Copper toxicity and cyanobacteria ecology in the Sargasso Sea

Abstract

The closely related cyanobacteria Synechococcus and Prochlorococcus have different distributions in stratified water columns in the northern Sargasso Sea. The abundance of Synechococcus is relatively uniform with depth, but Prochlorococcus cell numbers are low within shallow mixed layers and high in and below the thermocline. Because free cupric ion (free Cu 2ϩ ) concentrations are high (up to 6 pM) in shallow mixed layers and lower in deeper water, there is an inverse relationship between Prochlorococcus densities and the free Cu 2ϩ concentration. We explored the possibility of a causal underpinning for this relationship by examining the relative copper sensitivities of Prochlorococcus and Synechococcus in cultures and field populations. Prochlorococcus isolates from both the high-and low-light adapted ecotypes were inhibited at free Cu 2ϩ concentrations that had no effect on Synechococcus. However, the high-light adapted strains were more copper resistant than their low-light adapted counterparts. When copper was added to Prochlorococcus from environments where the in situ free Cu 2ϩ was low (in deeply mixed water columns and below the mixed layer in stratified conditions), net growth rates were substantially reduced and cells arrested in the G 1 and early S phases of the cell cycle. Prochlorococcus in shallow mixed layers were less sensitive to copper and were probably members of the copper-resistant high-light adapted ecotype. Synechococcus were relatively copper resistant across a range of environments in the Sargasso Sea. These observations are consistent with our hypothesis that copper plays a role in cyanobacteria ecology in the Sargasso Sea.Human activities have produced a measurable increase in the concentrations of trace metals in even remote environments. For instance, the atmospheric flux of copper to Greenland ice has increased by over an order of magnitude from the pre-Roman period to the present (Hong et al. 1996). 3 Corresponding author (chisholm@mit.edu). AcknowledgmentsWe thank P. Lam and H. Hsu for invaluable technical assistance in the laboratory and M. Saito for stimulating discussions. We also thank the captain and crew of the RV Oceanus for making the field experiments possible. Comments from two anonymous reviewers have been very helpful in revising the manuscript.

Toxicity of atmospheric aerosols on marine phytoplankton

“…The toxicity response was rapid, depressing F v /F m measurements (a measure of photosynthetic capacity) within 1 h and impairing cell growth within 24 h of Cu addition (see SI Text). These results are consistent with our field observations (toxicity at 0.4 g of Cu per g Chl a apparent within the first day of incubation) and with published results from the Sargasso Sea and other culture experiments (24,25). Similar experiments with nickel (Ni) or lead (Pb) (which were also higher in the African aerosols) did not result in similar toxicity responses as were seen with the field incubations.…”

Copper aerosols inhibit phytoplankton growth in the Mediterranean Sea

“…5 and the data displayed in However, this value has to be considered as an approximate threshold because the sensitivity to Cu toxicity varies among phytoplankton species (28). For example, diatoms are usually more tolerant than dinoflagellates to elevated Cu concentrations (19,30,31). In the western Mediterranean Sea, cyanobacteria, picoeukaryotes, diatoms, and flagellates belonging to different algal groups often coexist, although their relative abundances are marked by strong seasonal variations in that diatoms predominate in winter and nanoflagellates in spring and summer (32).…”

“…Merged satellite chlorophyll (Chl) data were used as an indicator of total phytoplankton biomass (18). Daily Chl variations (ΔChl) were calculated during Cu pulses, as the toxic response of phytoplankton to Cu was previously shown to be evident already on the first day of a Cu event (9,10,19). As seen in Fig.…”

The Impact of Atmospheric Dry Deposition Associated Microbes on the Southeastern Mediterranean Sea Surface Water following an Intense Dust Storm

“…This is consistent with the expected change derived from the PO 3− 4 addition (∼1.5 nM) and its subsequent carbon addition based on the C to chl-a ratio (see above). The relatively limited change in picophytoplankton abundance implies a possible top-down control of small-size autotrophs by grazers (Billen, 1990), dominance of larger-size algae such as microphytoplankton that utilized most of the leached nutrients/trace metals and compete with the picophytoplankton (Duarte et al, 2000), or toxicity of small-size autotrophs (Mann et al, 2002;Paytan et al, 2009). It may also hint at interactions between the ambient population and the airborne microbes that affect the abundance of small-size autotrophs.…”