The physiological-ecology of the cyanobacterium Microcoleus

Abstract

A study was carried out to determine how widespread N(_2) fixation is in the cyanobacterium Microcoleus, both in the laboratory and in the field. The research was extended to compare the influence of environmental variables on both Ng fixing and non-fixing strains of Microcoleus isolated from a range of habitats. Since the morphology of Microcoleus strains is likely to influence their physiology, attempts were made to grow them in vitro in a form morphologically akin to that in the field i.e. with a communal sheath. Limited success was achieved with Microcoleus D634 on incubating in standard medium supplemented with 516 mM Na and 125 mM Ca (salinity of 30% as shown by hydrometer) , where a thin, communal sheath was only found intermittently surrounding 2-3 trichomes. All five strains were mixohaline growing at salinities of 0.5 – 30 (40) % and surviving periods under euryhaline (30 – 40%) and polyhaline (>40%) conditions. The shorter the time of exposure the higher the salinity tolerated. Growth varied according to the ratio of Na(^+) to Ca(^2+), Na(^+) to K(^+) and Na(^+) to Mg(^2+). Despite many changes in the nutrient status of the medium (Na(^+), Ca(^2+), Mg(^2+), K(^+)) at varying PAR and temperature, under oxic and micro-oxic conditions, only one of the five strains (Microcoleus D778) fixed N(_2) as shown by acetylene reduction activity (ARA) and growth in the absence of combined N. No ARA was detected for Microcoleus mats at Gibraltar Point, over two diel cycles in August 1986; however, when ARA was measured at Church Island, Anglesey (from whence Pearson et al., (1979) isolated Microcoleus D778), over 6 diel periods between June and October, 1987, ARA was detectable at all times between 0.1 and 3.4 nmol C(_2)H(_4) cm(^-2) h(^-1) using 4 h incubation periods. A different response in ARA was found on each occasion; generally, high activity (with > 70% in the dark) was found on sunny days and low, fairly constant ARA during cloudy, overcast days. On incubating Microcoleus D778 in 86 mM Na (salinity of 5%) at 20ºC under 16:8 and 8:16 light (50 µmol photon m(^-2) s(^-1)):dark, the majority (72 and 92% respectively) of ARA occurred in the dark, whereas in 20:4 and 16:8 light: dark only 19 and 40% ARA occurred in the light. However, ARA over a 16:8 light:dark cycle varied with PAR, salinity and temperature. In addition, the optimum temperature for ARA varied according to pH and salinity. When DCMU was added to Microcoleus mats and to axenic cultures, ARA increased markedly; the precise value depending on PAR and preincubation conditions.