Regeneration of Trace Metals During Phytoplankton Decay: An Experimental Study

Trace metals act as important nutrients, and sometimes toxins, to phytoplankton and other marine microorganisms. When phytoplankton decay, the elements in their cells are released back into the water column through regeneration (also referred to as remineralization), one of the processes responsible for governing concentrations of dissolved trace metals and macronutrients in depth profiles. In order to experimentally study regeneration, controlled experimental incubations of mixed phytoplankton assemblages from the Gulf of Mexico (GoM) and monocultures of the diatom Pseudo-nitzschia dolorosa and the dinoflagellate Karenia brevis were placed in the dark and monitored as they decayed with naturally present bacteria. Over six months, samples were collected and analyzed for dissolved trace metals (Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb), macronutrients (phosphate, silicate, nitrate + nitrite (“N+N”), nitrite, and ammonium), chlorophyll a (Chl a), and particulate organic carbon and nitrogen (POC and PON) concentrations. In experiments with diatoms present (P. dolorosa and mixed assemblages), phosphate and silicate remained closely coupled during regeneration, and no delay in silicate regeneration was observed. Cadmium (Cd) regeneration mirrored that of phosphate, consistent with the nutrient-type distributions of Cd commonly observed in the global oceans. Zn (Zn) followed a similar trend, but was less tightly coupled with any of the macronutrients. Dissolved manganese (Mn), on the other hand, was briefly released at the onset of regeneration but was then drawn down through the remainder of the experiments, which was attributed to the formation of Mn-oxides. Of the other trace elements studied, lead (Pb) and iron (Fe) showed the most evidence of scavenging to Mn oxides during the regeneration, and a delayed increase in Fe was observed by the end of most of the experiments after Mn stabilized. In M2 and M3, copper (Cu) also followed a similar delayed increase to Fe, but was less pronounced. Cobalt (Co) and nickel (Ni) did not display clear evidence of scavenging to Mn-oxides, and in one of the mixed assemblages (M1), followed a more nutrient-like uptake and regeneration. For the rest of the experiments, Cu, Co, and Ni exhibited more conservative behavior (i.e., little change relative to the overall reservoir), though any biological uptake observed in the grow-out phase was generally returned to the dissolved fraction in the regeneration experiments. The results of these experiments fill an important gap in understanding the important role of regeneration in trace metal cycling and can ultimately be applied to understanding vertical distributions in ocean water columns