Biodiversity Among Haptophyte Algae

Biodiversity and Conservation 6, 131±152 (1997) Biodiversity among haptophyte algae R. W. JORDAN* Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, Yamagata, 990 Japan A. H. L. CHAMBERLAIN Microbial Physiology and Ecology Research Group, School of Biological Sciences, University of Surrey, Guildford, Surrey GU2 5XH, UK Received 31 October 1995; revised and accepted 24 January 1996 The division Haptophyta is represented only by about 300 extant species showing wide diversity in morphology, biochemistry and ecology. They have a world-wide distribution and are numerically important in phytoplankton populations in nearly all marine environments. Evidence from the geological record shows that they have been the major constituent of calcareous deposits since the Late Triassic and, as they have evolved quickly through time, their coccoliths have always shown wide morphological diversity. In today's oceans they occasionally produce extensive blooms, visible by satellite imagery, which have ecological impact. As a consequence of these blooms the haptophyte algae are now receiving greater attention, as their role in the global sulphur and carbon cycles may in¯uence the world's climate, and their potential as nuisance bloom algae have implications for commercial ®shing and the marine ecosystem. As it is likely that these organisms have always produced such blooms, these eects may have been in operation for the last 200 million years. Keywords: haptophyte; ultrastructure; morphology; bloom products; distribution; evolution. Introduction The freshwater representatives of the Haptophyta are few, but marine species constitute one of the most successful groups of living organisms with a well-documented fossil record (Tappan, 1980). Their ubiquitous distribution and ability to form large blooms has al- lowed them to be globally important, both as primary producers and, after death, as a component of sedimentary rock. The coccolithophorids are probably the most important of the haptophytes, as they produce calci®ed scales (coccoliths), which, on the cell's death, are liberated and transported to the sea ¯oor. These calcareous deposits are the major constituent of chalk and have their greatest period, both in abundance and species diversity, in the Late Cretaceous (65±95 Ma) (Bown et al., 1992; Young et al., 1994). Due to their rapid evolution, abundance, and the ease with which they can be observed, they have become one of the most widely used microfossil groups for stratigraphy. Thus, they are common tools for age determinations in oil exploration. Furthermore, as many species display biogeographic zonation and live in restricted environments, the distribution of their coccoliths in the sediments may also be useful in palaeoclimatic reconstructions. Hence, coccoliths are useful fossils in both stratigraphic and palaeoceanographic studies. In more recent times the extant coccolithophorids have gained attention mainly through their ability to form mesoscale blooms, whose re¯ectance via birefringent loose coccoliths can be observed with satellite imagery (Holligan et al., 1983; Balch et al., 1991). *To whom correspondence should be addressed. 0960-3115 Ó 1997 Chapman & Hall 132 Jordan and Chamberlain The processes of photosynthesis, calci®cation and respiration occurring in such large blooms as these have important implications for the global carbon cycle. In addition, blooms of Phaeocystis and Emiliania are involved in dimethyl sulphide (DMS) production and so their role in the global sulphur cycle is also important (Malin et al., 1992, 1994). Thus, haptophytes make a large contribution to the world's climate (Westbroek et al., 1993). Other haptophytes can also produce large blooms, some of which are known to cause economic problems either through toxin, mucilage or foam production (Underdal et al., 1989; Davidson and Marchant, 1992; Moestrup, 1994). In terms of diversity, there are now about 300 known extant species of haptophyte algae (Jordan and Green, 1994; cf Andersen, 1992; cf Vickerman, 1992), although undoubtedly many remain undescribed. While this number is relatively small, compared with diatoms for instance, diversity in haptophyte scale morphology is astounding. However, much of what is currently known about these organisms comes from observations on cultured material. In this respect the unmineralized taxa have proved easier to maintain than the coccolithophorids, and freshwater and coastal species better than open ocean taxa±see lists provided by the Culture Centre of Algae and Protozoa (Thompson et al., 1988), the Provasoli-Guillard National Centre for Culture of Marine Phytoplankton (Andersen, 1991), and the University of Texas (Starr and Zeikus, 1993). At present there are probably less than 10 species of coccolithophorids successfully maintained in culture throughout the world. However, Emiliania huxleyi grows easily in a variety of media under an enormous range of culture conditions, re¯ecting its very wide geographic range and, for this reason, it has become one of the most useful experimental marine organisms used today. As a consequence of this, more is known about this alga than probably any other marine species (for recent reviews see Westbroek et al., 1993; Heimdal et al., 1994). In the following sections haptophyte diversity will be discussed in terms of taxonomic concepts, internal and external structures, ecology and distribution in both water and sediments. A glossary of haptophyte terms has been published recently by Jordan et al., (1995). Taxonomic concepts In general, haptophytes may be identi®ed by a number of key characters (Table 1), and variation in some of these characters forms the basis for their separation at the ordinal level (Table 2). Another set of criteria can be used to separate them into three arti®cial groups relating to: (a) the extant unmineralized haptophytes, (b) the extant coccolithophorids, and (c) fossil coccoliths. In the extant unmineralized haptophytes the main em- phasis is on organic scale morphology, ¯agellar and haptonematal arrangement, and cell shape. For living coccolithophorids organic scale morphology is replaced by coccolith morphology, with importance placed on the combination of coccolith types and their arrangement on the cell (Jordan et al., 1995). The classi®cation of fossil haptophytes is restricted almost entirely to coccolithophorids and, moreover, to isolated coccoliths (coccospheres are generally rare in sediments). This means that most of the conventional methods of identi®cation employed for living haptophytes, as described above, cannot be used for the fossil forms, and their taxonomy is therefore constrained solely by in- formation on the coccolith morphology (Tappan, 1980; Perch-Nielsen, 1985). This approach leads to an overestimation of the number of species present and to a more diverse classi®cation system at all levels. Haptophyte algae 133 Table 1. Main characteristics of the division Haptophyta Chloroplasts with no girdle lamella Flagella, usually 2, equal or subequal, sometimes unequal; no tubular hairs (Pavlovophycidae have ®brous hairs and knobscales on longer ¯agellum±possibly modi®ed hairs rather than scales±see Cavalier-Smith, 1994) Eyespots found only in Pavlovophycidae, usually associated with invagination of plasmalemma; not associated with ¯agellar swelling (except in Diacronema vlkianum) Haptonema (or trace) present Unmineralized body scales basically a ®brillar 2-layered plate; many species with calci®ed scales (coccoliths). Silici®cation rare Reproduced in part from Green and Jordan (1994) Table 2. Main characteristics of the orders in the Haptophyta Subclass: Prymnesiophycidae Subclass: Pavlovophycidae Isochrysidales Prymnesiales Coccosphaerales Pavlovales or Coccolithophorales Unicellular, motile Unicellular, usually Unicellular, motile Unicellular or forming or non-motile, motile, sometimes or non-motile palmelloid masses sometimes non-motile. Cells ®lamentous free, occasionally colonial Flagella 2, equal or Flagella 2, equal or Flagella 2, equal or Flagella unequal, shorter subequal. One subequal. One subequal may be reduced, longer ¯agellum sometimes ¯agellum sometimes with investment of auto¯uorescent auto¯uorescent ®brous hairs, and knob-scales. Always heterodynamic Haptonema reduced Haptonema usually Haptonema Haptonema short, or absent conspicuous, often conspicuous or non-coiling well-developed and coiling reduced Body-scales small Body-scales various, Calci®ed scales Body-scales absent unmineralized one or more layers, (coccoliths) present (occasionally absent) and often several at some stage types of scale per cell during life cycle; non-mineralized scales also present Flagellar root system based on 3±4 microtubular roots Flagellar root system unique to group, based on two microtubular roots Mitotic spindle axis straight; no ®brous root MTOC Mitotic spindle axis V-shaped; ®brous root MTOC From Green and Jordan (1994). Jordan and Green (1994) assign taxa in the Prymnesiophycidae to only one order; the Prymnesiales. Some authors include the coccolithophorid genera, Emiliania, Gephyrocapsa and Reticulofenestra, in the Isochrysidales±thus in the above table, coccoliths may also be present in this order 134 Jordan and Chamberlain Over the last few decades a number of discoveries have changed our approach to the classi®cation scheme. One of the main advances has been the determination of a number of haptophyte life cycles by cultural studies (Billard, 1994). This has produced some interesting

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