different occasions and in all cases, massive blooms resulted. However, the blooms persisted for variable periods of time. An interesting observation made in some of these studies was that the concentration of the organic ligands increased over a short time span in order to match the concentration of added iron, thus implying biological origin and in view of their affinity for iron possibly being of a siderophore or siderophore-like nature. Significantly, heterotrophic bacteria were also found to markedly increase in number in the iron-induced blooms. Thus there is the element of synergism between phytoplankton and heterotrophic bacteria. Phytoplankton require iron (provided by bacterial siderophores), and heterotrophic bacteria require non-CO2 carbon sources (provided by phytoplankton). The dilute nature of the pelagic marine environment promotes large diffusive losses and renders the efficiency of the normal siderophore-based iron uptake strategies problematic. However, many heterotrophic marine bacteria do produce siderophores, albeit with properties different from those produced by terrestrial organisms. Many marine siderophores are surface-active and tend to form molecular aggregates, for example aquachelins. The presence of the fatty acyl chain renders the molecules with a high surface activity and an ability to form micelles. Thus, when secreted, these molecules bind to surfaces and to each other, thereby slowing the rate of diffusion away from the secreting organism and maintaining a relatively high local siderophore concentration. Phytoplankton have high iron requirements and yet the majority (and possibly all) do not produce siderophores. Phytoplankton can, however, obtain iron from siderophore complexes by the aid of membrane-bound reductases and certainly from iron(II) generated via photochemical decomposition of iron(III) siderophores. Thus a large proportion of iron (possibly all iron) absorbed by phytoplankton is dependent on bacterial siderophore production. Plant pathogens Most plant pathogens invade the apoplasm by releasing pectolytic enzymes which facilitate the spread of the invading organism. Bacteria frequently infect plants by gaining entry to the tissue via the stomata. Having entered the plant they spread and multiply in the intercellular spaces. With bacterial vascular diseases, the infection is spread within the plants through the xylem. Once within the plant, the bacteria need to be able to scavenge iron from the two main iron-transporting ligands, nicotianamine and citrate. To do this they produce siderophores, thus the enterobacterial Erwinia chrysanthemi produces two siderophores, chrysobactin and achromobactin. Xanthomonas group of plant pathogens produce xanthoferrin siderophores to scavenge the iron. Like in humans, plants also possess siderophore binding proteins