Photosynthetic picoeukaryotes (PPEs) are single celled algae of <3μm diameter,\ud present in both marine and freshwater environments. Marine PPEs have begun to\ud gain increasing recognition as important, ubiquitous primary producers, after largely\ud being overlooked in favour of the more numerous picocyanobacteria for many years.\ud Molecular studies have shown the group to be extremely diverse. However, most\ud molecular studies have used PCR with general primers targeting the nuclear 18S\ud rRNA gene to construct clone libraries and have been dominated by heterotrophic\ud picoeukaryotes. To overcome this problem, more recent molecular studies have\ud targeted the 16S rRNA gene of marine algal plastids, an approach which essentially\ud excludes heterotrophic organisms.\ud In this PhD thesis, the molecular diversity of the PPE community was analysed over\ud broad spatial scales using both 16S and 18S rRNA gene markers to begin to draw\ud global conclusions on the phylogenetic composition of this group and identify the\ud major players in marine CO2 fixation. Moreover, distributions of various PPE classes\ud were also analysed along a range of cruise transects with dot blot hybridisation of\ud PCR amplified DNA using class-specific plastid 16S rRNA gene targeted\ud oligonucleotide probes. All major ocean basins were analysed, encompassing a range\ud of nutrient regimes and latitudes.\ud The dot blot hybridisation approach revealed that the classes Prymnesiophyceae and\ud Chrysophyceae appeared to be ubiquitous and dominated the PPE community\ud throughout large expanses of the global ocean. Furthermore, these classes showed\ud strongly complementary distributions along some of the transects analysed. Clone\ud library construction demonstrated that both classes are comprised of an array of\ud genetic lineages, many with no close cultured counterpart. For one cruise transect,\ud the extended Ellett Line in the North Atlantic Ocean, a fluorescent in situ\ud hybridisation approach was used as a PCR-independent assessment of the PPE\ud community. This approach largely supported the dot blot hybridisation data.\ud Other classes, Cryptophyceae, Pinguiophyceae, Pelagophyceae, Eustigmatophyceae,\ud Pavlovophyceae, Trebouxiouphyceae, Chlorarachniophyceae and Prasinophyceae\ud clade VI, were detected at lower abundances by dot blot hybridisation, with some\ud classes restricted to specific sites. Multivariate statistics indicated that the\ud distribution patterns of PPE classes were influenced by both temperature and nutrient\ud concentrations. However, at the global scale, a large proportion of the variation in\ud dot blot hybridisation data could not be explained by the environmental parameters\ud measured. It is likely that the classes harbour different ecotypes which are\ud individually influenced by environmental factors. Furthermore, biotic parameters not\ud measured in this work e.g. viral lysis, predation or parasitic infection may have been\ud important in controlling the PPE community
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