Interaction between the bacterial and phytoplanktonic inorganic nitrogenous nutrition
The present work investigates the inorganic nitrogenous nutrition of the phytoplanktonic community and its association with the bacterial role in the remineralisation and/or utilisation of inorganic nitrogen, in the waters of the Menai Strait. The field study lead to the recognition of four distinct phases in the seasonal cycle, based on measurements of primary plankton production and metabolism of nitrate and ammonium by the algae and bacteria. These were first the net autotrophic phase (April and May), followed by the net heterotrophic phase (June), then the secondary net autotrophic phase (July and August) and finally, the regenerative phase (remainder of the year). The phytoplankton seasonal pattern was characterised by nitrate-based production in the spring followed by ammonium-based production in summer and autumn, associated with the ambient availability of these nutrients. Bacteria accounted for up to 70% of the total inorganic nitrogen taken up with nitrate contributing heavily to bacterial production (up to 80%) early in the year. Whilst the maximum contribution of the heterotrophs to the total uptake was observed during the net heterotrophic phase, the lowest (<25%) was during the net autotrophic periods of the seasonal cycle. Calculations suggest that a maximum of 67% of algal ammonium uptake came from bacterial recycling. Only in June was there evidence of competition between algae and bacteria for nitrogen which is believed to have contributed to the sharp decline of the phytoplankton, at this time of the year. There was no evidence for nutrient limitation of the algae at other times of the season. There was also evidence for uptake of nitrate and ammonium in conditions of total darkness by both the bacteria and the algae, with bacteria accounting for a maximum of-75% the total nitrate and ammonium uptake. Ammonium was preferred over nitrate by the autotrophs and the heterotrophs. However, early in the year, when both primary and bacterial production were nitrate based, nitrate was utilised equitably with its environmental availability. For increasing concentrations of ammonium, bacteria clearly rejected the more oxidised form of inorganic nitrogen. The seasonal variation of 13C (as NaH13CO3) uptake by the populations studied was also investigated and there was strong evidence for heterotrophic uptake. Over the diel cycle, the contribution of bacteria to the total 13 C uptake amounted to a maximum of 44% during the net heterotrophic phase. In dark conditions, this percentage increased to a value of 60%, measured on the same occasion. The ambient availability of inorganic nitrogen seemed to influence the proportion of carbon fixed by the autotrophs that was inferred to be transferred to the bacteria. The assumed carbon transference from the autotrophs to the heterotrophs was highest in the spring, following a period of sufficiency of inorganic nitrogen, and lowest during summer (despite peaks of primary production), associated with conditions of ambient nitrogen depletion. It is widely hypothesised that bacteria take up inorganic nitrogen in order to be able to utilise carbon-rich, nitrogen-deficient compounds and so maintain a constant C:N cell quota. From a series of laboratory experiments, it was observed that the C:N ratio of the dissolved organic matter (DOM) used by the bacteria set the boundary on the role of these organisms as inorganic N net consumers or remineralisers. The laboratory work was interpreted as showing that when DOM was the major nitrogen source for growth, bacteria excreted the excess nitrogen as ammonium whereas at high C:N ratios, bacteria took up and conserved nitrogen. In the latter circumstances, when nitrogen was limiting, bacteria appeared to increase respiratory losses in order to dispose of the excess carbon. By contrast at low C:N ratios, carbon conservation occurred. It was concluded that if the release of DOM by the algae is the prime source of organic material for the bacteria, then this switching of the inorganic nitrogen metabolism of the bacteria, will be ultimately driven by the algae. These results were used to interpret the seasonal dynamics of the phytoplanlctonic and bacterial inorganic nitrogen metabolism in the Menai Strait.