Rare earth element behaviour in seawater under the influence of organic matter cycling during a phytoplankton spring bloom – A mesocosm study

Rare earth elements (REEs) are used as powerful proxies for a variety of oceanic processes. The understanding of their biogeochemical behaviour in the marine environment is therefore essential. While the influence of OM-cycling on REE patterns in seawater is considered as insignificant, it has been shown that algae and bacteria provide good sorption surfaces for REEs and that components of the dissolved OM pool are able to complex REEs, thus potentially altering their behaviour. To investigate the impact of bio-associated processes on REEs in the bio-productive marine environment, we conducted an indoor mesocosm experiment that mimicked a phytoplankton spring bloom in the neritic coastal North Sea. The incubation period of 38 days covered two distinct phytoplankton bloom phases (diatoms followed by Phaeocystis sp.) and an interjacent bacterioplankton maximum. All dissolved REEs (dREEs) except samarium showed similar temporal concentration patterns, which were closely connected to the bloom succession. The concentration patterns were shaped by the ‘phytoplankton-shuttle’, which summarizes adsorption processes on phytoplankton-derived particulate OM (POM) and resulted in decreasing dREE concentrations alongside chlorophyll-a and POM maxima. The ‘heterotrophic-shuttle’ resulted in increasing dREE concentrations likely linked to heterotrophically mediated regeneration of POM and associated desorption processes. The effect of these processes on dREEs resulted in enhanced fractionation of light REEs (LREEs) relative to heavy REEs (HREEs) during adsorption processes and decreased fractionation as a result of desorption. At times of high dissolved organic carbon (DOC) concentrations, we observed a stabilization of especially dHREEs likely in organic complexes. To test the potential influence of DOC on dREEs, we used a PHREEQC model approach that revealed dREE complexation with components of the DOC pool and an increase in complexation with atomic mass of the REEs. That is, at high DOC concentrations OM-dREE complexation leads to an effective and preferential buffering of dHREE against adsorption. Our findings reveal that OM-cycling influences concentration patterns of dREEs via ad- and desorption processes as well as organic complexation with parts of the OM pool, suggesting these processes can have a significant impact on dREE concentrations in the natural marine environment under high OM conditions

Highlighting Theodor W. Engelmann's “Farbe und Assimilation” [Color and Assimilation]

In 1883, Theodor Wilhelm Engelmann, a German scientist, wrote his essay “color and assimilation” (Ger.: “Farbe und Assimilation”) describing the state of the art in photosynthesis research, his recent findings, and further assumptions based upon his presented results. Nearly 140 years later, many of his assumptions were proven correct. By his still well‐known bacteria experiments using aerotactic, heterotrophic bacteria, he identified the chloroplasts as the location in which photosynthesis and oxygen production takes place. Furthermore, by evaluating the effects of different light spectra, he constructed the first action spectra that demonstrated the implication of the “green gap” of chlorophylls. He further posited that accessory photosynthetic pigments existed to extend the absorption range of chlorophyll. Although infrequently cited, his work was foundational for current ecological research of the vertical appearance of algae species within the underwater gradient in light spectrum due to specific harvesting of different light spectra, hence complementary chromatic adaptation of communities. This short retrospective highlights this piece of literature that represents an early step toward our current understanding of ecological competition for light spectra

Changes in spectral quality of underwater light alter phytoplankton community composition

Light is a fundamental resource for phytoplankton. To utilize the available light, most phytoplankton species possess pigments in taxon‐specific combinations and quantities, which in turn result in a specific use of certain wavelengths. This optimizes the light use efficiency, allows for a complementary use of light, and may be an additional driver for community structure. While the effects of light intensity on phytoplankton biomass production and community composition have been intensively studied, here we focused on the effects of specific light spectrum quality (thus light color) on a natural phytoplankton community. In a controlled mesocosm experiment we reduced the supplied wavelength range to its blue, green, or red part of the light spectrum and compared the responses of each treatment to a full spectrum control over 28 d. Highest community growth rates were observed under blue, lowest under red light. Light absorption by the communities showed adaptation toward the supplied wavelength range. Community composition was significantly affected by light quality treatments, driven by Bacillariophyta and Chlorophyta, whereas pigment composition was not. Furthermore, lower species richness but higher evenness occurred when communities were exposed to red light compared to the full spectrum. We expected the response of phytoplankton communities to changes in the light spectrum to be driven by a combination of species sorting and pigment acclimation; however, the effect of species sorting turned out to be stronger. Our study showed that, even if species might acclimate, changes in the available light spectrum affect primary production and phytoplankton community composition

Concentrations, patterns and organic complexation of dissolved rare earth elements during an artificially induced phytoplankton spring bloom

In order to investigate the influence of organic matter (OM) on rare earth element (REE) distributions and patterns in the marine environment we monitored concentrations of dissolved REEs (dREEs) during an artificially induced spring bloom. Our mesocosm approach mimicked a neritic North Sea water body. Three biological replicates (P2-P4) were inoculated with a phytoplankton and associated bacterial community, which was retrieved in March 2018 from the southern North Sea. The incubation was monitored for 38 days. The experiment additionally covered the investigation of two biota-free controls. A variety of parameters were sampled, the results of some are published by Mori et al. (2021). Samples for dREE analyses were taken at intervals of 1-5 days. Preconcentration, analysis and quantification of dREEs followed the method described by Behrens et al. (2016). In order to investigate possible complexation of dREEs with components of the dissolved OM pool, a PHREEQC model was written that simulated chemical speciation of the dREEs in the mesocosms. A new databank was created that includes stability constants for complexes of dREEs with the main inorganic ligands (Cl⁻, SO₄⁻, OH⁻, CO₃⁻) as well as with the strong organic ligand desferrioxamine B (DFOB) after Christenson and Schijf (2011). The model outcome includes concentrations of inorganic and organic dREE complexes as well as abundances as free ions (REE3+) and total dREE concentrations. Additionally, we calculated the proportions of the different complexes to the total dREE pool. We used two different approaches for the PHREEQC model approach that followed Schijf et al. (2015) and were characterized by the concentration of the strong organic ligand and the resulting proportion of organic complexes to the dREE pool. The 'High-DOC' approach results in a maximal proportion of organic REE-DOC complexes of 40%, the 'Low-DOC' approach results in maximum of 10% organic complexes. To keep an eye on variations in carbonate complexes, total alkalinity (TA) was monitored as well. TA was sampled daily, for the analysis we used a multiscan GO microplate spectrophotometer (Thermo Scientific) and followed the method described by Sarazin et al. (1999)

Fractionation patterns, normalized concentrations and standards of dissolved rare earth elements during an artificially induced phytoplankton spring bloom

To investigate the influence of organic matter (OM) on rare earth element (REE) distributions and patterns in the marine environment we monitored concentrations of dissolved REEs (dREEs) during an artificially induced spring bloom. Our mesocosm approach mimicked a neritic North Sea water body. Three biological replicates (P2-P4) were inoculated with a phytoplankton and associated bacterial community, which was retrieved in March 2018 from the southern North Sea. The incubation was monitored for 38 days. The experiment additionally covered the investigation of two biota-free controls. A variety of parameters were sampled, the results of some can be found in Mori et al. (2021; doi:10.1016/j.gca.2021.08.002). Samples for dREE analyses were taken at intervals of 1-5 days. Preconcentration, analysis and quantification of dREEs followed the method described in Behrens et al. (2016; doi:10.1016/j.marchem.2016.08.006.). Concentrations were normalized to the Post Archaen Australian Shale (Rudnick and Gao, 2003). We further normalized concentrations throughout the experiment to initial concentrations (T0-normalization) and calculated loss and gains of dREEs

Investigation of Total Alkalinity during an artificially induced phytoplankton spring bloom

To investigate links between biological, biogeochemical and physical parameters, we closely monitored an artificially induced spring bloom. Our mesocosm approach mimicked a neritic North Sea water body. Three biological replicates (P2-P4) were inoculated with a phytoplankton and associated bacterial community, which was retrieved in March 2018 from the southern North Sea. The incubation was monitored for 38 days. The experiment additionally covered the investigation of two biota-free controls. A variety of parameters were sampled, the results of some can be found in Mori et al. (2021; doi:10.1016/j.gca.2021.08.002). Total alkalinity was sampled daily. For the analysis we used a multiscan GO microplate spectrophotometer (Thermo Scientific) and followed the method described by Sarazin et al. (1999; doi:10.1016/S0043-1354(98)00168-7)

Concentration of dissolved rare earth elements during an artificially induced phytoplankton spring bloom

To investigate the influence of organic matter (OM) on rare earth element (REE) distributions and patterns in the marine environment we monitored concentrations of dissolved REEs (dREEs) during an artificially induced spring bloom. Our mesocosm approach mimicked a neritic North Sea water body. Three biological replicates (P2-P4) were inoculated with a phytoplankton and associated bacterial community, which was retrieved in March 2018 from the southern North Sea. The incubation was monitored for 38 days. The experiment additionally covered the investigation of two biota-free controls. A variety of parameters were sampled, the results of some can be found in Mori et al. (2021; doi:10.1016/j.gca.2021.08.002). Samples for dREE analyses were taken at intervals of 1-5 days. Preconcentration, analysis and quantification of dREEs followed the method described in Behrens et al. (2016; doi:10.1016/j.marchem.2016.08.006.)

Organic complexation of dissolved rare earth elements during an artificially induced phytoplankton spring bloom- a PHREEQC model approach

A PHREEQC model was written to investigate complexation of dREEs with components of the dissolved OM pool. Based on the data measured during a mesocosm experiment (maybe link the original dataset) the model simulated chemical speciation of the dREEs in the mesocosms. A new databank was created that includes stability constants for complexes of dREEs with the main inorganic ligands (Cl⁻, SO₄⁻, OH⁻, CO₃⁻) as well as with the strong organic ligand desferrioxamine B (DFOB) after Christenson and Schijf (2011). The model outcome includes concentrations of inorganic and organic dREE complexes as well as abundances as free ions (REE^3+) and total dREE concentrations. Additionally, we calculated the proportions of the different complexes to the total dREE pool. We used two different approaches for the PHREEQC simulations that followed Schijf et al. (2015; doi:10.1016/j.marchem.2015.06.010) and were characterized by the concentration of the strong organic ligand and the resulting proportion of organic complexes to the dREE pool. The 'High-DOC' approach results in a maximal proportion of organic REE-DOC complexes of 40%, the 'Low-DOC' approach results in maximum of 10% organic complexes. To keep an eye on variations in carbonate complexes, total alkalinity (TA) was monitored as well. TA was sampled daily, for the analysis we used a multiscan GO microplate spectrophotometer (Thermo Scientific) and followed the method described by Sarazin et al. (1999; doi:10.1016/S0043-1354(98)00168-7)