Importance of integrating mixoplankton into marine ecosystem policy and management-Examples from the Marine Strategy Framework Directive.

Marine plankton capable of photosynthesis and predation ("mixoplankton") comprise up to 50% of protist plankton and include many harmful species. However, marine environmental management policies, including the European Union Marine Strategy Framework Directive (MSFD) and the USEPA, assume a strict dichotomy between autotrophic phytoplankton and heterotrophic zooplankton. Mixoplankton often differ significantly from these two categories in their response to environmental pressures and affect the marine environment in ways we are only beginning to understand. While the management policies may conceptually provide scope for incorporating mixoplankton, such action is rarely implemented. We suggest that the effectiveness of monitoring and management programs could benefit from explicit implementations regarding the ecological roles and impact of mixoplankton. Taking the MSFD as an example of marine management guidelines, we propose appropriate methods to explicitly include mixoplankton in monitoring and marine management. Integr Environ Assess Manag 2024;00:1-18. © 2024 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Corrigendum to 'Modeling mixoplankton along the biogeochemical gradient of the Southern North Sea' Ecological Modelling 458 (2021) 0304-3800/109690.

[This corrects the article DOI: 10.1016/j.ecolmodel.2021.109690.].

Modeling mixoplankton along the biogeochemical gradient of the Southern North Sea.

The ecological importance of mixoplankton within marine protist communities is slowly being recognized. However, most aquatic ecosystem models do not include formulations to model a complete protist community consisting of phytoplankton, protozooplankton and mixoplankton. We introduce PROTIST, a new module for the aquatic ecosystem modelling software Delft3D-WAQ that can model a protist community consisting of two types of phytoplankton (diatoms and green algae), two types of mixoplankton (constitutive mixoplankton and non-constitutive mixoplankton) and protozooplankton. We employed PROTIST to further explore the hypothesis that the biogeochemical gradient of inorganic nutrient and suspended sediment concentrations drives the observed occurrence of constitutive mixoplankton in the Dutch Southern North Sea. To explore this hypothesis, we used 11 1D-vertical aquatic ecosystem models that mimic the abiotic conditions of 11 routine monitoring locations in the Dutch Southern North Sea. Our models result in plausible trophic compositions across the biogeochemical gradient as compared to in-situ data. A sensitivity analysis showed that the dissolved inorganic phosphate and silica concentrations drive the occurrence of constitutive mixoplankton in the Dutch Southern North Sea.

A dataset on trophic modes of aquatic protists.

BACKGROUND: An important functional trait of organisms is their trophic mode. It determines their position within food webs, as well as their function within an ecosystem. For the better part of the 20th century, aquatic protist communities were thought to consist mainly of producers (phytoplankton) and consumers (protozooplankton). Phytoplankton cover their energy requirements through photosynthesis (phototrophy), while protozooplankton graze on prey and organic particles (phagotrophy). However, over the past decades, it was shown that another trophic group (mixoplankton) comprise a notable part of aquatic protist communities. Mixoplankton employ a third trophic mode by combining phototrophy and phagotrophy (mixotrophy). Due to the historical dichotomy, it is not straightforward to gain adequate and correct information on the trophic mode of aquatic protists. Long hours of literature research or expert knowledge are needed to correctly assign trophic modes. Additionally, aquatic protists also have a long history of undergoing taxonomic changes which make it difficult to compare past and present literature. While WoRMS, the World Register of Marine Species, keeps track of the taxonomic changes and assigns each species a unique AphiaID that can be linked to its various historic and present taxonomic hierarchy, there is currently no machine-readable database to query aquatic protists for their trophic modes. NEW INFORMATION: This paper describes a dataset that was submitted to WoRMS and links aquatic protist taxa, with a focus on marine taxa, to their AphiaID and their trophic mode. The bulk of the data used for this dataset stems from (routine) monitoring stations in the North Sea and the Baltic Sea. The data were augmented and checked against state-of-the-art knowledge on mixoplankton taxa by consulting literature and experts. Thus, this dataset provides a first attempt to make the trophic mode of aquatic protists easily accessible in both a human- and machine-readable format.

Solutions for ecosystem-level protection of ocean systems under climate change.

The Paris Conference of Parties (COP21) agreement renewed momentum for action against climate change, creating the space for solutions for conservation of the ocean addressing two of its largest threats: climate change and ocean acidification (CCOA). Recent arguments that ocean policies disregard a mature conservation research field and that protected areas cannot address climate change may be oversimplistic at this time when dynamic solutions for the management of changing oceans are needed. We propose a novel approach, based on spatial meta-analysis of climate impact models, to improve the positioning of marine protected areas to limit CCOA impacts. We do this by estimating the vulnerability of ocean ecosystems to CCOA in a spatially explicit manner and then co-mapping human activities such as the placement of renewable energy developments and the distribution of marine protected areas. We test this approach in the NE Atlantic considering also how CCOA impacts the base of the food web which supports protected species, an aspect often neglected in conservation studies. We found that, in this case, current regional conservation plans protect areas with low ecosystem-level vulnerability to CCOA, but disregard how species may redistribute to new, suitable and productive habitats. Under current plans, these areas remain open to commercial extraction and other uses. Here, and worldwide, ocean conservation strategies under CCOA must recognize the long-term importance of these habitat refuges, and studies such as this one are needed to identify them. Protecting these areas creates adaptive, climate-ready and ecosystem-level policy options for conservation, suitable for changing oceans.

Use of 16S ribosomal DNA for delineation of marine bacterioplankton species.

All of the marine bacterioplankton-derived 16S ribosomal DNA sequences previously deposited in GenBank were reanalyzed to determine the number of bacterial species in the oceanic surface waters. These sequences have been entered into the database since 1990. The rate of new additions reached a peak in 1999 and subsequently leveled off, suggesting that much of the marine microbial species richness has been sampled. When the GenBank sequences were dereplicated by using 97% similarity as a cutoff, 1,117 unique ribotypes were found. Of the unique sequences, 609 came from uncultured environmental clones and 508 came from cultured bacteria. We conclude that the apparent bacterioplankton species richness is relatively low.

Modeling the increase of nodularin content in Baltic Sea Nodularia spumigena during stationary phase in phosphorus-limited batch cultures.

Nodularin (Nod), produced by the brackish/marine cyanobacterium Nodularia spumigena, is a potent hepatotoxin, tumor promoter and is possibly carcinogenic to mammals. It is structurally and toxicologically related to the microcystins, produced by Microcystis aeruginosa in fresh water. A better understanding of the kinetics of Nod production might provide an insight into the physiological and ecological function of cyanobacterial hepatotoxins. The present study presents a simple model simulating the concentration of Nod in N. spumigena KAC66 during phosphorus-limited growth. The main assumption of the model is that the Nod production rate is proportional to the chlorophyll-a (Chla) concentration. The model was tuned to data from phosphorus-limited batch cultures of N. spumigena KAC66 at saturating light and was able to predict 96% or more of the variation in both Chla and Nod concentration. No significant effect of available nitrogen source was found on the Chla-specific Nod production rate although specific growth rates were higher in ammonium and nitrate grown cultures compared to cultures grown with N(2) as the sole nitrogen source. Literature data on microcystin production by M. aeruginosa in phosphorus-limited chemostats fitted the model predictions well, except at very low dilution rates (0.1 day(-1)). The good fit with the proposed model to our own and literature data suggests that the production of hepatotoxic cyanotoxins is not regulated upon growth reduction due to phosphate limitation.

GENETIC AND PHYSIOLOGICAL VARIATION IN PIGMENT COMPOSITION OF EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) AND THE POTENTIAL USE OF ITS PIGMENT RATIOS AS A QUANTITATIVE PHYSIOLOGICAL MARKER.

Genetic variation of pigment composition was studied in 16 different strains of Emiliania huxleyi (Lohm.) Hay et Mohler in batch culture. Distinct strain-dependent differences were found in the ratios of fucoxanthin, 19'-hexanoyloxyfucoxanthin, and 19'-butanoyloxyfucoxanthin, hampering the use of these individual pigments as a taxonomic marker at the species level. The molar ratio of total carotenoids to chl a, however, was constant for all strains tested. In addition, the pigment composition of one axenic strain (L) of E. huxleyi at different growth rates in light-, nitrate-, and phosphate-limited continuous cultures was analyzed quantitatively. The pigments fucoxanthin and 19'-hexanoyloxyfucoxanthin correlated closely under all conditions. From steady-state rate calculations, it is hypothesized that 19'-hexanoyloxyfucoxanthin is synthesized from fucoxanthin, with light as a modulating factor. The net rate of synthesis of diatoxanthin depended both on the concentration of diadinoxanthin (its partner in the xanthophyll cycle) and on light, illustrating its photoprotective function in the xanthophyll cycle. In axenic strain L, the ratio of total fucoxanthins to chl a correlated strongly with photon flux density and can potentially be used to assess the physiological status with respect to irradiance in field populations. In multispecific bloom situations, the ratio of diadinoxanthin plus diatoxanthin to total fucoxanthins could be used as an alternative indicator for the light-dependent physiological state of E. huxleyi, provided that no other chromophytes are present. Application of these correlations to mesocosm data from the literature has so far provided no evidence that E. huxleyi blooms form only at inhibiting light levels, as previously suggested.

Effect of phytoplankton cell size on transient-state nitrate and ammonium uptake kinetics.

SUMMARYThe uptake of nitrate or ammonium (at a concentration of 10 mol I-1) by marine phytoplankton was studied in relation to cell size. Initial specific nitrate uptake rates by small (35000 m3) and large (130000 m3) cells of the diatom Ditylum brightwellii did not differ significantly. However, the larger cells maintained a high uptake rate for a longer time. Therefore, they accumulated nitrate in a higher biomass-specific pool than the smaller cells. In the dark, this effect was even more pronounced. Two smaller diatom species, Lauderia borealis (7474 m3) and Thalassiosira pseudonana (98 (m3), had lower initial specific nitrate uptake rates and lower intracellular pools. Transient-state ammonium uptake did not result in accumulation of large intracellular pools of ammonium. Theoretically, and on the basis of the presented results, we stress the dualistic functional role of the vacuole. A large vacuole is an effective way for larger algal species to possess a minimum cell nutrient quota/cell surface ratio which is in the range of smaller species. Furthermore, by functioning as a storage reservoir it reduces inhibition of the uptake rate by cytoplasmic accumulated nutrients. The effect of the latter mechanism is that larger algal species are better at nitrate uptake under fluctuating conditions. These results imply that, in nitrogen-controlled marine systems, resource competition under fluctuating nutrient concentrations can only lead to a shift towards larger phytoplankton species if nitrate rather than ammonium is the main nitrogen source. From theoretical considerations it is argued that the maximum growth rate of algae is determined by nutrient assimilation properties rather than by photosynthetic capacity.