organism that incorporates BrdU (ingests bacteria) and has chloroplasts (capable of photosynthesizing) is identified as a mixoplankton. While the BrdU method itself cannot provide absolute abundance data, the abundance of taxa identified as mixoplankton via BrdU experiments can be determined for any taxa that were also identified in corresponding microscopy samples (Millette et al, 2021;Dobbertin da Costa et al, 2024). The limited application of BrdU labeled prey to date has used labeled bacteria (Fay et al, 2013;Gast et al, 2018;Millette et al, 2021;Dobbertin da Costa et al, 2024).…”

“…While the BrdU method itself cannot provide absolute abundance data, the abundance of taxa identified as mixoplankton via BrdU experiments can be determined for any taxa that were also identified in corresponding microscopy samples (Millette et al, 2021;Dobbertin da Costa et al, 2024). The limited application of BrdU labeled prey to date has used labeled bacteria (Fay et al, 2013;Gast et al, 2018;Millette et al, 2021;Dobbertin da Costa et al, 2024). However, eukaryotic prey could be labeled with BrdU as well in order to target mixoplankton, but this approach will need testing before application to in situ samples.…”

“…One approach that might be used to help identify mixoplankton within previously collected samples is BrdU labeled prey experiments. Repeated BrdU labeled bacteria experiments can help to develop a list of known mixoplankton taxa in a region where a long-term dataset exists and help identify at what time of the year or under what conditions these taxa are actively engaging in mixotrophy (Dobbertin da Costa et al, 2024). On its own, this data could expand our understanding of mixoplankton's biogeography, however, when combined with historical datasets, this data can exponentially expand our estimations of active mixoplankton's temporal and spatial variability.…”

“…As a result, this approach can either underestimate or overestimate their "true" abundance, resulting instead in a "potential" estimation of mixoplankton abundance (Stamieszkin et al, 2024). Methods such as fluorescent-labeled prey, LysoTracker, or BrdU-labeled prey have the potential to identify active mixoplankton in situ (Fay et al, 2013;Millette et al, 2021;Dobbertin da Costa et al, 2024), providing critical new information to re-analyze historical long-term datasets. However, in order for this to be attempted, scientists must first properly assign active mixoplankton as such.…”