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Diatoms are responsible for 40% of ocean production and are strongly limited by the lack of iron salts in the sea. The shortage of this important nutrient may explain a surprising discovery: the oceanic diatom Thalassiosira oceanica uses a copper-containing plastocyanin for electron transport. All other chlorophyll c-containing taxa, including coastal diatoms, use the iron-containing cytochrome c6 instead. The use of these metalloproteins matches the availabilities of copper and iron in the ocean, and this new discovery suggests that copper is a potentially important nutrient in the open sea. The supply of some essential metals to pelagic ecosystems is less than the demand, so many phytoplankton have slow rates of photosynthetic production and restricted growth1. The types and amounts of metals required by phytoplankton depends on their evolutionary history2 and on their adaptations to metal availability3,4, which varies widely among ocean habitats. Diatoms, for example, need considerably less iron (Fe) to grow than chlorophyll-b-containing taxa2, and the oceanic species demand roughly one-tenth the amount of coastal strains5,6,7. Like Fe, copper (Cu) is scarce in the open sea, but notably higher concentrations of it are required for the growth of oceanic than of coastal isolates8. Here we report that the greater Cu requirement in an oceanic diatom, Thalassiosira oceanica, is entirely due to a single Cu-containing protein, plastocyanin, which—until now—was only known to exist in organisms with chlorophyll b and cyanobacteria. Algae containing chlorophyll c, including the closely related coastal species T. weissflogii, are thought to lack plastocyanin and contain a functionally equivalent Fe-containing homologue, cytochrome c6 (ref. 9). Copper deficiency in T. oceanica inhibits electron transport regardless of Fe status, implying a constitutive role for plastocyanin in the light reactions of photosynthesis in this species. The results suggest that selection pressure imposed by Fe limitation has resulted in the use of a Cu protein for photosynthesis in an oceanic diatom. This biochemical switch reduces the need for Fe and increases the requirement for Cu, which is relatively more abundant in the open sea.

Iron is unique among biologically essential trace metals in having a higher particulate than dissolved concentration in ocean surface waters1. Uptake of dissolved iron is generally considered to be the norm for phytoplankton, as even the smallest iron-bearing particles are unavailable for transport into cells2,3. But the oceanic dissolved fraction is so small, and the particulate fraction so inert2, that phytoplankton production is limited by a dearth of available iron in some regions4. Here we use incubation experiments to show that Ochromonas sp., a common photosynthetic flagellate from the Pacific Ocean, can obtain iron directly in particulate form, by ingesting bacteria. Iron acquisition is highly efficient; Ochromonas assimilates 30% of the ingested ration, acquiring a high intracellular iron concentration and maintaining a significantly faster growth rate than when iron is provided in the dissolved phase. Phytoplankton capable of such phagotrophy (so-called mixotrophic species) may thus be able to assimilate iron in both particulate and dissolved forms in the ocean. Moreover, when iron availability is limited, the iron ‘cost’ of growth is diminished because Ochromonas derives a greater fraction of its energy from the bacteria. Analysis of standing stocks and clearance rates of plankton in the equatorial Pacific shows that the iron flux through mixotrophic flagellates can amount to 35–58% of the total Fe uptake by the entire autotrophic community. Our results suggest that the phagotrophic ingestion of bacteria may be an effective adaptive strategy for photosynthetic organisms to obtain iron for growth in iron-limited regions of the sea.

IN the oceans, many trace metals show a surface depletion relative to deep waters that is typical of the principal algal nutrients, N, i and Si, and is therefore presumed to result from biological uptake at the sea surface and regeneration at depth. Among trace metals, cadmium has an especially acute surface depletion1,2, and shows the best correlation with a major algal nutrient (P) 1–3. But the biological reason for Cd surface depletion is particularly puzzling, because unlike other surface-depleted trace elements (for example, Fe, Ni, Co, Zn and Cu), Cd is not known to be required by organisms. However, because Cd can substitute for Zn in some metalloenzymes in vitro4 and in vivo5, we hypothesized that Cd might promote the growth of Zn-limited phytoplankton. Marine phytoplankton are limited by a free Zn ion activity of 10–11.5 M (refs 6, 7), which is similar to the activity estimated for ocean surface waters8 as a result of the low concentration and organic complexation of Zn in the oceans. We now report that, in sea water with low Zn concentration, mimicking conditions of the ocean surface waters, Cd stimulates the growth of the marine diatom Thalassiosira weissflogii by substituting for Zn in certain macromolecules. The substitution is highly effective, in that Zn-defAcient cells can grow at 90% of their maximum rate when supplied with Cd. We also find that Co can substitute for Zn (although less efficiently than Cd), indicating that Co could be an important nutrient for algal growth for reasons other than its role in vitamin B12 (ref. 9).

Species of neritic eukaryotic marine phytoplankton were investigated during 1982 for hydroxamate-type siderophore production under iron-sufficient and iron-deficient culture conditions. Three of the 5 Prorocentrum species examined produced siderophores. Prorocentrin, the extracellular hydroxamate-type siderophore isolated from P. minimum, was also produced by P. mariae-lebouriae and P. gracile. P. maximum and P. micans grew poorly in iron-deficient medium and did not produce intracellular or extracellular hydroxamate-type siderophores. Thalassiosira pseudonana and Dunaliella tertiolecta produced extracellular siderophores under iron-deficient conditions, but siderophore production was not detected in the other two species, Skeletonema costatum and Olisthodiscus luteus. Each species which produced extracellular Csaky-positive hydroxamate showed a similar pattern of production. Under iron-sufficient conditions there was no measurable siderophore found either intracellularly or extracellularly. Under iron-deficient culture conditions hydroxamate-type siderophore was produced 1 to 2 d after the cessation of growth in the stationary phase. Production was over a short period of time (1 to 2 d) and the siderophore did not remain in the medium. The rate of siderophore disappearance from the medium was similar to the rate of production. Each species which produced siderophores showed an increase in in vivo fluorescence coincidental with the disappearance of the extracellular siderophore from the culture medium. There was no corresponding increase in in vivo fluorescence in iron-sufficient cultures. It is suggested that in vivo fluorescence may be used as a screening procedure for determining hydroxamate-type siderophore production in eukaryotic phytoplankton. An hypothesis on the iron uptake mechanism is proposed.

A comparison between the diacetyl monoxime and urease methods for measuring dissolved concentrations of urea in seawater was conducted in artificial seawater, phytoplankton-culture filtrate and both natural and ureaspiked field samples from coastal and oceanic enviroments during 1984. The urease technique underestimated urea concentrations in unbuffered photoplankton-culture filtrate as a result of the inhibition of the urease enzyme, causing the incomplete hydrolysis of urea in these samples. Factors responsible for inhibiting urease included pH, seawater ions, and possibly extracellular metabolites produced in unialgal cultures. Seawater type and time of sample collection were important variables affecting urea measurement by the urease method, and recovery of internal standards ranged from 40 to 100%. Increasing the heating time of the urease assay, or the concentration of urease added to the seawater samples increased the amount of urea determined by the urease method. However, measured values were still less than the concentration of the urea internal standards. The diacetyl monoxime method was suitable for urea determinations in all the seawater samples we examined; it was easily automated, and the results were accurate and reproducible. This modified technique is recommended for measuring disolved concentrations of urea in seawater.