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

Abstract

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.

Multi-Endpoint Analysis of Cerium and Gadolinium Effects after Long-Term Exposure to Phaeodactylum tricornutum

“…According to Neira et al [22], heavy REEs tend to remain in solution-forming complexes usually unavailable for organisms, while light REEs (such as Ce) are most likely to be assimilated by them, posing potential biological implications. In general, in seawater, the toxicity of Ce and Gd compounds may be influenced by the interaction of the metal cation (Ce 3+ or Gd 3+ ) with other conjugated anions (e.g., SO 4 2− or Cl − ) [23]. REE concentrations in the ocean generally decrease from surface waters to mid depths (ranging from 6 to 10 m), before increasing near the sea bottom [22,24], and according to Akagi et al [25], the diatoms' presence may be a major pathway of REE transport to deeper layers.…”

The Global Biogeochemical Cycle of the Rare Earth Elements

“…Haley et al (2014) used the term "bio-active pool" to describe the influence of biological Global Biogeochemical Cycles 10.1029/2024GB008125 processes observed in the distribution of dissolved REEs, which is attributed to bacterial activity. Sutorius et al (2022) demonstrated, by incubation experiment, that the development of phytoplanktonic and associated bacterial development significantly impacts REE distribution through sorption processes and complexation with organic matter, although this effect is only observed during bloom periods.…”

“…Sutorius et al. (2022) demonstrated, by incubation experiment, that the development of phytoplanktonic and associated bacterial development significantly impacts REE distribution through sorption processes and complexation with organic matter, although this effect is only observed during bloom periods.…”

Extensive and diverse lanthanide-dependent metabolism in the ocean

“…5). Inputs of Ln to the surface ocean are highly spatially heterogeneous including terrigenous input from rivers 39 , wind-blown dust, subaqueous volcanism and upwelling of nutrient-rich deep water 40 , while outputs canonically include precipitation, and adsorption to clay minerals 41 and planktonic biomass 42 , in addition to biological uptake as described here. We focus our discussion on three possible outputs of Ln from the surface ocean: 1.…”