J. Phycol. 41, 874–879 (2005) r 2005 Phycological Society of America DOI: 10.1111/j.1529-8817.2005.000100.x
NOTE
A GENETIC MARKER TO SEPARATE EMILIANIA HUXLEYI (PRYMNESIOPHYCEAE) MORPHOTYPES1 Declan C. Schroeder,2 Gaia F. Biggi,3 Matthew Hall, Joanne Davy4 Marine Biological Association, Citadel Hill, Plymouth, PL1 2PB, UK
Joaquı´n Martı´nez Martı´nez Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH, UK
Anthony J. Richardson5 Sir Alister Hardy Foundation for Ocean Science, Citadel Hill, Plymouth, PL1 2PB, UK
Gillian Malin School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
and William H. Wilson Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH, UK
Emiliania huxleyi (Lohm.) Hay and Mohler is a ubiquitous unicellular marine alga surrounded by an elaborate covering of calcite platelets called coccoliths. It is an important primary producer involved in oceanic biogeochemistry and climate regulation. Currently, E. huxleyi is separated into five morphotypes based on morphometric, physiological, biochemical, and immunological differences. However, a genetic marker has yet to be found to characterize these morphotypes. With the use of sequence analysis and denaturing gradient gel electrophoresis, we discovered a genetic marker that correlates significantly with the separation of the most widely recognized A and B morphotypes. Furthermore, we reveal that the A morphotype is composed of a number of distinct genotypes. This marker lies within the 3 0 untranslated region of a coccolith associated protein mRNA, which is implicated in regulating coccolith calcification. Consequently, we tentatively termed this marker the coccolith morphology motif.
Key index words: calcification; calcium binding protein; coccolith morphotypes; coccolithophore; coccoliths; Emiliania huxleyi Abbreviations: CMM, coccolith morphology motif; CP, calcium binding polysaccharide; DGGE, denaturing gradient gel electrophoresis; GPA, calcium binding protein with a high glutamic acid, proline, and alanine content; UTR, untranslated region Emiliania huxleyi (Lohm.) Hay and Mohler (Prymnesiophyceae) is an important species with respect to past and present marine primary productivity, sediment formation, and climate change. It is the most numerous coccolithophore in our oceans and can occur in extensive blooms covering thousands of square kilometers that are visible from satellites (Holligan et al. 1993, Iglesias-Rodriguez et al. 2002a). The involvement of E. huxleyi in global biogeochemical cycles of both carbon and sulfur makes it a key player in the earth’s climate system (Charlson et al. 1987, Malin et al. 1994, Iglesias-Rodriguez et al. 2002a). Consequently, E. huxleyi has been a subject of interest for many ecologists, biostratigraphers, oceanographers, and paleoceanographers (Young and Westbroek 1991). There is considerable variation among E. huxleyi isolates with respect to coccolith morphology, physiological properties, and immunological properties of the polysaccharide associated with coccoliths, and they are currently separated into five morphotypes where A and B are the best characterized and most widely
1
Received 7 October 2004. Accepted 26 April 2005. Author for correspondence: e-mail
[email protected]. 3 Present address: School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG. 4 Present address: The Centre for Marine Studies, University of Queensland, Brisbane, QLD 4072, Australia. 5 Present address: Department of Mathematics, University of Queensland, St. Lucia, Qld, 4072, Australia, and CSIRO Marine Research, P.O. Box 120, Cleveland, Qld, 4163, Australia. 2
874
GENOTYPING E. HUXLEYI MORPHOTYPES
FIG. 1. Emiliania huxleyi scanning electron micrographs. (a) CCMP 1516 type A morphotype (scale bar, 2 mm), (b) CCMP 1516 coccolith (scale bar, 1 mm), (c) Ch25/90 type B morphotype (scale bar, 2 mm), and (d) Ch25/90 coccolith (scale bar, 1 mm). Note the larger size of the type B coccosphere and coccolith compared with that of the type A.
recognized (van Bleijswijk et al. 1991, Young and Westbroek 1991, Medlin et al. 1994, Young et al. 2003). Previous attempts to identify a genetic marker to underpin these differences observed between the A and B morphotypes (Fig. 1) failed to find any interclonal variations in the E. huxleyi DNA sequences coding for either the small subunit rRNA gene or the more rapidly evolving untranscribed RUBISCO rbcLrbcS spacer region (Medlin et al. 1994). The lack of genetic variation does not support the separation of the A and B morphotypes at the species level; however, other molecular techniques such as RAPD and microsatellites have shown ample evidence of interclonal variation (Medlin et al. 1994, Iglesias-Rodriguez et al. 2002b). Despite this, these two techniques still do not resolve the A and B morphotypes definitively. Coccolith formation takes place in a dedicated intracellular vesicle with the reticular body supplying vesicle membrane and coccolith constituents including an acidic, water-soluble, calcium binding polysaccharide (CP) (Corstjens et al. 1998). The CP is thought to play a role in coccolith formation, possibly by controlling crystal nucleation and crystal growth. Antibodies raised to the CP of type A do not generally cross-react with type B and vice versa. However, weak cross-reactions were observed in a few E. huxleyi isolates and some diatoms and dinoflagellates (van Bleijswijk et al. 1991, Corstjens et al. 1998). Hence, the use of this immunological method for A and B morphotype differentiation requires additional validation. The antibodies raised against an intracellular protein associated with CP in a type B morphotype did,
875
however, cross-react with intracellular fractions of A and B (Corstjens et al. 1998). The antiserum was used to screen the expressed proteins from a cDNA library of a type A morphotype (strain L), and a gene encoding a protein with calcium-binding motifs, designated GPA (due to its high glutamic acid, proline, and alanine content), was identified. It is thought that GPA is involved in nucleating the calcium carbonate crystals during coccolith development or delivery of calcium to the coccolith vesicle (Corstjens et al. 1998). We therefore set out to determine whether the mRNA transcript coding for the GPA protein contained any significant differences that may be related to the A and B morphotypes. The GPA coding sequence from strain L is situated between two untranslated regions (UTRs), and primers were designed to the GPA cDNA of strain L (Fig. 2a). Two oligomers, GPAF1 (5 0 -GAGGAGGAGAAGCCGAGCCT-3 0 ) and GPAR1 (5 0 -CTTGAATCCTCTGTGCTGAGCGAG-3 0 ), corresponding to the sequence positions 897–915 and 1382–1405, respectively, of the GPA mRNA of strain L (AF012542), were used to amplify the DNA extracted from the
FIG. 2. (a) A graphic representation of the GPA mRNA (Corstjens et al. 1998). The regions spanning 1–18 bp and 1103– 1458 bp represent the 5 0 UTR and 3 0 UTR, respectively. The GPA coding region (gray) lies between these two UTRs. The arrows indicate the location of the oligonucleotide primers used in this study. (b) Image of a DGGE gel of PCR fragments amplified from 15 Emiliania huxleyi isolates using these two primers (Table 1). (c) An illustration representing the DGGE gel in b. The ovals indicate genotype CMM I, squares genotype CMM II, circles genotype CMM III, and triangles genotype CMM IV. The slanted and vertical stripes indicate the identical CMM I alleles and the horizontal stripes indicate the identical CMM IV alleles. The stars indicate the alleles where no or partial sequence data are currently available for strains 92D and 92E, respectively.
876
DECLAN C. SCHROEDER ET AL.
TABLE 1. Emiliania huxleyi isolates used in this study.
Other names
CCMP 373
1960
BT6, CSIRO-CS-57
Sargasso Sea
A
I
AY629169
Ch24/90
1990
Texel A, Ch24
North Sea
A
I
AY629180
215 L
2002 1959/68/80
LN, Lsc, Ln, S
English Channel Oslo fjord
A A
I I III
AY629179 AY629173 AY629174
92E
1992
English Channel
A
I IV
CCMP 372 CCMP 1516
1987 1991
Sargasso Sea North Pacific
A A
IV III IV
Ch25/90
1990
North Sea
B
IV II
AY629178 AY750878 AY629175 AY629166 AY629167 AY629167 AY629181
92D
1975
English Channel
B
II
AY629177
CCMP 370 CCMP 379
1959 1950/57
North Sea English Channel
un un
I I
AY629168 AY629172
CCMP 374
1989
Gulf of Maine
un
I IV
5_90_25b
1990
N. Atlantic
un
I
AY629170 AY629171 AY629176
CCMP 1516 (b)a Texel B, Ch 25
451 B, F451 P-92A, 92Aa, UTEX 1016, CCAP920/1A
Cell type
Accession number
Isolation date
CCMP 1A1
Origin
CMM group
Clone designation
Reference/source
Medlin et al. 1996, Steinke et al. 2000 Medlin et al. 1996, van Bleijswijk 1996 PCC Young and Westbroek 1991, Medlin et al. 1996, van Bleijswijk 1996 PCC PCC Steinke et al. 2000 PCC Green et al. 1996, Medlin et al. 1996, van Bleijswijk 1996 Young and Westbroek 1991, Green et al. 1996, Medlin et al. 1996 Steinke et al. 2000 Steinke et al. 2000 Steinke et al. 2000 PCC
a
Additional isolates tested in this study. PCC, Plymouth culture collection; un, unknown.
E. huxleyi isolates (Schroeder et al. 2002). The oligonucleotide primers were successfully used to amplify the approximately 500-bp PCR products from DNA extracted from 15 E. huxleyi isolates (Table 1), but no PCR products were produced from three prymnesiophytes, Gephyrocapsa oceanica, Coccolithus pelagicus, and Phaeocystis globosa, and a prasinophyte, Micromonas pusilla (data not shown). The fact that the primers failed to amplify sequence in G. oceanica, the species considered to be the most closely related to E. huxleyi, gives us confidence regarding the potential utility of these primers for field studies. The denaturing gradient gel electrophoresis (DGGE) of the PCR products, which was conducted using 35% to 55% linear denaturing gradient 6% polyacrylamide gels (Schroeder et al. 2003), revealed that the amplified fragments were different in a range of clones, demonstrating significant interclonal variation (Fig. 2b). Four isolates (L, CCMP 374, CCMP 1516, and 92E) contained two alleles, whereas the other 11 isolates (CCMP 370, CCMP 373, CCMP 379, 215, 92A, 92D, Ch24/90, Ch25/90, 5_90_25b, CCMP 1516 (b), and CCMP 372) contained a single allele. Sequence analysis of the apparent second allele of 92D revealed that both alleles were identical, and thus the two alleles were in fact an artifact of the mung bean nuclease treatment. Subsequent DGGE and sequence analysis confirmed this observation (data not shown).
Bands were excised from the DGGE gel as described in Schroeder et al. 2003 (Fig. 2c), sequenced using a BigDye Terminator v3.1 cycle sequencing kit (Applied Biosystems, Manchester, UK) on an ABI 3100 capillary sequencer (Applied Biosystems) and the data for each fragment was aligned using ClustalW (http:// www.clustalw.genome.ad.jp/). The sequence alignment corresponding to the coding region of GPA revealed a single base-pair substitution at position 962 (star, Fig. 3) that was only present in the two B morphotype strains (92D and Ch25/90); the remaining A morphotype strains (Table 1) were identical in the coding region of GPA. Given that only two genotypes had a single base-pair substitution in this region and none of the other eight did, there is a very small probability (less than 5%) that by chance they would correspond to the two individuals with B morphotype ( 5 1/10C2 5 0.0222, where 10C2 represents the number of combinations that two individuals can be drawn from the 10 individuals tested). It is thus highly likely that position 962 separates the A and B strains. In addition, the guanine to adenosine substitution is silent because the ACG and ACA codons both code for the amino acid threonine. The alignment also revealed differences in the 3 0 UTRs that allowed the alleles to be separated into four genotypes based on a 32-bp stretch of sequence; we designated this the coccolith morphology motif (CMM; Fig. 3).
GENOTYPING E. HUXLEYI MORPHOTYPES
877
FIG. 3. Multiple sequence alignment of the amplified GPA PCR products with the sequence of the primers omitted. The amplified GPA coding region is shaded in gray, and the location of the four coccolith morphology motifs (CMM I, II, III, and IV) is identified by a rectangular box. The oval with slanted stripes represents the consensus sequence of the five identical alleles from strains CCMP 374, L, CCMP 370, Ch24/90, and 5_90_25b. The oval with vertical stripes represents the consensus sequence of the two identical alleles from strains CCMP 379 and 92A. The triangle with horizontal stripes represents the consensus sequence of the two identical alleles from isolates CCMP 1516 and CCMP 1516(b). The numbers 1 and 2 in brackets represent the top and bottom alleles, respectively. Variations in sequence composition are highlighted in bold and hyphens. The star indicates base pair position 962.
Strains with the genotype CMM I are CCMP 374, L, CCMP 370, Ch24/90, 5_90_25b, CCMP 379, 92A, CCMP 373, 215, and 92E (ovals, Fig. 2c). The DGGE suggests that the lower alleles of strains CCMP 374 and L and the single alleles of strains CCMP 370, Ch24/90, and 5_90_25b are similar (slanted stripes, Fig. 2c), whereas the sequence data confirmed they are identical (Fig. 3). The lower allele of strain L was identical to the previously characterized allele of strain L (Corstjens et al. 1998), although this is the first report of a second allele in strain L. Strains CCMP 374 and L had different second alleles based on both DGGE (Fig. 2b) and sequence data (Fig. 3), which allowed these two isolates to be differentiated. However, the three strains (CCMP 370, Ch24/90, and 5_90_25b) with identical single alleles could not be differentiated. Similarly, the single alleles of strains CCMP 379 and 92A comigrated on DGGE (Fig. 2b), and their sequence data confirmed they are identical in sequence (Fig. 3). The similarity between these two strains was expected because they originated from the same isolate, although they were sourced from different culture collections (Table 1), but DGGE (vertical stripes, Fig. 2c) and sequence homology (Fig. 3) shows they are different from other
strains with CMM I alleles. The single alleles of strains CCMP 373 and 215 and the lower allele of strain 92E make up the rest of this CMM I genotype; they are also different from each other and the other members that contain CMM I (ovals no shading, Fig. 2c). Therefore, six of the nine strains had a unique 3 0 UTR allele. The CMM I consensus sequence for the members of this genotype is shown in Figure 3. Strains 92D and Ch25/90 both contained alleles that differed from those of the CMM I group; hence, they were assigned to the genotype CMM II (squares, Fig. 2c). The DGGE evidence suggested a similar sequence composition (Fig. 2b), and the sequence data revealed a single base-pair difference between the two alleles (Fig. 3). Therefore, both strains had unique 3 0 UTR alleles. The lower and upper alleles of stains CCMP 1516 and L, respectively, were also grouped together into a different genotype, CMM III (circles, Fig. 2c). Again, both DGGE analysis (Fig. 2b) and sequencing (Fig. 3) showed unique 3 0 UTR alleles for both strains. The upper alleles of strains CCMP 1516 and CCMP 374 and the single alleles of strains CCMP 1516 (b) and CCMP 372 were grouped into a fourth genotype, CMM IV (triangles, Fig. 2c). Partial sequence data of
878
DECLAN C. SCHROEDER ET AL.
the 3 0 UTR of the upper allele of strain 92E also revealed that this allele belongs to CMM IV (data not shown). DGGE (Fig. 2b) and sequence data revealed that the single allele of strain CCMP 1516 (b) and the top allele of strain CCMP 1516 were identical (Fig. 3). Despite the indistinguishable allele shared between the two CCMP 1516 strains, the extra allele in the one strain is surprising because they originated from the same isolate. Of the 15 isolates tested, the 3 0 UTR allele confirmed the expected commonality of strains CCMP 379 and 92A while calling into question the identity of the two CCMP 1516 isolates. Of the 13 strains, 10 had unique alleles and hence could be differentiated (Table 1). The two B morphotypes, strains Ch24/90 and 92D, contained alleles that grouped into CMM II, whereas all the known A morphotypes, CCMP 373, Ch24/90, 215, L, 92E, and CCMP 372, had alleles with CMM signature sequences that fell into the other three groups (Table 1). This difference between A and B morphotypes is consistent with the difference observed in the GPA coding region. Therefore, CMM could serve as an interclonal marker and could also provide a useful tool to differentiate between the A and B morphotypes. In addition, where EM and immunological evidence is lacking or if the isolates are either naked or motile (e.g. strains CCMP 370, CCMP 379, CCMP 374, and 5_90_25b), the CMM marker could be used to differentiate them into CMM genotypes and hence A or B morphotypes (Table 1). This study raises the question of whether there is a link between the function of the 3 0 UTR of the GPA mRNA transcript and coccolith morphology. Regulation of translation initiation by UTR has been described in a number of kingdoms (Gray and Wickens 1998, Lai 2002). A review by Gray and Wickens (1998) described how mRNA localization signals in the 3 0 UTR can dramatically influence translational activation and repression in animal cells. Modulations of the initiation machinery can regulate both specific mRNAs and overall translation rates and thereby affect cell growth and phenotype (Gray and Wickens 1998). The mechanism by which repressors inhibit translation through the 3 0 UTR is not well understood. However, four mechanisms have been described in the literature. First, repressor elements are postulated to form a nucleation site for large protein complexes that make the mRNA inaccessible to the translational machinery (Wolffe and Meric 1996). Second, 3 0 UTR binding proteins may directly interrupt the interaction of the two ends of the mRNA or prevent its activity during initiation (Ostareck-Lederer et al. 1994). Third, repressors could bind to the 3 0 UTR and place an mRNA in a microenvironment in which translation is inefficient (Decker and Parker 1995). Finally, repressor proteins bound to the 3 0 UTR can directly modulate accessibility of the mRNA to the cytoplasmic polyadenylation or deadenylation apparatus (Standart and Jackson 1994). Potential mechanisms of de-repression observed in animal systems include loss or mod-
ification of the repressor and recruitment of an activator (Standart 1992). Micro-RNAs have also implicated in mediating negative posttranslational regulation in Caenorhabditis elegans, Drosophila, and higher plants by binding to several classes of sequence motifs (Lai 2002, Reinhart et al. 2002). Similarly, the 3 0 UTR of GPA mRNA might regulate levels of GPA in the E. huxleyi cell. The CMM signature sequence may be the specific protein binding recognition sequence involved in the modulation of the initiation machinery, which in turn controls the overall morphology of the coccolith. Alternatively, the CMM signature sequence might be part of a set of regulatory sequences, repressor and activator proteins that ultimately control the development of the coccolith within the coccolith vesicle. The variety of genotypes within the A morphotype may also reflect the variation of coccolith morphologies observed within this morphotype. The degree of calcification (i.e. the amount of calcite incorporated in the coccolith) is thought to account for this variation in coccolith morphology (Young and Westbroek 1991, Buitenhuis et al. 1999, Paasche 2002). Environmental conditions such as temperature and dissolved inorganic carbon availability are traditionally thought to be the key factors that influence the degree of calcification in E. huxleyi, but the nature of this relationship is not clear (Young and Westbroek 1991, Buitenhuis et al. 1999, Paasche 2002). We suggest that temperature and dissolved inorganic carbon availability could directly or indirectly influence the regulation of the 3 0 UTR CMM region, which in turn regulates the levels of GPA present in the cell. This ultimately affects the degree of coccolith calcification and hence coccolith morphology. The isolates analyzed in this study propose a link between the 3 0 UTR of GPA and coccolith morphotypes. A word of caution is nonetheless warranted here; the presence of the 3 0 UTR of GPA mRNA and the variation found in the CMM does not necessarily equate to it having a role in the formation of the A and B morphotypes. The relationship between CMM and E. huxleyi morphotypes may still prove to be purely coincidental, however compelling the correlation. In addition, the number of CMM genotypes observed in this study will probably not represent all the A and B morphotypes because it is likely that additional CMM genotypes will be added as more E. huxleyi strains are sequenced. A more comprehensive coupled taxonomy and genotype study is warranted to fully resolve this relationship, specifically the relationship between allele number and coccolith morphotype, and to develop robust molecular tools that would allow differentiation of different genotypes in field populations and cultures. This would permit more detailed investigations of the inherent variation in calcification and dimethyl sulfide production, and how these might alter with or drive future climate change. We are grateful to Professor Colin Brownlee and Dr. Alison Taylor for critical review of the manuscript. We thank
GENOTYPING E. HUXLEYI MORPHOTYPES
Peter Bond, Roy Moate, and Keith Ryan from the Plymouth Electron Microscope Centre at the University of Plymouth for their assistance. Thanks also to the Plymouth Culture Collection for supplying some of the E. huxleyi cultures outlined in Table 1. The research was supported by the Marine and Freshwater Microbial Biodiversity (M&FMB) community programme, funded by the Natural Environmental Research Council of the United Kingdom (NERC). D. C. S. is a Marine Biological Association of the UK Research Fellow. W. H. W. is supported in part by a Marine Biological Association of the UK Research Fellowship funded by grant in aid from the NERC and through the NERC-funded core strategic research programme of Plymouth Marine Laboratory. G. M. is a NERC Advanced Research Fellow.
Buitenhuis, E. T., de Baar, H. J. W. & Veldhuis, M. J. W. 1999. Photosynthesis and calcification by Emiliania huxleyi (Prymnesiophyceae) as a function of inorganic carbon species. J. Phycol. 35:949–59. Charlson, R. J., Lovelock, J. E., Andreae, M. O. & Warren, S. G. 1987. Oceanic phytoplankton, atmospheric sulfur, cloud albedo and climate. Nature 326:655–61. Corstjens, P., van der Kooij, A., Linschooten, C., Brouwers, G. J., Westbroek, P. & deVrind-de Jong, E. W. 1998. GPA, a calciumbinding protein in the coccolithophorid Emiliania huxleyi (Prymnesiophyceae). J. Phycol. 34:622–30. Decker, C. J. & Parker, R. 1995. Diversity of cytoplasmic functions for the 3 0 untranslational region of eukaryotic transcripts. Curr. Opin. Cell Biol. 7:386–92. Gray, N. K. & Wickens, M. 1998. Control of translation initiation in animals. Annu. Rev. Cell Dev. Biol. 14:399–458. Green, J. C., Course, P. A. & Tarran, G. A. 1996. The life-cycle of Emiliania huxleyi: a brief review and a study of relative ploidy levels analysed by flow cytometry. J. Mar. Syst. 9:33–44. Holligan, P. M., Fernandez, E., Aiken, J., Balch, W. M., Boyd, P., Burkill, P. H., Finch, M., Groom, S. B., Malin, G., Muller, K., Purdie, D. A., Robinson, C., Trees, C. C., Turner, S. M. & Vanderwal, P. 1993. A biogeochemical study of the coccolithophore, Emiliania huxleyi, in the North-Atlantic. Glob. Biogeochem. Cycle 7:879–900. Iglesias-Rodriguez, M. D., Brown, C. W., Doney, S. C., Kleypas, J., Kolber, D., Kolber, Z., Hayes, P. K. & Falkowski, P. G. 2002a. Representing key phytoplankton functional groups in ocean carbon cycle models: coccolithophorids. Glob. Biogeochem. Cycle 16, doi: 10.1029/2001GB001454. Iglesias-Rodriguez, M. D., Saez, A. G., Groben, R., Edwards, K. J., Batley, J., Medlin, L. & Hayes, P. K. 2002b. Polymorphic microsatellite loci in global populations of the marine coccolithohorid Emiliania huxleyi. Mol. Ecol. Notes 2:495–97. Lai, E. C. 2002. Micro RNAs are complementary to 3 0 UTR sequence motifs that mediate negative post-transcriptional regulation. Nat. Genet. 30:363–4.
879
Malin, G., Liss, P. S. & Turner, S. M. 1994. Dimethyl sulphide: production and atmospheric consequences. In Green, J. C. & Leadbeater, B. S. C. [Eds.] The Haptophyte Algae. Clarenden Press, Oxford, pp. 303–20. Medlin, L., Barker, G. L. A., Baumann, M., Hayes, P. K. & Lange, M. 1994. Molecular biology and systematics. In Green, J. C. & Leadbeater, B. S. C. [Eds.] The Haptophyte Algae. Clarenden Press, Oxford, pp. 393–411. Medlin, L. K., Barker, G. L. A., Campbell, L., Green, J. C., Hayes, P. K., Marie, D., Wrieden, S. & Vaulot, D. 1996. Genetic characterisation of Emiliania huxleyi (Haptophyta). J. Mar. Syst. 9:13–31. Ostareck-Lederer, A., Standart, N. & Thiele, B.-J. 1994. Translation of 15 lipoxygenase mRNA is inhibited by a protein that binds to a repeated sequence in the 3 0 untranslated region. EMBO J. 13:1476–81. Paasche, E. 2002. A review of the coccolithophorid Emiliania huxleyi (Prymnesiophyceae), with particular reference to growth, coccolith formation, and calcification-photosynthesis interactions. Phycologia 40:503–29. Reinhart, B. J., Weinstein, E. G., Rhoades, M. W., Bartel, B. & Bartel, D. P. 2002. MicroRNAs in plants. Genes Dev. 16:1616–26. Schroeder, D., Oke, J., Malin, G. & Wilson, W. H. 2002. Coccolithovirus (Phycodnaviridae): characterisation of a new large dsDNA algal virus that infects Emiliania huxleyi. Arch. Virol. 147:1685–98. Schroeder, D. C., Oke, J., Hall, M., Malin, G. & Wilson, W. 2003. Virus succession observed during an Emiliania huxleyi bloom. Appl. Environ. Microbiol. 69:2484–90. Standart, N. 1992. Masking and unmasking of maternal mRNA. Semin. Dev. Biol. 3:367–79. Standart, N. & Jackson, R. J. 1994. Regulation of translation by specific protein/mRNA interactions. Biochimie 76:867–79. Steinke, M., Malin, G., Turner, S. M. & Liss, P. S. 2000. Determinations of dimethylsulphoniopropionate (DMSP) lyase activity using headspace analysis of dimethylsulphide (DMS). J. Sea Res. 43:233–44. van Bleijswijk, J., Vanderwal, P., Kempers, R., Veldhuis, M., Young, J. R., Muyzer, G., de Vrind-de Jong, E. & Westbroek, P. 1991. Distribution of two types of Emiliania huxleyi (Prymnesiophyceae) in the northeast Atlantic region as determined by immunofluorescence and coccolith morphology. J. Phycol. 27:566–70. van Bleijswijk, J. D. L. 1996. Ecophysiology of the calcifying marine alga Emiliania huxleyi. Ph.D. Thesis, Rijksuniversitet, Groningen, The Netherlands, 71 pp. Wolffe, A. P. & Meric, F. 1996. Coupling transcription to translation: a novel site for the regulation of eukaryotic gene expression. Int. J. Biochem. Cell Biol. 28:247–57. Young, J. R., Geisen, M., Cros, L., Kleijne, A., Sprengel, C., Probert, I. & Østergaard, J. B. 2003. A guide to extant coccolithophore taxonomy. J. Nannoplankton Res. Special Issue 1, pp. 1–125. Young, J. R. & Westbroek, P. 1991. Genotypic variation in the coccolithophorid species Emiliania huxleyi. Mar. Micropaleontol. 18:5–23.
