Aerosol trace metal leaching and impacts on marine microorganisms

Abstract

Metal dissolution from atmospheric aerosol deposition to the oceans is important in enhancing and inhibiting phytoplankton growth rates and modifying plankton community structure, thus impacting marine biogeochemistry. Here we review the current state of knowledge on the causes and effects of the leaching of multiple trace metals from natural and anthropogenic aerosols. Aerosol deposition is considered both on short timescales over which phytoplankton respond directly to aerosol metal inputs, as well as longer timescales over which biogeochemical cycles are affected by aerosols.

Fig. 1

Estimate of iron limitation regions and sources of soluble iron. Estimate of ocean iron limitation regions for diatoms, small phytoplankton, and diazatrophs, based on an ocean biogeochemistry model (a, b); iron is limiting for small phytoplankton (light mauve); iron is limiting for diazotrophs and small phytoplankton (purple); iron is limiting for small phytoplankton, diatoms, and diazatrophs (deep purple); iron is limiting for diazatrophs (gray-blue); iron is limiting for diatoms and diazatrophs (blue). This is shown for the current climate (a), and in the current climate with no atmospheric deposition of iron (b). See Supplementary Figure 1 for more information. Overall, most of the world’s ocean depend on atmospheric iron inputs. Diazotrophs are not present in the model at high latitudes where temperatures are below ~15 °C. The solid yellow line represents the mean location of this temperature control on the diazotrophs. Model-based source apportionment (c) for soluble iron deposition into the oceans using R. Scanza et al. (manuscript in preparation) and ref.. The dominant source for each region is identified by color (North Africa: dark blue; East Asia: light green; North America: pink; Australia: yellow; South Africa: red, South America: green). If the soluble iron is contributed substantially from two source regions, the colors are mixed with the dominant source providing the base color, and the secondary source the points. In regions where one source dominates, but the dominant source is not dust, but combustion, black diagonal stippling is applied (in the Southern Hemisphere, near Africa, South America and Micronesia)

Fig. 2

Schematic of aerosol metal dissolution and impacts on the ocean. Metal-bearing aerosols are emitted from natural (mineral dust, volcanoes, biological particles, and biomass burning) and anthropogenic activities (industrial combustion, agriculture, and land conversion), mostly over land. Combustion sources tend to have more soluble metals than the larger mineral dust sources. Some of the aerosols are transported to ocean regions, and can become processed so that the metals are more soluble by acids, photochemical and/or cloud processes, before being deposited to the oceans. In the oceans, the metals in the particles experience different chemical conditions, and can become more or less soluble, due to changes in acidity and ligands, for example. The metals can be cycled through bacteria, phytoplankton and zooplankton, and/or sink through the ocean to be scavenged in the deep ocean, or be deposited into the sediment. Sedimentary sources, riverine inputs, and hydrothermal vents can be important for many metals, as sources of new metals in the oceans

Fig. 3

Variations in aerosol metal composition, solubility, and evolution. a Estimates of the percent of the atmospheric source for different metals; b estimates of the mass percentage of different metals in dust concentrations, from observations and model simulations in dust storm events (from ref. ). c Measurements of metal solubility from ocean cruises with standard deviations shown as error bars^,. d Measurements of iron solubility and pH (blue) as a function of travel time from the source region from ocean cruise (data taken from ref. )

Fig. 4

Spatial distribution of metals in aerosols. Distribution of iron concentrations (R. Scanza et al. (manuscript in preparation) and ref. ) (a), iron solubility (R. Scanza et al. (manuscript in preparation) and ref. ) (c), and copper concentrations (e) in models and observations. Iron solubility in measurements in the atmospheric aerosols (a) and modeled values (b), and scatter plot comparing the observations versus the model for total iron (b), solubility of iron (d), and copper (f). Vertical bars in b, d represent one standard deviation above and below the annual mean values, based on daily averages within a chemical transport model. The mean and standard deviation are calculated in log-normal space, as the distribution of the values is closer to a log-normal distribution than a Gaussian distribution

Fig. 5

Ocean dissolution processes. a Elemental molar fractions of metals in bulk aerosol material collected In Eilat, Israel along the coast of the Gulf of Aqaba, Red Sea, and in the sea water-soluble fractions after 10 min and 7 days of leaching. Metals are arranged from most to least abundant based on the abundance in the bulk aerosol. Data were published previously in Mackey et al.; see reference for sample collection and methodological details. b Three dissolution modes were observed for different metals. Gradually dissolving metals show increased dissolved concentrations over time, whereas nearly all soluble metal is released immediately for rapidly dissolving metals. For particle reactive metals, the dissolved concentrations first increase but then decrease due to sorption onto particles and/or wall loss. c Schematic of the percent change in soluble concentration of several aerosol metals over time based on measurements after 10 min and 7 days of leaching. Positive values (above the horizontal line) indicate more metal dissolves with longer leaching (gradual dissolution), while negative values (below the horizontal line) indicate declining concentrations with time (particle reactive). Values near zero indicate rapid dissolution where the concentration of dissolved metal remains relatively constant over time

Fig. 6

Biological responses of phytoplankton and bacteria to aerosol trace metals. Trace metals are required as co-factors for many biogeochemically important molecules. Certain aerosol trace metals (a) have been shown to stimulate production of nitrogenase^,, alkaline phosphatase and metal-binding ligands^,,, and these molecules may remain within the cells (proportion of helix which is green) or be exuded into the sea water (b), where they catalyze chemical reactions that influence biogeochemical cycles (c). The cycling of N, P, and trace metals in turn affects the carbon cycle by influencing cellular growth rates. See Supplementary table 2 for compilation of studies, which this figure syntheses