Regulation of heterocyst frequency in Baltic Sea Aphanizomenon sp.

Abstract

Heterocyst frequencies in Baltic Sea Aphanizomenon sp. were similar along a strong nutrient gradient from the discharge point of a sewage treatment plant at the head of the Himmerfjärden bay to the open sea. Filaments lacked heterocysts in winter and for over a month after the spring bloom had depleted combined nitrogen in the surface layer. Heterocyst-free filaments in spring contained granulate structures that decreased in abundance simultaneously as colony nitrogen content decreased, but δ15N remained unchanged, indicative of storage of fixed nitrogen in overwintering Aphanizomenon sp. filaments. Heterocyst formation was initiated when water temperature was sufficient to form a shallow seasonal pycnocline that allowed filaments to be exposed to enough light to initiate growth and a subsequent intracellular shortage of nitrogen. During the growth season, heterocyst frequency varied significantly with maximum values in early summer (May), lower values in mid-August that coincided with maximum temperatures and an increase in late summer. Heterocyst frequencies decreased with increased temperatures, suggesting a more efficiently functioning nitrogenase enzyme. Based on data from three seasons, filament C:P ratios did not correlate with heterocyst frequencies, neither did reduced heterocyst frequencies coincide with high dissolved inorganic nitrogen concentrations. Increased heterocyst frequencies, however, resulted in decreased C:N ratios, probably as more heterocysts likely increase nitrogen fixation.