Nitrogen fixation in cyanobacteria, an optimal photoperiod

Nitrogen fixation is one of the most important ocean processes, it can support up to 40% of total primary production in the marine environment. In the tropics, this process is carried out by unicellular cyanobacterium such as Trichodesmiun and Richelia, which are thought to survive on tropical temperatures. This study investigates the idea that these sort of bacteria, particularly Cyanothece sp., also thrive in these areas due to the light regime of tropical regions. It investigates how the ratio of light to dark is important to balance the processes of photosynthesis and nitrogen fixation which are generally separated in organisms by means of a circadian clock.

For this study, Cyanothece sp. strain BG 04351 was isolated from Bahama Islands in the Atlantic Ocean and grown in continuous culture at 26ᵒC. 3 different light regimes were tested to simulate tropical (12L:12D), winter temperate (8L:16D) and summer temperate (16L:8D) light patterns. Nitrogen fixation and photosynthetic activity were measured with each regime. Nitrogen fixation was measured using a nitrogenase activity assay which measures the reduction of acetylene gas to ethylene. Whereas photosynthetic activity was measured by using a Walz MINI-PAM to measure in-situ fluorescence in the light period and maximum fluorescence in the dark period, which in turn can be used to work out the quantum yield of photosystem II.

In each treatment, population dynamics reached equilibrium within a few days of the experiment. Irrespective of the light period, all treatments showed nitrogenase activity at 12-16 hours demonstrating that the 24h biological clock is highly conserved in terms of the onset of nitrogen fixation.

Nitrogen fixation was initiated in the tropical light regime around the onset of darkness, for activity of nitrogenase to then peak at 6.5-9 hours and return to 0 by the end of the dark period. This condition appears to be optimal for Cyanothece, as it provides the perfect balance of available carbon storage to nitrogen for the cells’ metabolic rate. This therefore becomes an important factor in determining the ecological distribution of this microbe and begins to explain why unicellular diazotrophs are so successful in tropical and subtropical latitudes.

Sub-optimal conditions for Cyanothece were shown to be the summer and winter temporal light variations. During the winter temperate light regime, nitrogenase activity was initiated after 8 hours of dark and lasted for another 8h; no nitrogen fixation happened in the light period. There was a lower net nitrogen fixation due to the lack of carbon reserves. The summer temperate regime showed nitrogenase activity to start in the light period and to peak at 1.5-4 hours into the dark period, this confirms the knowledge that Cyanothece can fix nitrogen in the light. However, the total nitrogen fixation in this regime was a third less than that of the tropical regime.

The nocturnal quantum yield in Cyanothece was shown to be in close synchrony with nitrogenase activity; this was hypothesised to be due to supply of ATP in the cell from the electron transport chain. It is thought that nitrogen fixation is the most energy consuming process in the dark period, therefore is limited by the supply of ATP.

This study is an insight into understanding the niche in which Cyanothece fits in tropical regions, it gives an insight into the functional plasticity of this particular microbe and reveals how although it is best suited to tropical regions, it can adapt its circadian rhythm to survive in sub-optimal light conditions. Further study would be applicable in the area to see how other environmental factors such as temperature and nutrient levels would also affect the Cyanothece and its ecological niche.

Rabouille S., Van de Waal D. B., Hans C.P. Matthijs H. C. P. and Huisman J. (2013) Nitrogen fixation and respiratory electron transport in the cyanobacterium Cyanothece under different light/dark cycles. FEMS Microbiology Ecology, 87, 3: 630-638.