FULL DESCRIPTION
Oligotrophic ocean water systems
Open water systems, including lakes and the oceans, cover the majority of the surface of Earth, and provide primary forces for driving the processes essential to maintain our planet in a habitable state. The oceans, which have the highest cellular production rate of any ecosystem on the planet, are vast oligotrophic environments. Despite the low level of nutrients in oligotrophic ocean waters, microbial numbers persist on the order of 0.5 - 5 x 10 5 cells ml -1 . As a result, marine microorganisms contribute a large proportion of the world's biosphere in terms of carbon, nitrogen and phosphorous. Furthermore, of the three largest microbial habitats (seawater, soil and sediment-soil subsurface), the rates of cellular activity and turnover are highest in the open ocean. In this oligotrophic environment, prokaryotes dominate in terms of biomass and play an essential role in regulating the accumulation, export, re-mineralisation and transformation of the world's largest pool of organic carbon. The fixation of carbon, nitrogen and phosphorus by bacteria, and their conversion into particulate matter forms the basis of the microbial food web in the oceans. While these are critically important processes in aquatic environments, they are poorly characterized.

There are global consequences for these microbial processes since the downward flow of particles is the most efficient means of transporting CO 2 fixed by primary production to marine sediments, thus sequestering it from the atmosphere. The balance between particle degradation, regenerating CO 2 via respiration, and burial, is a critical factor affecting climate change, and increases in ocean oligotrophy are forceast as a consequence of global warming. It is therefore important to increase our understanding of the genetic make-up and eco/physiology of the important classes of marine bacteria. Gaining this understanding is critical for future predictive modelling of interdependent marine ecosystems, ranging from phytoplankton to fish and whales.

S. alaskensis
S. alaskensis was isolated as one of the most numerically abundant bacteria from Alaskan waters ( e.g. strain RB2256), the North Sea and the North Pacific ( e.g. strain AFO1) over a period spanning ten years, demonstrating it is one of the most common culturable inhabitants from these environments. This indicates it is an important contributor to microbial biomass in these marine waters. A broad range of studies by the international community aimed at understanding the basis of microbial oligotrophy have focused on S. alaskensis . These studies have shown that its capacity to thrive in oligotrophic environments is linked to unique genetic and physiological properties which are fundamentally different from those of the well studied bacteria such as Escherichia coli.

Characteristics that promote S. alaskensis as a model oligotroph include a constant ultramicro-size (<0.1 µm 3 ), irrespective of whether it is growing or starved, that provides it with a mechanism for avoiding predation, and a high surface to volume ratio to enhance nutrient uptake. This is coupled with the ability to utilize low concentrations of nutrients using high affinity, broad specificity uptake systems ( e.g. highest reported rates of alanine transport for any bacterium) and the ability to simultaneously take up mixed substrates. Based on the Michaelis-Menten constants for substrate transport (K t ) and the available concentrations of mixed amino acids in the ocean, S. alaskensis is predicted to have an in situ doubling time equivalent to experimentally determined doubling times for microorganisms in oligotrophic waters. Its abundance at the time of sampling and geographical distribution, indicates that it is likely to be an important contributor in terms of biomass and nutrient cycling in marine environments.

Importantly, S. alaskensis is amenable to laboratory experimentation growing in liquid and on solid medium, in a wide-range of culture media, and using batch (nutrient excess) and chemostat (nutirent limitation) cultivation. Proteomic, genetic and biochemical methods have been established for probing its physiology.

Ultramicrobacteria (nanobacteria), such as S. alaskensis, have been reported in a range of aquatic, terrestrial and clinical samples, and in fossils; many of which are controversial. The reports have raised questions about the minimum size of a free-living cell. The astrobiology community has been particularly interested, as the minimum cell-size has important implications for cellular evolution and for the search for extraterrestrial life. S. alaskensis is a useful model for these purposes, as it has been shown, for example, to achieve maximum rates of growth with 200 ribosomes per cell.

Importantly, S. alaskensis is amenable to laboratory experimentation growing in liquid and on solid medium, in a wide-range of culture media, and using batch (nutrient excess) and chemostat (nutirent limitation) cultivation. Proteomic, genetic and biochemical methods have been established for probing its physiology.