Biogeochemical cycling of molybdenum and thallium during a phytoplankton summer bloom: A mesocosm study

Molybdenum (Mo) and thallium (Tl) are known as conservative-type elements in open ocean settings, despite their involvement in bio-cycling processes. In coastal oceans like the southern North Sea, however, positive and negative anomalies of dissolved Mo and Tl concentrations occur during certain time periods of the year, which are characterized by intensive organic matter cycling. The main motivation of the present study was to identify potential drivers for the non-conservative behavior of Mo and Tl. For this purpose, we conducted an indoor mesocosm experiment with natural seawater and sediment (including a natural microorganism community) and applied close to natural light and tidal (diurnal) conditions. After an incubation time of 35 days, we initialized a storm event to examine its influence on organic matter as well as nutrient and trace metal cycling. The temporal pattern of the inorganic macronutrients (N-species, dissolved phosphorous, dissolved silicate) as well as dissolved and particulate organic matter was highly dependent on the interplay of the phytoplankton and its associated bacteria bloom. Our results suggest that the redox-sensitive trace metals manganese (Mn), vanadium (V) and iron (Fe) were involved in bio-cycling processes. While temporal pattern of dissolved Mn and V were likely induced by active (macro-)nutrient assimilation rather than redox induced phase changes, dissolved Fe was present as organo-metallic complex and shielded from the flocculation as metal oxide. Our results further reveal positive Mo and negative Tl anomalies, especially during pre-storm conditions. The additional input of Mo was derived from the oxidation of reducing bottom sediments. Thereby, the degree as well as the rate of Mo-input was dependent on the composition of the background sediment. In the water column Mo was not only present in its dissolved oxidized form but was also stabilized by organic (algae-detritus, ligands) and inorganic (aluminosilicates) binding partners, preventing its (re-)deposition. Negative Tl anomalies were found to be induced by its immobilization by organic (algae-detritus, ligands) and inorganic (aluminosilicates) carrier phases in the water column prior to its deposition and potential fixation in the sulfidic bottom sediments. Particles derived from the storm event did not have any considerable effect on dissolved organic nor inorganic compounds, as they (re-)deposited before significant remineralization processes could take place.