The interaction between zinc deficiency and copper toxicity as it affects the silicic acid uptake mechanisms in Thalassiosira pseudonana l

Limnol. Oceanogr., 26(l), 1981, 67-73 The interaction between zinc deficiency and copper toxicity as it affects the silicic acid uptake mechanisms in Thalassiosira pseudonana l John G. Rueter, Jr.,2 and F. M. M. Morel Department of Civil Engineering, Massachusetts Institute of Technology, Cambridge 02139 Abstract Zinc-deficient cultures of Thalassiosira pseudonana exhibited reduced silicic acid uptake rates. Copper toxicity decreased the silicic acid uptake rate at any zinc concentration. This resulted in the uptake rate being a function of the ratio in the medium of the cupric ion activity to the zinc activity rather than of either metal activity separately. These results are consistent with a proposed mechanism for the interaction between silicic acid and cupric ion activity involving a zinc-dependent active site. In work to be published elsewhere (Rueter and Morel in prep.) we found a competitive relationship between cupric ion activity and silicic acid concentration as they affect growth, silicic acid uptake, and copper uptake in the marine diatom Thalassiosira pseudonana (3H) (Hust.) Hasle and Heimdal. The result of this competitive interaction was that relatively high cupric ion activities resulted in higher rates of copper uptake and lower rates of silicic acid uptake. Conversely, high silicic acid concentrations could reverse the effect of copper toxicity, causing lower copper uptake rates and near maximal silicic acid uptake rates. We have considered a mechanism for this interaction in which an active site for silicic acid uptake is inhibited by the cupric ion activity and protected by high substrate (silicic acid) concentrations. This form of inhibition is similar to nonreversible competitive binding to the active site that has been studied in other enzyme systems (Zeffren and Hall 1973). We also thought that the binding of copper at this active site was one possible pathway for copper to enter the cell. In order to explain this proposed mechanism, we considered that zinc could be involved in silicic acid uptake. Zinc was This research was supported by NSF grant OCE 77-0900 to F. M. M. Morel and NOAA grant NA79AA-D-00077. 2 Present address: P.O. Box 751, Department of Biology, Portland State University, Portland, Oregon 97207. 67 suspected initially from circumstantial observations and evidence in the literature. Our original observation was that zinc-limited diatoms, Thalassiosira weissjlogii (Grun.) (= T. jluviatilis Hust.) (Anderson et al. 1978), morphologically resembled the copper-inhibited or silicate-starved T. pseudonana (Rueter and Morel in prep.). This suggested that the symptoms of copper toxicity might be equivalent in some cases to Zn deficiency, due to the inactivation of zinc sites by copper. Copper has been shown to displace zinc from high molecular weight compounds and the amount of displacement is directly related to the toxic effect of either Cu or Cd to many organisms (Brown 1978). Similarly the ratio of cop- per to zinc in the medium can determine the growth response in Euglena (Price and Quigley 1966). We investigate here our primary hypothesis that zinc was involved in a silicic acid uptake site by examining the silicit acid uptake rate in silicon-limited cultures grown in a range of zinc concentrations and also a secondary hypothesis that the zinc was exchangeable for copper. We thank S. Chisholm for advice and help on the manuscript and R. Collier for developing the methods used for copper analysis. Methods Thalassiosira pseudonana (3H) was obtained from R. R. L. Guillard and

68 Rueter and Morel Table 1. Zinc ion activity in Aquil for given total Table 2. Cupric ion activity in Aquil for given added zinc concentrations. pzn + computed for total copper concentrations. pcu* calculated for 5x lo- M and 5x 10 B M EDTA. 5x 10-fi M EDTA. pzn + 13.4 13.1 12.5 12.1 11.6 11.5 11.1 10.5 9.5 8.5 5x10 none B 10-l) 3x 10-g 4 x 10-g 10-H 4x 10-H 4x 10-7 4 x 10-G I Zn concn 5x10 a none (assume 5x 10.I )) 4x10 L, 4x10 H 4x10 7 maintained in f/2 (Guillard and Ryther 1962) with synthetic ocean water (Morel et al. 1979). For experimental work the cultures were grown at 22 C, illuminated continuously from below at 95 PEinst. rn-. s-l, and grown in variations of the medium Aquil (Morel et al. 1979). For the present study we used 3 x lo- M NO,, 2 x lo- M P04, 2 x lo- M Si(OH)4, and 5 x lo- M EDTA plus trace metals with no zinc: zinc was added in the amounts shown in the figures. The normal preparation of Aquil includes sterilization through a 0.4-pm Nuclepore filter and then aseptic transfer to autoclaved polycarbonate flasks of distilled water. The special preparation of Aquil used here to reduce zinc contamination included the following precautions: medium was passed through a Teflon column containing Chelex 100 (instead of the glass column normally used); the flasks and pipette tips were washed in distilled nitric acid and rinsed with Corning distilled water immediately before use; all transfers were made in a laminar flow hood; no part of the labware was ever autoclaved or exposed to a flame (i.e. standard sterile techniques could not be used); and the cultures were grown in closed flasks wrapped in plastic bags and only opened in the laminar flow hood. Most of these precautions were the result of a search for contamination sources with measurements on the atomic absorption spectrophotometer. pcu* z Cu 13 none 11.2 10-ti M 10.1 4x10 fi M 8.9 5x10- M Si(OH), was determined by centrifuging the sample (5 min at 1,600 x g) to remove the cells and using a reduced volume modification of the method described by Strickland and Parsons (1972). Alkaline phosphatase activity was measured by following the evolution of nitrophenol from p -nitrophenylphosphate at 4 10 nm (Kuenzler and Perras 1965). The acid-soluble copper was determined by digesting the centrifuged pellet from about 50 ml of culture with 40% freshly distilled (Vycar) nitric acid for 1 h and then reading directly on a model 360 Perkin-Elmer atomic absorption spectrophotometer with a HGA-2100 graphite furnace. Cells were counted with a Fuchs Rosenthal hemocytometer. The silicic acid uptake data were obtained from two types of experiments. In the first we grew cells to stationary phase due to silicic acid limitation at four zinc concentrations, spiked them with 10 PM Si(OH), and copper to pcu* = 8.9, and followed the uptake over the next 4 h. The cell specific uptake rate could be computed from the velocity and cell number since the cell concentrations did not vary greatly over the uptake interval. In the second type of experiment we followed the uptake of silicic acid during the exponential phase of growth. The initial Si(OH), in these cultures was 20 PM. The velocity of uptake could be computed directly for any interval of uptake, or the cell specific uptake rate could be computed as the growth rate times the final average cell quota. These computations depend on the value of the growth rate since the cell number changes rapidly during the uptake interval. Two other metabolic activities were examined in silicic acid-limited cultures

Cu:Zn to silicate uptake 69 pcu*= 8a9 o(cu : Zn**) a. II ] Q. I I I I I IO 9 8 7 6 5 PZn TOTAL Fig. 1. Uptake rate of silicic acid as a function of log total zinc concentration at two cupric ion activities. Silicic acid-limited cultures in stationary phase of cell density (2.46 & 0.2) x lo* cells-liter- that were grown in four concentrations of zinc were then spiked with two concentrations of copper to equal pcu* = 13 and pcu* = 8.9. Uptake rate was determined by disappearance from media over 4 h after a spike of 10 PM Si(OH),. Background zinc concentration was assumed to be lo-l0 M. pzn + ranges from 13.1 to 8.1 (Table 1: 5 x lo- M EDTA). at stationary phase that had been grown in different concentrations of zinc. The cultures were spiked with 10 PM Si(OH),, and then with either pnpp or with copper (pcu* = 8.9), to determine alkaline phosphatase activity over 20 h or copper uptake over 4 h. The chemical speciation of the medium Aquil was calculated with the computer program MINEQL (Westall et al. 1976). The zinc ion and copper ion activities in these experiments are the result of the total zinc and copper concentrations added to the media, as shown in Tables 1 and 2. Results In the first set of experiments there was no difference in the growth rate or final cell yield of 7. pseudonana cultures in response to total added zinc from 0 to Fig. 2. Uptake rate of silicic acid as a function of ratio of cupric ion to zinc ion activities. Data are same as in Fig. 1 plotted against minus log of ratio of copper to zinc ion activities. To calculate zinc ion activity we assumed that there was no significant depletion of zinc from medium. On this figure, higher activities of copper and lower activities of zinc are toward left. 5 x lo- M. This indicates that the background con tamination i n the normal preparation of Aquil mi nus zinc (designed to be very low in all trace metals) was sufficient for maximum growth rate. Although neither the growth-rate during exponential phase nor the plateau density was limited by zinc, the potential uptake rate of these stationary phase cultures was higher in the higher zinc cultures (Fig. 1, 0). These stationary phase cultures were silicic acid-limited and the uptake rate was determined by disappearance of a 10 PM Si(OH), spike. This means that the growth rate in these cultures was not related to the maximum silicic acid uptake rate at stationary phase. When copper was added to give a cupric ion activity of 1O-s.g (pcu* = 8.9), the uptake rate of silicic acid was again a direct function of the original zinc added to the medium, but Si(OH), uptake rates were uniformly inhibited by Cu for all values of Zn (Fig. 1, 0). This indicates that zinc concentrations up to 5 x lo- M can ameliorate the effect of copper toxicity. In this experiment for the least favorable conditions -high copper and lowest zinc-there was a net negative up-

Rueter and Morel IO! i r - i II > 01 E v) - u 0 IO o No Zn + IO+ Zn A 3x10dgZn 0 lome Zn i L!-L-L----A- I 0 I 3 IO Zn x109m TOTAL 10: I I I I 2 3 4 DAYS Fig. 3. Cell concentration of T. pseudonancl as a function of time for four zinc concentrations. A very clean preparation technique was used to lessen background zinc contamination (see methods). take of Si(OH), (fl ux of silicic acid out of the cell). Our hypothesis predicts that the inhibition by copper should be related to the ratio Cu:Zn. As predicted, if the data from Fig. 1 are transformed there is a linear relationship between the silicic acid uptake rate and the log of the ratio of the copper to zinc activities (Fig. 2). Using extraordinary care in media preparation (see methods), we could reduce the zinc contamination to such a level that Zn was actually limiting the growth rate and cell yield of T. pseudonana (Fig. 3). We also determined silicic Fie. 4. Silicic acid udtake velocities for cultures groin in four differeni zinc concentrations. Cells were in the middle of exponential phase (from day 1.5 to 2.5: Fig. 3) increasing from an average of 1.3 x lo4 cells *ml- to 10.2 x lo4 over the 26.5-h interval. Uptake velocity calculated by change in external silicic acid concentration. Absolute value of concentration of zinc at an extradolated zero UPtake velocity is used as an estimate for background contamination of zinc in media. acid uptake rates in this experiment in the 26.5-h interval from 1.5 to 2.5 days after inoculation into media of different zinc concentrations and activities. This experiment was designed to have lower cell numbers than before to allow the assumption of as small as possible a change in the zinc concentration due to cellular uptake. The uptake rate shows a hyperbolic relationship to the original zinc concentration in the media (Fig. 4). The uptake rates per cell for exponential phase cultures are higher than for stationary phase cells (Fig. l)-2540 x 10-l pmol. cell- * h-l for stationary phase. The background contaminant zinc concentration can be estimated by extrapolating the relationship between the uptake and the total zinc concentration to

Cu:Zn to silicate uptake 71 l,5r p (Cu*+: ZnZ+ ) z - -Ll-Lld IO II 12 13 14 p Zn*+ Fig. 5. Silicic acid uptake rate and alkaline phosphatase activity in same culture as a function of zinc activity in media. Silicon-limited cultures grown to same stationary phase density (2 x 1Oj cells -ml- ) in different zinc concentrations and 5 x lo- M EDTA. Silicic acid uptake rate per cell determined from growth phase of cultures and alkaline phosphatase activity determined at stationary phase by spiking with pnpp and sampling after 20 h. All values are compared to uptake or utilization rate of normal Aquil activity of zinc (which equals 10-10. M). the abscissa to calculate the additive intercept (Fig. 4). Simply, it is assumed that if this amount of zinc could be removed from the medium a zero uptake rate would be expected. The total concentra- Fig. 6. Copper per cell as a function of minus log of ratio of copper to zinc ion activities. 4 siliconlimited culture was spiked with copper to pcu* = 8.9 and zinc to concentrations of lo-, lo- M, and none added, and then with silicic acid. Cultures with an average cell density of 19.9 x 10 cells. ml- were harvested after 4 h and analyzed for acid-soluble copper per cell. Zinc activities were computed and background concentration of 5 x 10-l M was assumed. The no copper control (a) is plotted as a background copper estimate. tion of zinc that corresponds to this intercept is (0.7 2 0.2) x lo- M. Zinc seems to be strongly implicated in the silicic acid uptake mechanism. There are two additional supporting pieces of evidence that the uptake system for silicic acid is zinc-dependent. The first is the demonstration that silicic acid uptake behaves in a way similar to that of other zinc-dependent systems. Alkaline phosphatase is a zinc metalloenzyme present in T. pseudonana and has a convenient assay. The activity of alkaline phosphatase and the silicic acid uptake

72 Rueter und Morel rate show the same trend with respect to the original zinc activity in the medium (Fig. 5). Additional supporting evidence is from the uptake of copper. If copper uptake depends on the displacement of zinc by copper at a zinc-active site (as proposed by our model), then the rate of displacement should depend on the ratio of copper to zinc activities. Copper per cell decreased with increased activity of zinc in the medium at pcu* = 8.9 (Fig. 6, X) and, in addition, the copper per cell at pcu* = 13 (with no added copper) (Fig. 6, 0) also gives a reasonable value if plotted according to its Cu:Zn value. Discussion All of the results presented here are consistent with the hypothesis that silicic acid uptake is mediated by a zinc-dependent system which is inactivated by copper. The biochemical isolation of the enzyme responsible for this uptake will be necessary to prove the hypothesis. However, the fact that zinc is related to the activity of the enzyme may facilitate its isolation and characterization. Although the evidence suggests that the site of the Cu:Zn interaction is at the Si transport step, interactions at many steps in the cell metabolism might ultimately be manifested in the silicic acid uptake rate. The net result would be the same, but until the actual mechanism is determined this interaction is most simply modeled as a single zinc-dependent system for silicic acid uptake. The ratio of copper activity to zinc activity in the medium seems to be an important parameter for determining silicic acid uptake. Although our experiments were designed to minimize zinc uptake into the cells, at the low zinc concentrations added slight uptake could reduce the zinc activity drastically, resulting in a higher susceptibility to copper than predicted from the ratio of cupric ion to original zinc ion activity. The effect of such a mechanism was minimal in these experiments, as seen from the linearity of the uptake rate as a function of the ratio of the activities of Cu and Zn which seems to be conserved even for the lowest Zn activities (Fig. 2). The negative uptake rates that were seen in stationary phase cultures under the least favorable copper and zinc combination (Figs. 1 and 2) can be related to two previous observations. First, the actual dissolution rate may depend on trace metal absorption onto the frustule, as shown by Lewin (1961) for other divalent trace metals. Second, the net uptake rate for all the other copper to zinc ratios may include a dissolution component that is less than the uptake rate, as suggested by Nelson et al. (1976). Trace metal conditions that result in higher dissolution rates are just as important in the biogeochemical cycle of silicon as the effect on the silicic acid uptake rate. Our results also show that T. pseudonana can achieve maximum growth at lower zinc activities than can other species. An addition of lo- M zinc with 5 x lo- M EDTA resulted in a pzrp+ of 12.1 and fully restored the cells to maximal growth rate (Fig. 3). Using Aquil media, Anderson et al. (1978) found that T. weissflogii showed growth limitation starting at pzn 3+ = 11. Thus it takes an order of magnitude less zinc activity for full growth of T. pseudonana than it takes to limit the growth of T. weissjlogii. Although cell size may play a role in this difference, the mechanism is not obvious. This variation between similar species in their response to zinc activities adds another dimension to the interaction between phytoplankton and the metals in their environment and, in particular, illustrates the complications introduced by the interaction between toxic and nutritional metals. Although previous work with copper toxicity (Sunda and Guillard 1976; Morel et al. 1978; Rueter and Morel in prep.) did not ad- dress the zinc interaction, as long as the zinc concentrations were uniform in any study the responses observed should be due only to copper. Zinc is involved in the uptake of silicic acid in T. pseudonana. This can be modeled as a zinc-active site that has three

Cu.-Zn to silicate uptake 73 characteristics: cells show reduced uptake of silicic acid with less zinc, Cu competes with Zn for the active site and disables the site, and copper enters the cell as a function of the ratio of copper to zinc ion activities. These results are consistent with our model for a silicic acid uptake mechanism (Rueter and Morel in prep.). References ANDERSON, M. A., F. M. MOREL, AND R. R. GUIL- LARD. 1978. Growth limitation of a coastal diatom by low zinc ion activity. Nature 276: 70-71. BROWN, D. A. 1978. Toxicology of trace elements: Metallothionein production and carcinogenesis. Ph.D. thesis, Univ. British Columbia. 226 p. GUILLARD, R.R., AND J. H. RYTHER. 1962. Studies on marine planktonic diatoms. 1. CycZoteZZa nana Hustaedt and Dentonula confervacea (Cleve) Gran. Can. J. Microbial. 8: 229-239. KUENZLER, E. J., AND J. P. PERRAS. 1965. Phosphatases of marine algae. Biol. Bull. 128: 271-284. LENIN, J. C. 1961. The dissolution of silica from diatom walls. Geochim. Cosmochim. Acta 21: 182-195. MOREL, F. M., J. G. RUETER, D. M. ANDERSON, AND R. R. GUILLARD. 1979. Aquil: A chemically defined phytoplankton media for trace metal studies. J. Phycol. 15: 135-141. MOREL, N. M.,J. G. RUETER, AND F. M. MOREL. 1978. Copper toxicity to Skeletonema costatum. J. Phycol. 11: 4348. NELSON, D. M.,J.J. GOERING, S. S. KILHAM,AND R. R. GUILLARD. 1976. Kinetics ofsilicic acid uptake and rates of silica dissolution in the marine diatom Thalassiosira pseudonana. J. Phycol. 12: 246-256. PFUCE,~. A., AND J. W. QUIGLEY. 1966. A method for determining quantitative zinc requirements for growth. Soil Sci. 101: 11-16. STRICKLAND, J. D., AND T. R. PARSONS. 1972. A practical handbook of seawater analysis, 2nd ed. Bull. Fish. Res. Bd. Can. 167. SUNDA, W. G., AND R. R. GUILLARD. 1976. Relationship between cupric ion activity and the toxicity of copper to phytoplankton. J. Mar. Res. 34: 51 l-529. WESTALL, J.C.,J.L. ZACHARY,AND F.M. MOREL. 1976. MINEQL, a computer program for the calculation of chemical equilibrium composition of aqueous systems. Mass. Inst. Technol. Water Quality Lab. Tech. Note 18. ZEFFREN, E., AND P. L. HALL. 1973. The study of enzyme mechanisms. Wiley. Submitted: 11 October 1979 Accepted: 3 July 1980