Calcium belongs in the second of these categories. It does have an immediate and direct relevance to the species that deploy calcium salts (usually carbonate in the form of calcite) in skeletal structures. Among the most notable of these are the marine coccolith-producing hap-tophytes. For such organisms, there is a specific and finite requirement for calcium but the amounts dissolved in sea water are, globally, generally uniform. Of the mean solute content (35 ± 3 g kg-1: Section 2.2.1), calcium accounts for just over 1% by mass (typical calcium content, ~0.4 g L-1, i.e. ~10-2 M, or 20 meq L-1) (see Fig. 4.7). Any preference of particular species for particular water masses is unlikely to be governed by differences in ambient calcium concentrations. Among fresh waters, however, calcium is frequently the dominant cation (<120 mg L-1, <6 meq L-1), although concentrations (and specific 'hardnesses') in individual water bodies vary throughout the available range. The importance of calcium hardness resides in its relation to the anions; electrochemical balance in fresh waters is often contributed, substantially or in part, by bicarbonate ions. Being derived from the salt of

Figure 4.7

The major ionic constituents dissolved in sea water. Redrawn with permission from Harvey (1976).

Figure 4.7

The major ionic constituents dissolved in sea water. Redrawn with permission from Harvey (1976).

a weak acid, they allow the strongly alkaline ions to press pH above neutrality, although this is resisted by the presence of free carbon dioxide in solution. Dissociation of the bicarbonate to release free CO2 serves to buffer the water at a mildly alkaline level, as well providing an additional resource of photosynthetic DIC (Section 3.4.1; see Fig. 3.17). This crucial participation in the carbon-dioxide-bicarbonate system means that calcium can have a strong selective influence among phytoplankton that are variously sensitive to pH and carbon sources and, ultimately, those with an acknowledged capacity for carbon concentration (CCM; see Section 3.4.2). In this way, many chrysophyte species seem to be confined to soft-water (low-Ca) systems (unless there are local sources of CO2 from organic fermentation), while many Cyanobacteria are supposed to have an affinity for calcareous waters. Indeed, a large number of cyanobacterial genera is reputed to be relatively intolerant of acid conditions (pH <6.0; Paerl, 1988; Shapiro, 1990). The mechanism underpinning the observation has not been satisfactorily explained. The generality about pH sensitivity in these organisms is doubtless confounded by the extreme tolerance of high pH by many planktic Cyanobacteria, yet some species (e.g. of Merismopedia, Chroo-coccus) are plainly able to function adequately in notably acidic waters (pH >5.5; author's observations). The green volvocalean alga Phacotus, whose single cells are enclosed in a calcified envelope, is a true calcicol and its presence in sediment cores is taken to indicate highly calcareous phases in the development of the lake whence the cores were extracted (Lund, 1965).

Magnesium is the second most abundant (to sodium) cation in sea water (Fig. 4.7) and is the other common divalent cation (with calcium) in freshwater. Although an essential component of the chlorophylls, magnesium - a single atom is chelated at the centre of a tetrapyrole ring - is not known to limit phytoplankton production in nature. Supposing Mg to be under 3% of the mass of a chlorophyll molecule of 894 da, then even a very large phytoplankton population (1000 mg chla m-3 is sustained by 30 mg Mg m-3, or ~1.25 |M Mg) is unlikely to challenge the availability in the majority of natural waters.

Similarly, sodium and potassium are rarely considered to have much influence on algal composition, save through their impact on ionic strength and on the effects of ionic osmosis across the cell membrane. Marine algae equipped to deal with a medium containing some 10.7 g Na L-1 (i.e. ~0.47 M, or 470 meq L-1) would simply burst if immersed in a soft, oligotrophic lake water containing between (say) 0.1 and 0.5 meq (or 2-10 mg Na) L-1. Conversely, phytoplankters from dilute fresh waters shrivel and lyse in sea water. Neither would tolerate the extreme salinities of certain endorheic inland waters; the dissolved sodium content of the Dead Sea is ~1090 meq (25 g, or 1.1 mol) L-1. The cationic strength is compounded by some 140 meq (5.5 g) L-1 potassium and 2540 meq (58 g) L-1 calcium. At the other extreme, minimum sodium concentrations required by algae and Cyanobacteria are to be found in the literature, the highest of these being 4-5 mg Na L-1 (say 0.2 meq L-1) for a cultivated Anabaena (Kratz and Myers, 1955). However, many studies on other algae and Cyanobacteria report much lower thresholds than this (Reynolds and Walsby, 1975).

The requirements of phytoplankton for potassium are probably similarly non-controversial. This is despite its well-known importance as a constituent of agricultural fertilisers (acknowledging soil deficiencies) and, in both the sea and in many fresh waters, it being the least abundant of the four major cations, yet the most abun dant of the four in cytoplasm. A recently published experimental study (Jaworski et al., 2003) has reappriased the situation. The authors were unable to show any dependence of the growth rate of the freshwater diatom Asterionella on potassium concentrations above 0.7 | M (27.5 | g K L-1), while yields were only diminished in cultures in which the initial concentration was below this level. Another diatom (Diatoma elonga-tum) and a cryptomonad (Plagioselmis nannoplank-tica) also showed no dependence on potassium initially supplied at >0.8 |M (31 |g K L-1). We may conclude that the regulation of phytoplank-ton growth by potassium or sodium is unlikely to occur naturally.

It is relevant to remark briefly on another suggestion in the early ecological literature (Pearsall, 1922) that the species composition of the freshwater phytoplankton might be sensitive to the variable ratio of monovalent to divalent cations (M : D). Pearsall (1922) deduced that diatoms are more abundant in relatively calcareous waters (M : D < 1.5) whereas many desmids and some chrysophyceans occur in softer water. Talling and Talling (1965) showed desmids increased in major African lakes of high alkalinity (>2.5 meq L-1) but where sodium rather than calcium is the dominant cation. The observations are, broadly, upheld by later work but the underpinning mechanisms can be explained by sensitivity to the carbon sources in the media concerned.

4.6.2 Anions

Apart from the crucial behaviour of bicarbonate, the major anions (chloride, sulphate) do not appear to limit algal production. Chloride is, by mass, the most abundant of the dissolved ions in sea water (~19.3 g L-1, i.e. ~0.54 M, or 540 meq L-1) (Fig. 4.7) and the principal agent in its salinity and halinity. Among softer fresh waters, it is generally the dominant anion (0.1-1 meq L-1, 4-40 mg Cl L-1) but, elsewhere, it may be less abundant than either bicarbonate or sulphate or both. The anions influence the medium and, in extreme, distinguish the properties of highchloride, high-sulphate and carbonate-hydroxyl (soda) lakes. Sulphate also normally saturates the sulphur uptake requirements of algae down to concentrations of 0.1 meq L-1 (4.8 mg SO^-, 0.05 mM).

In the context of sulphur biogeochemistry, this is an appropriate point to mention the biological production of dimethyl sulphide (DMS). This volatile compound evaporates from the sea to the air, where it constitutes the main natural, biogenic source of atmospheric sulphur. As the molecules in the air act as condensation nuclei, DMS production has consequences on the radiative flux to the ocean surface. In the 1980s, when the DMS fluxes were first recognised, excitement was engendered by the idea that the release by marine microalgae of such a substance might contribute to the regulation of global climate. It was cited as a practical demonstration of Lovelock's (1979) Gaia principle, with living systems creating and regulating the planetary conditions for their own survival. Since then, it has been recognised that the source of the DMS is a precursor osmolyte, dimethyl-sulphonioproponiate (DMSP), which is synthesised by marine microalgae and bacteria as a counter to excessive water loss. Measurements noted by Malin et al. (1993) showed correlations between DMS concentrations ranging between 1 and 94 nmol L-1 (mean 12) and those of the DMSP precursor and between the concentrations of DMSP and chlorophyll a during a summer bloom of coccolithophorids in the northeast Atlantic Ocean. The volatile DMS metabolite is released mainly as a consequence of the operation of the marine food web (Simo, 2001). Using 35S-labelled DMSP, Kiene et al. (2000) have shown that DMSP supports a significant part of the carbon metabolism of the marine bacterio-plankton and that it impinges upon the availability of chelated metals (see Section 4.5.2). The arguments for and against the tenancy of the Gaian hypothesis of supra-organismic regulation of the planetary biogeochemistry notwithstanding, it is plain that the DMSP-DMS metabolism plays a significant role in the ecological structuring of the oceans.

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