1. Ectocarpus pathogens
Despite their small size and ephemeral life stages, filamentous brown algae have been frequently reported to be plagued by various pathogens, including viruses (Müller et al., 1998) and eukaryotic parasites of different phylogenetic lineages: oomycetes, chytrids and hyphochytrids (Andrews, 1976; Küpper & Müller, 1999; Müller et al., 1999) and by parasites related to the Plasmodiophorea (Karling, 1944; Maier et al., 2000). In addition, numerous historical records described ectocarpoids with abnormal sporangia or vegetative cells suspected to contain unknown parasites (Rattray, 1885; Müller et al., 1998).
The oomycete Eurychasma dicksonii has been described mainly in wild populations of Pylaiella littoralis (Küpper & Müller, 1999), but it displays a broad host range and infects various brown algae, including Ectocarpus (Müller et al., 1999), in which it was initially described by Wright (1879). There is a current effort to set up a defined pathosystem using E. siliculosus and E. dicksonii, and Ectocarpus strains have been shown to exhibit differential susceptibility to a defined Eurychasma strain. Conversely, several Eurychasma strains exhibit different host specificities, suggesting coevolution of the two species (Gachon et al., 2007). The molecular bases of resistance and virulence are under investigation.
Chytrids were described earlier by Petersen (1905), and the hyphochytrid Anisolpidium ectocarpii was described by Karling (1943) and Johnson (1957). Like E. dicksonii, Chytridium polysiphoniae (Chytridiomycota) is ubiquitous and can infect many hosts, including E. siliculosus and E. fasciculatus (Müller et al., 1999). Interestingly, its negative effects on photosynthesis of its host was described at the cellular level in the related ectocarpoid P. littoralis using fluorescence kinetic microscopy (Gachon et al., 2006). Recently, the 18S rRNA genes of Chytridium polysiphoniae and Eurychasma dicksonii were sequenced and used to clarify their phylogenetic affiliations (Küpper et al., 2006). The plasmodiophorean Maullinia ectocarpii is an obligate intracellular parasite of Ectocarpus spp. (Maier et al., 2000). However, the extent to which this infection occurs in nature and its effect on algal fitness are presently unknown.
Viral infections represent by far the most studied phenomenon in E. siliculosus (Müller, 1996; Müller & Knippers, 2001). Until the late 1980s, most reports of virus infections in brown algal tissues were based on electron microscopy studies, which sporadically described ‘virus-like particules’ (VLPs). Viruses were obtained in culture for the first time from a New Zealand strain of E. siliculosus after lysis of host cells, allowing evaluation of their infection potential (Müller, 1991; Müller et al., 1990). Virus infections were found in approx. 50% of the individuals of a given natural population (Dixon et al., 2000; Müller et al., 2000) and were shown to occur worldwide in correlation with the cosmopolitan distribution of E. siliculosus (Müller, 1991; Sengco et al., 1996).
The viruses that infect different ectocarpoid algae exhibit considerable variability in size and diameter and, in general, display a high degree of host specificity (Müller et al., 1998). However, several instances of trans-specific infection have been described, for example between EsV-1 (Ectocarpus siliculosus virus-1) and Kuckuckia kylinii (Müller, 1992; Müller & Schmid, 1996) and also between EfasV-1 (Ectocarpus fasciculatus virus-1) and E. siliculosus (Müller et al., 1996; Sengco et al., 1996). Interestingly, EsV-1 and EfasV-1 are the most similar of the brown algal viruses in terms of their genome size (Müller et al., 1996).
The EsV-1 virus specifically infects the single-celled gametes or spores, that is, the only cells in the life history that lack a cell wall (Maier & Müller, 1998). Following infection, a single copy of the viral DNA appears to integrate into the host genome (Delaroque et al., 1999). The viral DNA is then transmitted, via mitotic divisions, to all the cells of the developing alga. This has been confirmed by regenerating algae from protoplasts derived from virus-infected gametophytes (Kuhlenkamp & Müller, 1994). Despite the fact that they carry the integrated virus, vegetative cells do not produce viral particles (Müller et al., 1998). Viral particles are only produced in reproductive organs (sporangia and gametangia) of mature algae from where they are released to infect a new generation of zoids. In addition to these cycles of re-infection, the viral genome can also be transmitted to progeny through meiosis, in which case it segregates as a Mendelian factor and is inherited by half of the progeny (Müller, 1991; Bräutigam et al., 1995). The pathogenic character of viral infections has been unambiguously confirmed, but this association's main impact is on reproductive success. Plant sterility varies from partial (Müller et al., 1990) to total (Müller & Frenzer, 1993), but no significant difference in photosynthesis, respiration and growth rate were observed in infected gametophytes or sporophytes (Del Campo et al., 1997). This contrasts with the reduced photosynthetic performance of Feldmannia species infected with FsV (Robledo et al., 1994).
The EsV-1 genome is a circular DNA molecule of a relatively large size (335 kbp) for a phycodnavirus (Van Etten & Meints, 1999; Van Etten et al., 2002) with double-stranded regions interrupted by single-stranded regions (Lanka et al., 1993; Klein et al., 1994). Both EsV-1 and the related Feldmania irregularis virus (FirrV-1) have been sequenced (Delaroque et al., 2001, 2003). EsV-1 contains approx. 231 genes with a wide range of predicted functions, including DNA metabolism, signalling, transposition, DNA integration and polysaccharide metabolism (Delaroque et al., 2000a,b, 2003). It has also been proposed that the ability of the virus to integrate into its host's genome could be exploited to develop a transformation vector for a wide range of brown algae, including E. siliculosus (Henry & Meints, 1994, Delaroque et al., 1999). However, the complex integration pattern of the virus into the algal genome will considerably complicate this task (N. Delaroque, pers. comm.). A microarray has been constructed to analyse EsV-1 gene expression (Declan Schroeder, pers. comm.) and it will be particularly interesting in the future to couple the analysis of viral and genome-wide host gene expression during viral infection.
The development of genomic tools provides a new context to investigate the possible genetic basis of the coevolution between some pathogens and brown algae. The search for inducers of defense responses and resistance against parasites is also still ongoing as, in contrast to kelps, Ectocarpus does not react with an oxidative burst upon recognition of alginate fragments (Küpper et al., 2002a).
2. Abiotic stresses
Ectocarpus siliculosus is able to exploit a wide range of habitats and environmental conditions (see Section II.2: Distribution). This feature seems likely to be based at least as much as on a high intrinsic genetic variability as on a general physiological toughness, as illustrated by work carried out on copper and saline stress responses.
Interspecific variations in copper tolerance have been observed between different strains of E. fasciculatus and E. siliculosus, with the latter being the most tolerant (Morris, 1974). Differences have also been observed among E. siliculosus strains that are differently exposed to copper in their natural habitat (Russell & Morris, 1970; Hall, 1981). Cu2+ interferes with the general process of photosynthesis in brown algae, and particularly in E. siliculosus, by competing with magnesium for metal-binding sites in the chlorophyll molecules (Küpper et al., 2002b). A study of the mechanism of tolerance to copper and other heavy metals suggested a co-tolerance to copper, cobalt and zinc, and provided evidence for an exclusion mechanism to explain the particularly low sensitivity of E. siliculosus copper-tolerant strains (Hall et al., 1979; Hall, 1980, 1981). However, as yet there is no clear explanation for the intraspecific variation with respect to this trait within this species.
It has been suggested that the ability of some E. siliculosus strains to tolerate copper may be useful for the development of bioassays in which this alga is used for monitoring marine antifouling characteristics of copper-based materials (Hall & Baker, 1985, 1986). Copper chloride has been used to inhibit E. siliculosus infestations in tank cultures of Gracilaria gracilis (Van Heerden et al., 1997).
Russell & Bolton (1975) reported the occurrence of salinity ecotypes within E. siliculosus. This study was extended by Thomas & Kirst (1991a,b), who showed that large differences in photosynthesis, accumulation of osmotically active compounds (mannitol; Davis et al., 2003) and vitality occur between E. siliculosus isolates from different geographic locations following changes in salinity. They also observed that sporophytes were more salt-tolerant than gametophytes, irrespective of their level of ploidy.
Detailed investigations are necessary to decipher the physiological and cellular bases of salt and heavy metal tolerance in E. siliculosus. Mutagenesis and transcriptomic approaches will thus help to better understand the mechanisms involved in osmotic and oxidative adaptation, and to explain how these algae can cope with such a wide range of environmental conditions.