Seawater is not pure water; as everyone who has swum in it knows it has a salty taste. These salts comprise a range of chemicals that are dissolved in the sea, the most common being chloride and sodium, followed by sulphate, magnesium, and calcium. In addition, there is a long list of less common 'salts', which include many of the key nutrients for plankton, such as phosphate, nitrate, and iron. The main source of marine salts is the weathering of rocks on land and the subsequent addition of chemicals released from these rocks to the sea via rivers or wind-blown dust, along with some nutrients from hydrothermal vents.30 All of these chemicals are at low concentrations in seawater and this has made them difficult to study in the past. Indeed, it was not until after the First World War that accurate methods for measuring the phosphate and nitrogen components (nitrite, nitrate, ammonium) were devised, mainly by William Atkins, Hildebrand Harvey, and Leslie Cooper, working in Plymouth on the south coast of England. They were able to show that these nutrients were in such short supply that they limited the production of phytoplankton, which in turn limited the food available for the small animals of the zooplankton. Indeed, they likened the seas of southern England to 'a closely grazed pasture'.22,23
An obvious way to attempt to prove that an ecological system is limited by nutrients is to experimentally add more and record the results. We will describe such experiments for iron (a key limiting nutrient in approximately one-third of the surface area of the world's oceans31) and postpone the discussion of other important nutrients (especially nitrate and phosphate) until the next section.
Iron (chemical symbol Fe) is widely used by organisms in a variety of enzyme systems and since iron is the fourth most abundant element in the Earth's crust one might guess that access to it by microbes is straightforward. The problem for the plankton is that under oxidizing conditions iron is very insoluble above pH 4 (seawater usually has a pH of around 8), and in the open ocean, away from the continental shelves where rivers can wash in new supplies, biologically available iron is very scarce. The main input of iron to these oceanic waters is from dust blown from arid areas such as the Sahara desert,32 although in the Southern Ocean upwelling deep water also appears to be an important source.33 These facts have interesting implications for the marine ecology of the past. For example, during glacial periods there was increased aridity in many areas and therefore iron-rich dust was entering the ocean at a greater rate. This may have caused an increased removal of carbon dioxide from the atmosphere by increased plankton growth, so reducing the 'greenhouse effect' and making the glacial climate even colder.33 This has led to ambitious proposals of modifying the amount of iron in the modern ocean as a way of combating human-caused global warming—we will briefly discuss this idea later after we have summarized the relevant experiments.
Interestingly, the current problems plankton experience with access to iron may not have existed in the distant past. Early in the history of life, iron may have been less limiting for marine plankton as the ocean was less oxygen-rich, since photosynthesis had not yet greatly increased the oxygen concentration of the atmosphere. Consequently, any iron would have been in an oxidation state, which was more available to life (because this less-oxidized iron is more soluble in water), potentially allowing greater biomass of photosynthetic bacteria in these early oceans.34 Atmospheric oxygen levels are thought to have been exceedingly low until the 'Great Oxidation Event' approximately 2-2.4 billion years ago (a billion being one thousand million), although photosynthetic microbes probably evolved rather earlier than this, but did not immediately lead to an oxygen-rich atmosphere and ocean.35,36 Indeed, the deep ocean does not appear to have become oxygen-rich until around 551 million years ago (based on the molybdenum chemistry of the rocks of the time).37
Our understanding of the role of iron in ocean ecology is relatively recent. At the beginning of the 1990s ship-based experiments, where iron was added to marine water samples in onboard laboratories, suggested that this element was often important in limiting the growth of phytoplankton. However, such small-scale experiments are difficult to interpret—are the results an artefact of the artificial conditions in the research ship's laboratory or do they represent what really happens in the open ocean? Owing to this, larger-scale ('mesoscale', bigger than laboratory scale but smaller than whole-ocean scale) iron-enrichment experiments were started, where iron was added to patches of ocean and the effects monitored. As of early 2007 there had been 12 such experiments carried out around the world, with a range of 350-2,820 kg of iron added to the water depending on the details of the particular experiment.31 All of these experiments led to increased plankton primary production, with diatoms often being the organism that responded the most to this treatment. One problem with these experiments is that they are of relatively short duration. Recently, Stéphane Blain and colleagues38 attempted to address this by studying plankton at a natural, and so more long-lived, upwelling of iron-rich water near the island of Kerguelen in the Southern Ocean. They estimated that the amount of carbon being exported to deep water (effectively a measure of plankton productivity) was at least 10 times greater than that seen in the short-term mesoscale experiments, probably in part due to the more steady input of iron from deep water (rather than being added as a large experimental 'pulse'). So, plankton productivity was certainly benefiting from the presence of iron.
The simplest way to view these experiments is to suggest that in much of the ocean phytoplankton production is simply limited by iron. However, this may often be an oversimplification as different limiting factors can interact with each other, and not all species react the same way. For example, as iron is needed for the construction of some of the proteins involved in photosynthesis, there is an interaction between light levels and iron requirements. In the late 1990s, William Suda and Susan Huntsman39 showed that in culture (i.e. growing the microbes in the laboratory) levels of iron that were not limiting at high light levels were limiting when there was less light and hence a need for the plankton to produce more photosynthetic machinery to capture the available photons. In addition, the picophytoplankton outperform larger phytoplankton in low iron conditions because iron uptake varies with cell surface area, so that smaller cells can get proportionately more iron per unit time from seawater. In addition, their absolute requirement for iron per cell is much lower—giving a system that can work at much lower ambient iron levels.
As we have mentioned in the context of ice age dust, on a large enough scale these increases in plankton production could have global implications by possibly reducing the amount of carbon dioxide in the atmosphere. Some marine plankton can have other effects on the Earth's climate too, for example, by increasing the production of sulphur-rich chemicals (dimethyl sulphoniopropionate and its break-down product dimethyl sulphide), which are important chemicals in cloud formation.40
Because of the role of plankton in removing atmospheric carbon dioxide, some people have suggested adding iron to the oceans to try and combat 'global warming'; however, there are currently large uncertainties over the likely effects of such large-scale human interventions. For example, it is currently not known whether most of the extra plankton are recycled after their death in the upper layers of the ocean—so releasing their carbon back to the atmosphere—or if enough sink into the depths to effectively remove their carbon from the atmosphere. In addition, the rate of loss of iron from the upper waters is poorly known.31,33 Because of these, and other uncertainties, few of the scientists who have been involved in these experiments are currently enthusiastic about this as an approach to addressing 'global warming'.41
As well as the uncertainties listed earlier, there is the problem that iron fertilization does not benefit all marine organisms. During an exceptionally long hot summer in the southern hemisphere in 1997, there were extensive fires affecting large areas of Indonesia.42 The dust and ash this created added large amounts of iron to the surrounding seas and caused dramatic increases in phytoplankton, which led to extensive mortality in coral and fish due to oxygen shortages when the plankton decomposed.43 This dramatically illustrates the links between terrestrial vegetation (and its effects on dust levels and fire frequency—described in the previous chapter), and the limitation of phytoplankton growth by nutrient shortages, which gives us a planet dominated by a blue sea.
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