Incorporation of the trace elements Mn, Fe, Co, Ni, Cu and Zn into phytoplankton, and the chemical, biological and biochemical mechanisms which regulate this process, are of central importance to our understanding of the biogeochemical functioning of the oceans. These trace elements (TE) have the potential to control ocean productivity, ecosystem structure and the utilization of macronutrients in large regions of the global ocean. Significant effort has been expended to study the response of cellular trace element quotas to environmental conditions in laboratory cultures, but measurements of cellular trace element quotas and stoichiometries in natural biogenic particulate material are rare and sorely needed to constrain and evaluate biogeochemical models. The international GEOTRACES program was initiated to advance our knowledge of the concentrations and physical and chemical speciation of trace elements in the global ocean.
In this project, researchers at the Bigelow Laboratory for Ocean Sciences will measure the Al, Si, P, Mn, Fe, Co, Ni, Cu, Zn and Cd contents of individual phytoplankton function groups and bulk particulate matter across biogeochemical gradients in the surface waters of the Atlantic Ocean using a combination of bulk and microanalytical elemental techniques. Samples will be collected from the surface mixed layer and deep chlorophyll maximum layer during the GEOTRACES North Atlantic Survey Section cruise using a combination of GoFlo bottles, in situ pumping and a towed ¡®fish¡¯. Individual phytoplankton cells representing major functional groups (diatoms, autotrophic flagellates, cyanobacteria, coccolithophores) will be analyzed with synchrotron x-ray fluorescence analysis (SXRF). Particulate matter collected on 0.45-¦Ìm polyethersulfone membranes will be digested and analyzed with high resolution inductively coupled plasma mass spectrometry (ICP-MS). In this way, the response of phytoplankton trace element composition to natural gradients in dissolved and particulate macro- and micronutrient supply can be resolved. Additionally, three methods commonly used to better characterize bulk particulate matter (oxalate/EDTA rinse, weak acetic acid digestion, correction for lithogenic material with TE:Al ratios) will be applied and their effectiveness evaluated through comparisons with direct SXRF measurements of cellular trace elements. Single-cell analyses of diatoms and their frustules will enable evaluation of Zn and Ni as potential paleoceanographic proxies, as well as determination of synergistic and antagonistic relationships among macronutrients and micronutrients and taxa-dependent preferences for specific micronutrients, addressing several core GEOTRACES objectives.
Broader Impacts: The broader impacts of this study include a strong commitment to providing research experiences and mentorship to high school and undergraduate students as well as outreach to the general public through presentations at local organizations (e.g., libraries, churches, civic groups) and via the Caf¨¦ Scientifique program at Bigelow.
Ocean phytoplankton convert carbon dioxide into organic matter, some of which sinks from surface waters to the ocean depths. Through this process, phytoplankton ‘pump’ carbon from the atmosphere into the ocean and help to lower atmospheric carbon dioxide concentrations in the atmosphere. Phytoplankton in the ocean require a suite of nutrients to sustain their growth. In addition to major nutrients like nitrogen and phosphorus, plankton require metals such as iron, manganese, and zinc for their cellular processes. In regions with low inputs of these metals (for example, in regions far from dust sources), the growth of phytoplankton can be limited by metal availability. Despite the importance of metals to phytoplankton, there are very few measurements of the metal contents of phytoplankton in the ocean. In this study we used state-of-the-art microanalytical techniques to measure the metal contents of phytoplankton collected as part of a large international study to determine the forms and distributions of metals in the earth’s oceans (GEOTRACES). Cells were collected during 2 cruises across the Atlantic Ocean (Fig. 1) and then analyzed at the Advanced Photon Source synchrotron facility at Argonne National Laboratory in Illinois. This facility enables us to determine the spatial distributions and concentrations of most of the elements found within phytoplankton cells (Fig. 2). Our results show that some metals are maintained at remarkably consistent levels in cells over a wide range of environmental conditions, while others vary across geographic and chemical gradients. For example, nickel, cobalt and zinc were found at roughly the same levels in species of cells collected from different depths of the water column and from stations across the North Atlantic Ocean. However iron and manganese occurred at much higher levels in cells collected near the coast of North America. These waters overlay a wide, shallow continental shelf which can act as a source of metals to the water (and, we have discovered, the cells in these waters). Interestingly, phytoplankton cells collected from the eastern side of the North Atlantic off the coast of Africa did not have notably higher iron levels. This was surprising, since these waters are both close to the African continent and in a region that receives significant inputs of Saharan dust (that contains significant iron). Clearly the cells are able to control their iron contents. Additionally, we found that certain types of cells called diatoms have much higher levels of iron than other co-occurring species of phytoplankton. Diatoms have glass shells that make them more dense and more likely to sink from surface waters. Therefore, they may be more likely to remove iron from surface waters, as well. Finally, through comparisons with our single-cell element analyses we tested the utility of several acid leaches for estimating the concentrations of particulate metals that are associated with phytoplankton. We found that some (acetic acid leaches) are much more effective than others (oxalate soaks) for providing this valuable information. This project provided opportunities for the training of four undergraduate researchers, including opportunities for the students to assist with synchrotron analyses at the Advanced Photon Source. Additionally, several students attended international science meetings to present their findings. Project scientists also presented their findings at professional meetings, as well as to the general public through talks given as part of the Café Scientifique lecture series and through talks to civic and church groups. Finally, information gathered through this project was incorporated into course materials developed for undergraduate classes.
