Iron availability and the release of iron-complexing ligands by Emiliania huxleyi

The ubiquitous algal species, Emilania huxleyi, was incubated in sea water supplemented only with nitrate and phosphate (N and P) without chelating ag ents to control metal speciation. Growth was slow in a "low-iron" culture c ontaining 1.3 nM iron and was found to be iron-limited, growth-accelerating when a 1-nM iron addition was made. The growth rate in a "high-iron" cultu re (5.4 nM iron) was greater, reaching 0.4 div day(-1) but this culture too was found to have become iron-limited when a 9-nM iron addition was made o n day 17 of the incubation. Both cultures were found to release iron-comple xing ligands in excess of the iron concentration, 6 nM in the low-iron cult ure, and 10 nM in the high-iron culture. More ligands were produced after t he iron addition taking the ligand concentration to 11 nM in the low-iron c ulture. The data show that the ligands are released in response to the iron addition, when at least some of the iron had already been taken up. This t ype of release is contrary to the concept of a siderophore, which is suppos ed to be released in periods of lack of iron; however the increase in the l igand concentration is similar to that released by the natural community in response to the iron addition in the IRON-EX II experiment [Rue, E.L., Bru land, K.W., 1997. The role of organic complexation on ambient iron chemistr y in the equatorial Pacific Ocean and the response of a mesoscale iron addi tion experiment. Limnol. Oceanogr. 42, 901-910]. The enhanced growth in the cultures when more iron was added indicated that the organically complexed iron present in the cultures was not immediately available to the organism s (or at least not at sufficiently high rate), and that the organisms respo nded to freshly added, inorganic, iron. (C) 2000 Elsevier Science B.V. All rights reserved.