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