Regulation of amino acid uptake in marine unicellular cyanobacteria: light sensing and circadian clocks.

Lead Research Organisation: National Oceanography Centre
Department Name: Science and Technology


Cyanobacteria numerically dominate the tropical and subtropical regions of the world's oceans with Prochlorococcus being the dominant photosynthetic organism of the open ocean and Synechococcus dominating in oceanic boundaries at low latitudes. Recently it was observed that Prochlorococcus shows a pronounced diel (day-night) periodicity in uptake of dissolved amino acids compared to Synechococcus. These field observations have important ecological implications. For example in the tropical Atlantic Ocean the Prochlorococcus population contributes around 10% and 40% to the total bacterioplankton uptake of amino acids at midday and after dusk, respectively and therefore exerts a considerable pressure on other bacterioplankton populations by consuming a significant proportion of organic nitrogen in already nutrient-depleted oceanic waters. However, field observations alone are insufficient to understand the mechanism regulating organic nutrient uptake. To address this we propose to study this process using cyanobacterial cultures grown in controlled laboratory conditions and to test the hypothesis that the uptake of amino acids, and possibly other organic nitrogen containing compounds, by Prochlorococcus could be regulated either by light and/or a circadian clock. To do this we will use a combination of isotopic tracer and flow cytometric techniques. The cyanobacterial strains will be exposed to various light regimes (varying in quality, quantity, periodicity), loaded with isotopically-labelled precursor molecules, and the cells at various cell cycle stages will be flow sorted to determine cellular uptake rates of the isotopic tracers. The research team for this project will have a unique combination of expertise with marine skills in microbial biogeochemistry, flow cytometry, molecular ecology of cyanobacteria combined with terrestrial background skills in plant photobiology. All necessary capital equipment is available.


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Description The marine cyanobacterium Prochlorococcus, the most abundant phototrophic organism on Earth, numerically dominates the phytoplankton in nitrogen (N)-depleted oceanic gyres. Alongside inorganic N sources such as nitrite and ammonium, natural populations of this genus also acquire organic N, specifically amino acids. Here, we investigated using isotopic tracer and flow cytometric cell sorting techniques whether amino acid uptake by Prochlorococcus is subject to a diel rhythmicity, and if so, whether this was linked to a specific cell cycle stage. We observed, in contrast to diurnally similar methionine uptake rates by Synechococcus cells, obvious diurnal rhythms in methionine uptake by Prochlorococcus cells in the tropical Atlantic. These rhythms were confirmed using reproducible cyclostat experiments with a light-synchronized axenic Prochlorococcus (PCC9511 strain) culture and S-35-methionine and H-3-leucine tracers. Cells acquired the tracers at lower rates around dawn and higher rates around dusk despite > 10(4) times higher concentration of ammonium in the medium, presumably because amino acids can be directly incorporated into protein. Leucine uptake rates by cells in the S+G(2) cell cycle stage were consistently 2.2 times higher than those of cells at the G(1) stage. Furthermore, S+G(2) cells upregulated amino acid uptake 3.5 times from dawn to dusk to boost protein synthesis prior to cell division. Because Prochlorococcus populations can account from 13% at midday to 42% at dusk of total microbial uptake of methionine and probably of other amino acids in N-depleted oceanic waters, this genus exerts diurnally variable, strong competitive pressure on other bacterioplankton populations.
Exploitation Route Through publications and data archived at BODC
Sectors Education,Environment