Dedmer Van de Waal

Death is a common outcome of infection, but most disease models do not track hosts after death. Instead, these hosts disappear into a void. This assumption lacks critical realism, because dead hosts can alter host–pathogen dynamics. Here, we develop a theoretical framework of carbon-based models combining disease and ecosystem perspectives to investigate the consequences of feedbacks between living and dead hosts on disease dynamics and carbon cycling. Because autotrophs (i.e. plants and phytoplankton) are critical regulators of carbon cycling, we developed general model structures and parameter combinations to broadly reflect disease of autotrophic hosts across ecosystems. Analytical model solutions highlight the importance of disease–ecosystem coupling. For example, decomposition rates of dead hosts mediate pathogen spread, and carbon flux between live and dead biomass pools are sensitive to pathogen effects on host growth and death rates. Variation in dynamics arising from biologically realistic parameter combinations largely fell along a single gradient from slow to fast carbon turnover rates, and models predicted higher disease impacts in fast turnover systems (e.g. lakes and oceans) than slow turnover systems (e.g. boreal forests). Our results demonstrate that a unified framework, including the effects of pathogens on carbon cycling, provides novel hypotheses and insights at the nexus of disease and ecosystem ecology.

Land-water transition areas play a significant role in the functioning of aquatic ecosystems. However, anthropogenic pressures are posing severe threats on land-water transition areas, which leads to degradation of the ecological integrity of many lakes worldwide. Enhancing habitat complexity and heterogeneity by restoring land-water transition areas in lake systems is deemed a suitable method to restore lakes bottom-up by stimulating lower trophic levels. Stimulating productivity of lower trophic levels (phytoplankton, zooplankton) generates important food sources for declining higher trophic levels (fish, birds). Here, we study ecosystem restoration project Marker Wadden in Lake Markermeer, The Netherlands. This project involved the construction of a 700-ha archipelago of five islands in a degrading shallow lake, aiming to create additional sheltered land-water transition areas to stimulate food web development from its base by improving phytoplankton quantity and quality. We found that phytoplankton quantity (chlorophyll-a concentration) and quality (inversed carbon:nutrient ratio) in the shallow waters inside the Marker Wadden archipelago were significantly improved, likely due to higher nutrient availabilities, while light availability remained sufficient, compared to the surrounding lake. Higher phytoplankton quantity and quality was positively correlated with zooplankton biomass, which was higher inside the archipelago than in the surrounding lake due to improved trophic transfer efficiency between phytoplankton and zooplankton. We conclude that creating new land-water transition areas can be used to increase light and nutrient availabilities and thereby enhancing primary productivity, which in turn can stimulate higher trophic levels in degrading aquatic ecosystems.

Many marine planktonic ciliates retain functional chloroplasts from their photosynthetic prey and use them to incorporate inorganic carbon via photosynthesis. While this strategy provides the ciliates with carbon, little is known about their ability to incorporate major dissolved inorganic nutrients, such as nitrogen and phosphorus. Here, we studied how ciliates respond to different concentrations of dissolved inorganic nitrogen and phosphorus. Specifically, we tested the direct and indirect effects of nutrient availability on the ciliate Strombidium cf. basimorphum fed the cryptophyte prey Teleaulax amphioxeia. We assessed responses in the rates of growth, ingestion, photosynthesis, inorganic nutrient uptake, and excretion. Our results show that the prey changed its carbon content depending on the nutrient concentrations. Low inorganic nutrient concentrations increased S. cf. basimorphum growth and prey ingestion. The higher carbon content of the prey under these low nutrient conditions likely supported the growth of the ciliate, while the higher carbon:nutrient stoichiometry of the prey led to the higher ingestion rates. The low carbon content of the prey at high nutrient concentrations resulted in reduced growth of S. cf. basimorphum, which indicates that carbon acquired via photosynthesis in the ciliate cannot compensate for the ingestion of prey with low carbon content. In conclusion, our findings show S. cf. basimorphum is not able to utilize dissolved inorganic nitrogen and phosphorus for growth, and this species seems to be well adapted to exploit its prey when grown at low nutrient conditions.

Hellweger et al. (Reports, 27 May 2022, pp. 1001) predict that phosphorus limitation will increase concentrations of cyanobacterial toxins in lakes. However, several molecular, physiological, and ecological mechanisms assumed in their models are poorly supported or contradicted by other studies. We conclude that their take-home message that phosphorus load reduction will make Lake Erie more toxic is seriously flawed.

Dissolved oceanic CO₂ concentrations are rising as result of increasing atmospheric partial pressure of CO₂ (pCO₂), which has large consequences for phytoplankton. To test how higher CO₂ availability affects different traits of the toxic dinoflagellate Alexandrium ostenfeldii, we exposed three strains of the same population to 400 and 1,000 µatm CO₂, and measured traits including growth rate, cell volume, elemental composition, ¹³C fractionation, toxin content, and volatile organic compounds (VOCs). Strains largely increased their growth rates and particulate organic carbon and nitrogen production with higher pCO₂ and showed significant changes in their VOC profile. One strain showed a significant decrease in both PSP and cyclic imine content and thereby in cellular toxicity. Fractionation against ¹³C increased in response to elevated pCO₂, which may point towards enhanced CO₂ acquisition and/or a downscaling of the carbon concentrating mechanisms. Besides consistent responses in some traits, other traits showed large variation in both direction and strength of responses towards elevated pCO₂. The observed intraspecific variation in phenotypic plasticity of important functional traits within the same population may help A. ostenfeldii to negate the effects of immediate environmental fluctuations and allow populations to adapt more quickly to changing environments.

Using capture fishery-derived fish oil and fishmeal in aquafeeds is unsustainable. This study mimicked semi-intensive shrimp ponds, including primary producers, in mesocosm tanks. Fatty acid mass balances were computed to distinguish between diet-based and primary production-based LC-PUFA contributions to shrimp (Litopenaeus vannamei) production and meat quality. Performance and body fatty acid composition were compared of shrimp fed a commercial diet containing fish oil and fishmeal (control), with a fishmeal- and fish oil-free diet (low LC-PUFA diet: LOW). Six mesocosms were each stocked with 60 juvenile shrimp and randomly assigned to the two diets. After an 8-week grow-out period, biomass production, survival and proximate body composition were similar between diets. Control shrimp contained twice as much LC-PUFA and omega-3 fatty acids than LOW shrimp. Large quantitative losses (85%) were found in both treatments of the LC-PUFA-precursors alpha-linolenic acid (ALA) and linoleic acid (LA) that were being used as energy source by the shrimp instead for LC-PUFA synthesis. Whereas losses were also observed for eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the control group, there was a gain for these components in the LOW tanks. LOW shrimp sourced at least 32% of their total EPA gain and 15% of their total DHA gain from the algal-based food web. This quantitative analysis of the fate of major dietary fatty acids strongly suggests that the pond's primary production can provide shrimp additional LC-PUFA. Finding a balance between LC-PUFA contribution through formulated feed and natural production seems possible and deserves further research.

Along with increasing oceanic CO₂ concentrations, enhanced stratification constrains phytoplankton to shallower upper mixed layers with altered light regimes and nutrient concentrations. Here, we investigate the effects of elevated pCO₂ in combination with light or nitrogen-limitation on ¹³C fractionation (ε_p) in four dinoflagellate species. We cultured Gonyaulax spinifera and Protoceratium reticulatum in dilute batches under low-light (‘LL’) and high-light (‘HL’) conditions, and grew Alexandrium fundyense and Scrippsiella trochoidea in nitrogen-limited continuous cultures (‘LN’) and nitrogen-replete batches (‘HN’). The observed CO₂-dependency of ε_p remained unaffected by the availability of light for both G. spinifera and P. reticulatum, though at HL ε_p was consistently lower by about 2.7‰ over the tested CO₂ range for P. reticulatum. This may reflect increased uptake of (¹³C-enriched) bicarbonate fueled by increased ATP production under HL conditions. The observed CO₂-dependency of ε_p disappeared under LN conditions in both A. fundyense and S. trochoidea. The generally higher ε_p under LN may be associated with lower organic carbon production rates and/or higher ATP:NADPH ratios. CO₂-dependent ε_p under non-limiting conditions has been observed in several dinoflagellate species, showing potential for a new CO₂-proxy. Our results however demonstrate that light- and nitrogen-limitation also affect ε_p, thereby illustrating the need to carefully consider prevailing environmental conditions.