The planktonic marine cyanobacterium Trichodesmium occurs throughout the oligotrophic tropical and subtropical oceans. Trichodesmium fixes N2 and is a major contributor to the marine and global nitrogen (N) cycle. In the Great Barrier Reef (GBR) lagoon, Trichodesmium spp. often dominates the microplankton community. There is evidence that Trichodesmium growth has
increased in recent times and that this increased growth is more evident in regions impacted by river discharges. It is estimated that Trichodesmium supplies as much N to the GBR lagoon as do all of the rivers combined and hence its growth could well be a major contributor to eutrophication and hence the destruction of corals in the GBR lagoon. The factors promoting the growth and N2 fixation of Trichodesmium still remain poorly understood. This is because very few laboratories worldwide have been able to culture it. However it has been hypothesised that increased supply of essential nutrients (e.g. phosphorus (P), iron (Fe)) via run-off from coastal catchments could enhance Trichodesmium activity. Here I report results on the effects of various chemical and physical factors (i.e. salinity, light, Fe, P and combined nitrogen) on the growth and N2 fixation of a newly established
culture of Trichodesmium from the GBR lagoon.
Trichodesmium sp. actively grew over a wide range of salinities (22-43 psu). Maximum growth occurred in the salinity range of 33-37 psu, which is in good agreement with historical findings in the GBR lagoon. Chlorophyll α (Chl α) concentration and alkaline phosphatase activity (APA) were found to increase with salinity, but N2 fixation rate was reduced at salinities below and above the range for maximum growth. N2 fixation was dependent on the light intensity over the whole range tested (10-160 µmol quanta m-2 s-1) and the highest fixation occurred under white light when compared with blue, green,
yellow and red light. Trichodesmium may grow more efficiently under low irradiance (45-75 µmol quanta m-2 s-1) and under green and yellow light. Therefore these results suggest that Trichodesmium can grow well at depths of 50-65 m in clear ocean waters but that its optimum growth would be limited to shallower zones i.e. 3-15 m in the coastal regions. These results are in general agreement with the field observations that Trichodesmium often colonizes the top 100-200 m in open ocean waters but that the abundance of Trichodesmium is greatest at depths of 20-45 m in the open ocean and 5-10 m in coastal waters of GBR lagoon. The cellular concentrations of Chl α and phycobiliproteins (PBPs) declined with increasing light intensity; whereas the ratio of Chl
α : PBPs increased. Trichodesmium seems to exhibit some chromatic adaptation with respect to light intensity and light quality with the highest cellular concentrations of Chi α and PBPs occurring under low light conditions. The highest cellular concentrations of Chl α, PE (phycoerythrin) and PC (phycocyanin) occurred under blue, green and red light respectively.
Dissolved inorganic P (DIP) concentrations up to 1.25 µM significantly enhanced N2 fixation rates and concentrations up to 3.5 µM significantly enhanced growth rates. In both cultures and natural populations, APA was inhibited with increasing DIP and
dissolved organic phosphorus (DOP) concentrations. Trichodesmium also secretes soluble alkaline phosphatase into the medium. Fe additions also stimulated growth, N2 fixation, cellular Chl a concentrations, light-saturated Chl α-specific gross photosynthetic capacity (Pchiam) and dark respiration rate. These results suggest that P and Fe supply may be important factors in controlling population growth and bloom formation by Trichodesmium in coastal waters and the surface waters of the ocean.
Cell yields in media containing 10µM of combined nitrogen were similar to those for cultures grown in N-free medium. The addition of 10 µM of combined nitrogen
(NH4+, NO3-, urea) did not inhibit N2 fixation for the first sub-culture but did so for the fifth subculture. However, additions of 2 µM NH4+ did not have an effect on N2 fixation rates, suggesting Trichodesmium has the capacity to fix N2 in the presence of combined N sources and the inhibition of N2 fixation depends on the levels of combined nitrogen. In the latter stages of growth, Trichodesmium released 1 to 2 µM of NH4+ to N-free medium. In N-rich cells cellular PE was 3.5-fold higher than cellular Chl α, whereas cellular
PE was 2.1-fold higher when cells were N-deprived. Also, PE relative variations were higher than those of Chl α. These results suggest that the pool of PE is preferentially degraded to reduce the energy transfer from PE to Chl α for supporting the N demand and PE may play a role in Trichodesmium nitrogen reserves.