Regeneration of macronutrients and trace metals during phytoplankton decay: An experimental study

Abstract

Macronutrients and trace metals are incorporated into phytoplankton during growth and regenerated back into the water column when phytoplankton decay, a process that contributes to the distributions of dissolved trace metals and macronutrients in depth profiles. To study this, we incubated mixed Gulf of Mexico phytoplankton assemblages and monocultures of the diatom Pseudo-nitzschia dolorosa and the dinoflagellate Karenia brevis in the dark. Over 6 months, macronutrients (phosphate, silicic acid, nitrate + nitrite, nitrite, ammonium), chlorophyll-a, particulate organic carbon and nitrogen, and prokaryotes were monitored alongside dissolved manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb). Results were compared to depth profiles to evaluate the role of regeneration in trace metal cycling. In contrast to water-column distributions, silicic acid and phosphate were closely coupled in experiments containing diatoms, indicating a shared regeneration pathway. Nitrification and nitrifying prokaryotes were only observed near the end of a subset of the experiments. Of the trace metals, Cd was most tightly coupled with phosphate. Regeneration of Mn was followed by rapid drawdown, consistent with Mn-oxide formation. Iron (Fe), Cu, and Pb typically remained low until Mn was depleted, suggesting either scavenging to Mn-oxides or otherwise delayed regeneration of these elements. Cobalt (Co) and Ni were largely conservative, but behaved like nutrients in the experiment using more offshore water low in Cd and Zn. Although experimental conditions were limited in their representation of the water column, these incubations provide novel insight into macronutrient and trace metal regeneration in the oceans. At the base of the marine food web, phytoplankton require the macronutrients carbon (C), nitrogen (N), and phosphorous (P) to build cellular organic matter, and, for diatoms, silicon (Si) to build frustules (Redfield 1934; Brzezinski 1985). Phytoplankton also assimilate a suite of trace metals into their cells, including manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), cadmium (Cd), and lead (Pb) (

Characterizing the Roles of Biogeochemical Cycling and Ocean Circulation in Regulating Marine Copper Distributions

“…Recently, Hollister et al. (2020) measured remineralization timescales of trace elements and found delayed regeneration of Cu. Studies suggest that Cu assimilated by marine phytoplankton may be adsorbed on the outside of the cells, where it is complexed by the external organic matrix of the cells (Gonzalez‐Davila et al., 2000).…”

“…Recently, Hollister et al. (2020) showed that Cu remineralized slowly in seawater, suggesting that Cu‐rich particles would remineralize at much greater depth than normal biogenic particles. However, the implications of such deep remineralization on global marine Cu distributions have not been investigated.…”

“…Despite the existence of variability, the geological/global residence time for Cu is estimated to be ∼3,000-6,000 years (e.g., Boyle et al, 1977;Hayes et al, 2018;Little et al, 2014Little et al, , 2013Richon & Tagliabue, 2019;Roshan et al, 2020). Recently, Hollister et al (2020) showed that Cu remineralized slowly in seawater, suggesting that Cu-rich particles would remineralize at much greater depth than normal biogenic particles. However, the implications of such deep remineralization on global marine Cu distributions have not been investigated.…”

Defining the Realized Niche of the Two Major Clades of Trichodesmium: A Study on the West Florida Shelf

“…We note that the methodology we used for analyzing macronutrients does not incorporate techniques optimized for low-level concentration analysis, so PO 4 and N+N were excluded in later statistical analyses. Dissolved Fe samples were UV-oxidized, preconcentrated onto a Nobias PA1 chelating resin using an automated seaFAST-pico system, and the resulting eluents analyzed using standard addition on a Thermo Scientific Element XR Inductively Coupled Plasma Mass Spectrometer in medium resolution and counting mode at the University of South Florida (Hollister et al, 2020). Dissolved Ba was measured after a 1:50 dilution in 5% ultrapure nitric acid and quantified by standard addition within 24 hours of the dilution preparation directly on the Element XR.…”

Controls on the Distributions of Dissolved Cd, Cu, Zn, and Cu‐Binding Organic Ligands in the East China Sea

“…In contrast, Cu exhibits a hybrid‐type profile in which both nutrient‐type processes and scavenging control its distributions (Bruland, 1980; Roshan & Wu, 2018). Trace metal and macronutrients, such as phosphate and silicate, typically show similar distribution patterns in the ocean, as they are both utilized by phytoplankton and are subsequently regenerated (Hollister et al., 2020; Roshan et al., 2017). Consequently, many previous biogeochemical studies have investigated the relationships between Cd and Zn with phosphate as well as Cu and Zn with silicate on a global scale (Jakuba et al., 2012; Janssen & Cullen, 2015; Kim et al., 2017; Middag et al., 2018; Roshan & Wu, 2018; Vance et al., 2017).…”