Biology subjects

Schaum, E.

Publications and source records attributed to Schaum, E..

8 recordsLinked to original sources

bioRxiv2026

Exploring Warming Effects on lower food-web dynamics in the plankton of the River Elbe Estuarine Ecosystem in summer: Insights from a Mesocosm Experiment

Understanding how warming alters estuarine plankton communities is essential for predicting future changes in biodiversity and ecosystem functioning. We conducted a four-week indoor mesocosm experiment using natural summer plankton from the Elbe River to examine the effects of warming (+2 {degrees}C and +4 {degrees}C) on abiotic conditions and responses of the plankton community. In this study, oxygen concentrations, primary producer biomass (chlorophyll a, microphytoplankton) and microzooplankton abundances declined sharply during the first 10 days across all treatments while mesozooplankton abundances increased. This suggests a strong top-down control by mesozooplankton on lower trophic levels across all temperature treatments. Primary producers biomass and oxygen concentrations recovered after an initial decline, however to lower levels compared to the onset of the experiment while micro- and mesozooplankton remained low during the second half of the experiment. Nutrient dynamics indicated progressive remineralization, with increasing ammonium, NOx, and silicate concentrations, while phosphate concentrations remained low throughout the experiment. Complementary DNA and RNA metabarcoding revealed similar community turnover over time in all treatments and temperature effects became only pronounced at the end of the experiment. Overall, warming effects were subtle relative to the strong internal trophic dynamics likely caused by the artificial mesocosm setup. Our findings of changes in plankton community dynamics indicate that biotic interactions, changes in trophic diversity and other environmental factors, i.e. oxygen concentrations are likely the drivers of this estuarine system rather than warming alone.

ecology↗

bioRxiv2026

Use of organic compounds by phytoplankton - a synthesis across strains from different habitats and functional groups

Mixotrophy is emerging as a default nutritional strategy in phytoplankton, but research seems so far isolated and mostly focusing on single phytoplankton groups or strains. Here we combined data from 24 oceanic and 22 freshwater strains to analyze phytoplanktons ability to use dissolved organic compounds. The results emphasize that mixotrophy is ubiquitous in phytoplankton across functional groups and taxa isolated from various habitats and not strictly dependent on light or nutrient deficiencies. We propose that the prevalence of mixotrophy stems from phytoplanktons heterotrophic ancestry, while the divergence into different lineages partly explains variation in mixotrophic behavior. Differences found in phytoplankton originating from different habitats may result from adaptive responses to habitat conditions, but conclusions need to be drawn carefully in line with the limited number of taxa included per habitat and potential niche partitioning. This study underscores the ecological significance of heterogeneous nutritional strategies and the necessity to address existing knowledge gaps to finally draw conclusions about the impact of mixotrophy in the context of single ecosystems and on global scales. Graphical abstract textAnalyzing thousands of data points across 46 marine and freshwater strains, this study establishes mixotrophy as a ubiquitous metabolic strategy in phytoplankton. These findings challenge the autotrophic paradigm, demonstrating that organic matter acquisition is a default state with significant implications for our understanding of global carbon cycles. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=93 SRC="FIGDIR/small/703725v2_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@cd9939org.highwire.dtl.DTLVardef@6a4fdorg.highwire.dtl.DTLVardef@39f5a6org.highwire.dtl.DTLVardef@12e811a_HPS_FORMAT_FIGEXP M_FIG C_FIG

microbiology↗

bioRxiv2024

Picophytoplankton dominate phytoplankton communities in the Elbe estuary in terms of cell counts

(1) Picophytoplankton are important primary producers, but not always adequately recognized, e.g. due to methodological limitations. (2) In this study, we combined flow cytometry and metabarcoding to investigate seasonal and spatial patterns of picophytoplankton abundance and community composition in the Elbe estuary. (3) Picophytoplankton (mostly picoeukaryotes such as Mychonastes and Minidiscus) contributed on average 70 % (SD = 14 %) to the total phytoplankton counts. In the summer picocyanobacteria (e.g. Synechococcus) played a more significant role. The contributions of picophytoplankton to the total phytoplankton were particularly high from summer to winter as well as in the mid estuary. However, at salinities of around 10 PSU in the mixing area the proportion of picophytoplankton was comparably low (average 49 %, SD = 13 %). (4) Our results indicate that picophytoplankton prevail in the Elbe estuary year-round with respect to cell counts. Picophytoplankton could occupy important niche positions to maintain primary production under extreme conditions where larger phytoplankton might struggle (e.g. at high or low temperature and high turbidity), and also benefit from high nutrient availability here. However, we did not find evidence that they played a particularly significant role at the salinity interface. Our study highlights the importance of including picophytoplankton when assessing estuarine phytoplankton as has been suggested for other ecosystems such as oceans.

molecular biology↗

bioRxiv2023

Organic compounds drive growth in phytoplankton taxa from different functional groups

Phytoplankton are usually considered autotrophs by default, but an increasing number of studies shows that many taxa are able to also utilise organic carbon. Acquiring nutrients and energy from different sources might enable an efficient uptake of required substances and provide a strategy to deal with a varying resource availability, especially in highly dynamic ecosystems such as estuaries. In our study we investigated the effects of 31 organic carbon sources on the growth of 17 phytoplankton strains from the Elbe estuary spanning four functional groups. All of our strains were able to make use of at least 1 and up to 26 organic compounds for growth. Pico-sized green algae such as Mychonastes, Choricystis and Chlorella, as well as nano-sized green algae from the genus Monoraphidium in particular were positively affected by a high variety of substances. Reduced light availability, typically appearing in turbid estuaries and similar habitats, resulted in an overall poorer ability to utilise organic substances for growth, indicating that organic carbon acquisition was not primarily a specific strategy to deal with darkness. Our results give further evidence for mixotrophy being an ubiquitous ability of phytoplankton and highlight the importance to consider this trophic strategy in research.

ecology↗

bioRxiv2023

Co-evolutionary patterns shown in Ostreococcus-virus system from the Western Baltic Sea in freshly isolated hosts and viruses.

Marine viruses are a major driver of phytoplankton mortality and thereby influence biogeochemical cycling of carbon and other nutrients. In recent years, an understanding of the potential importance of phytoplankton-targeting viruses on ecosystem dynamics has emerged, but experimental investigations of host-virus interactions on a broad spatial and temporal scale are still missing. Here, we investigated in detail a phytoplankton hosts responses reacting to infections by species-specific viruses from i) distinct geographical regions and ii) different sampling seasons. Specifically, we used two species of picophytoplankton (1 {micro}m) Ostreococcus tauri and O. mediterraneus and their viruses (size ca. 100 nm), which represent systems well-known in marine biology, but almost entirely ignored in evolutionary biology. The strains stem from different regions of the Southwestern Baltic Sea that vary in salinity and temperature. Using an experimental cross-infection set-up, we show that in this natural system evolutionary history, and thus the timing of when hosts and their associated viruses coexisted, was the main driver of infection patterns. In addition species and strain specificity underline the present understanding of rapid host-virus co-evolution.

microbiology↗

bioRxiv2022

Predicting the unpredictable: heatwaves and history of variability shape phytoplankton community thermal responses within one generation

Predicting the effect of increased thermal unpredictability, for example in the shape of heatwaves on phytoplankton metabolic responses is ripe with challenges. While single genotypes in laboratory environments will respond to environmental fluctuations in predictable and repeatable ways, it is difficult to relate rapid evolutionary responses of whole communities to their ecological history. Previously experienced environments, including fluctuations therein, can shape an organisms specific niche as well as their responses to further environmental changes. This is a testable hypothesis as long as samples can be obtained where the environmental history is known, sufficiently diverse, and not obscured by confounding parameters such as day length and precipitation patterns. Here, we tested immediate (i.e. within one generation) metabolic temperature responses of natural phytoplankton assemblages from two thermally distinct regions in the Baltic Sea: the warmer and less predictable Kiel Area, and the overall colder and more predictable Bornholm Basin. Our approach allows us to investigate effects on immediate physiological time scales (responses curves), ecological and evolutionary processes on longer time scales (seasonal differences between basins) as well as mid-term responses during a natural occurring heatwave. We found evidence for a higher degree of phenotypic plasticity in samples from unpredictable environments (Kiel Area).

ecology↗

bioRxiv2021

Epidemiological dynamics of viral infection in a marine pico-eukaryote

Ostreococcus tauri is a ubiquitous marine pico-eukaryote that is susceptible to lysis upon infection by its species specific Ostreococcus tauri viruses (OtVs). In natural populations of O. tauri, costs of resistance are usually invoked to explain the persistence or reappearance of susceptible individuals in resistant populations. Given the low costs of resistance measured in laboratory experiments with the O. tauri/OtV system to date, the question remains of why susceptible individuals persist in the wild at all. Epidemiological models of host and pathogen population dynamics are one useful approach to understand the conditions that can allow the coexistence of susceptible and resistant hosts. We used a SIR (Susceptible-Infected-Resistant) model to investigate epidemiological dynamics under different laboratory culturing regimes that are commonly used in the O.tauri/OtV system. When taking into account serial transfer (i.e. batchcycle lengths) and dilution rates as well as different resistance costs, our model predicts that no susceptible cells should be detected under any of the simulated conditions - this is consistent with laboratory findings. We thus considered an alternative model that is not used in laboratory experiments, but which incorporates one key process in natural populations: host populations are periodically re-seeded with new infective viruses. In this model, susceptible individuals re-occurred in the population, despite low costs of resistance. This suggests that periodic attack by new viruses, rather than (or in addition to) costs of resistance, may explain the high proportion of susceptible hosts in natural populations, and underlie the discrepancy between laboratory studies and observations of fresh isolates. ImportanceIn natural samples of Ostreococcus sp. and its associated viruses, susceptible hosts are common. However, in laboratory experiments, fully resistant host populations readily and irreversibly evolve. Laboratory experiments are powerful methods for studying process because they offer a stripped-down simplification of a complex system, but this simplification may be an oversimplification for some questions. For example, laboratory and field systems of marine microbes and their viruses differ in population sizes and dynamics, mixing or migration rates, and species diversity, all of which can dramatically alter process outcomes. We demonstrate the utility of using epidemiological models to explore experimental design and to understand mechanisms underlying host-virus population dynamics. We highlight that such models can be used to form strong, testable hypotheses about which key elements of natural systems need to be included in laboratory systems to make them simplified, rather than oversimplified, versions of the processes we use them to study.

microbiology↗

bioRxiv2017

Thermal adaptation constrains the temperature dependence of ecosystem metabolism

Gross primary production (GPP) is the largest flux in the carbon cycle, yet its response to global warming is highly uncertain. The temperature sensitivity of GPP is directly linked to photosynthetic physiology, but the response of GPP to warming over longer timescales could also be shaped by ecological and evolutionary processes that drive variation community structure and functional trait distributions. Here, we show that selection on photosynthetic traits within and across taxa dampen the effects of temperature on GPP across a catchment of geothermally heated streams. Autotrophs from cold streams had higher photosynthetic rates and after accounting for differences in biomass among sites, rates of ecosystem-level GPP were independent of temperature, despite a 20 {o}C thermal gradient. Our results suggest that thermal adaptation constrains the long-term temperature dependence of GPP, and highlights the importance of considering physiological, ecological and evolutionary mechanisms when predicting how ecosystem-level processes respond to warming.

ecology↗