Coccolithophores concentrations

Particulate Calcium in WCR 82-B. Calcium is a major component of particulate matter and occurs largely as calcite (5). Most particulate Ca is small and is predominantly present in 10-20-jLtm sized coccolithophores in surface waters and in 1-5- im sized coccoliths below the euphotic zone (5). Individual coccoliths sink only fractions of a meter per day (2, 5). Hence, the <53-fim Ca concentration in waters below 100 m would be expected to show the least temporal variability of all particulate elements sampled. For this reason, Ca data from below 100 m was used for comparison of samples collected with GF and MQ filters and by the LVFS and MULVFS. 

In the earlier taxonomic studies of marine phytoplankton, only eukaryotes were determined, as the prokaryotes were too small to be measured using the available methodologies. Kimor etal. (1987) reported that the most common eukaryotes were coccolithophores, followed (numerically) by diatoms and dinoflagellates. On some occasions silicoflagellates were reported but they were rare. Large species >65 pm in diameter occasionally occurred at low concentrations. An example was the prasinophyte Halosphaera virtidis that was found at the DCM but not in the surface water. 

Coccolithophores synthesize different types of coccoliths during different life-cycle stages. Here we concentrate on the heterococcoliths associated with the diploid life stage. These heterococcoliths are formed from crystal units with complex shapes, in contrast to holococcoliths which are constructed out of smaller and simpler crystal constituents. Coccoliths are typically synthesized intracellularly (within a vesicle), probably one at a time, and subsequently extruded to the cell surface. The time taken to form a single coccolith can be less than 1 h for E. huxleyi. Coccoliths continue to be produced until a complete coccosphere covering (made up of maybe 20 coccoliths, depending on species) is produced. 

The first evidence that cadmium had a beneficial biological function came from growth data in laboratory cultures of the diatom Thalassiosira weissflogii [43,45]. As shown in Fig. 10, cultures of this coastal diatom grow slowly when the unchelated Zn concentration in the medium is reduced to about Zn = 3 pM. pM. These same cultures grow much faster when Cd is added to the medium at unchelated concentrations >5 pM [46]. This effect, which is particularly obvious at low Co concentrations, has now been observed in other families of marine phytoplankton. For example, Cd enhances the growth rate of the cosmopolitan coccolithophore Emiliana huxleyi when the unchelated Zn and Co concentrations in the medium are below 1 pM (Fig. 10) [42]. From similar laboratory studies, it appears that Zn, Cd, and Co can substitute for each other in many marine eukaryotic phytoplankton [47-51]. 

Figure 10 Growth rate of (A) diatom T. weissflogii and (B) coccolithophore E. huxleyi laboratory cultures under different unchelated zinc (Zn ) and unchelated cadmium (Cd ) concentrations [(A) T. W. Lane, F. M. M. Morel, unpublished results (B) Y. Xu, F. M. M. Morel, unpublished results].

It is well known that elevated Cd concentrations are toxic to phytoplankton and that different species have different sensitivity to Cd toxicity [36,41 6]. For example, in a comparison of phytoplankton taxa. Brand and coworkers showed that cyanobacteria were the most sensitive to Cd toxicity, and diatoms the least sensitive with coccolithophores and dinoflagellates having intermediate sensitivity [41]. Another study found no systematic differences among taxa [44]. Differences in sensitivity can also be found within the same genus for example, oceanic Thalassiosira species are more resistant to Cd toxicity than coastal ones [45]. The free Cd ion concentration that causes 50% reduction in growth rate ranges from a few pmol to several hundred nmol [41,44]. 

The coccolithophore E. huxleyi releases a variety of thiols into the external medium upon exposure to elevated Cd concentrations [90]. Cys and GSH were the primary thiols released by cells exposed to Cd only, whereas y-Glu-Cys was the primary thiol when Cu and Zn were also present at high concentrations. Like phytochelatins, these low molecular weight thiols released by phytoplankton may also serve as organic metal-complexing agents in surface seawater. 

Coccolithophores are, with rare possible exceptions, exclusively marine algae, chiefly planktonic although some benthonic forms are known. They secrete minute calcitic plate scales a few microns across. The mineralized plate scales are called coccoliths and may be extremely abundant in some soil parent materials, particularly chalk. Chalk commonly consists almost exclusively of the tests of planktonic foraminifers, discussed below, and coccoliths the two components may be present in approximately equal-weight abundances, or either component may strongly predominate. Although both kinds of fossil are calcitic, they have remarkably different resistance to solution, and local conditions may favor concentration of one or the other type in soils derived from a chalky parent material. A characteristic Cretaceous coccolith is illustrated in Figure 5. 

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