The identification of gas vacuoles and their abundance in the hypolimnetic bacteria of Arco Lake, Minnesota

The permeability of heterocysts to the gases nitrogen and oxygen

Heterocysts of the cyanobacterium Anabaena flos-aquae retina gas vacuoles for several days after differentiation. It is demonstrated that the rate of gas diffusion into a heterocyst that is near an overlying gas phase can be determined approximately from observations on the rate of gas pressure rise required to collapse 50% of its gas vacuoles. The mean permeability coefficient ( α ) of heterocysts O 2 and N 2 was found to be 0.3 s -1 . From this it was calculated that the average permeability ( k ) of the heterocyst surface layer is about 0.4 μm s -1 (within a factor of 2). This is probably within the range that could be provided by a few layers of the 26-C glycolipids in the heterocyst envelope. It is likely, but not proven, that the main route for gas diffusion is through the envelope rather than through the terminal pores of the heterocyst. From measurements of cell nitrogen content (2.7 pg). doubling time (3 days) and heterocyst: vegetative cell ratio (1:24) it was calculated that the average heterocyst fixed 5.9 x 10 -18 mol N 2 s -1 ; this must equal the diffusion rate of N 2 inside the average heterocyst that was 22% below the outside air-saturated concentration. the maximum N 2 fixation rate allowed by the estimated permeability coefficeint would be 2.7 x 10 -17 mol s -1 per heterocyst, slightly greater than the maximum calcualted N 2 fixation rate. The observed permeability coefficient is low enough for the oxygen concentration in the heterocyst to be maintained close to zero by the probable rate of respiration, providing an anaerobic environment for nitrogenase. The rate of O 2 diffusion will limit the N 2 -fixation rate in the dark by limiting the rate at which ATP is supplied by oxidative phosphorylation.

The phytoplanktonic ways of life

The adaptations of phytoplankton to life in suspension in water are considered with particular reference to hydromechanical factors ranging from molecular motion to ocean currents. The smallest phytoplankton, the picoplankton (0.2-2.0 μm), are the best adapted to the physico-chemical environment of the open waters of seas and lakes and, other things being equal, can out-compete the larger forms. The picophytoplankton are an autotrophic component in a microbial community, the ultraplankton, which also includes bacteria and flagellates up to about 20 μm in linear dimensions. This community is a highly dynamic and self-contained equilibrium system operating within a domain dominated by molecular diffusion. Within the photic zone it is limited, not by nutrient supply, but by its internal predator-prey relationships. It appears to be cosmopolitan, both in the sea and in freshwaters, to vary little either in time or space in species composition or in biomass concentration, and to contribute a minimum of organic carbon to higher trophic levels. There appears to be a fundamental divide in form and function between this and the microplankton, composed of organisms larger than about 20 μm. This community can only develop when nutrient levels are in excess of the concentrations required by the picoplankton. It is opportunistic, non-equilibrium in its dynamics, and highly variable in floristic composition and biomass concentration. Its life processes are dominated by turbulence. Nutrient supply is largely determined by turbulent eddy diffusion and movement of the organisms relative to the water mass. The microphytoplankton falls broadly into two types, one of which, exemplified by the diatoms, depends on turbulence to maintain it in the photic zone and the other, exemplified by the dinoflagellates and colony-forming cyanobacteria, relies on motility or buoyancy control to position it in a relatively stable water column so as to have best access to light and nutrients. The waxing and waning of microplankton populations is largely determined by hydrography and their floristic compositions by the interactions of the daily and seasonal rhythms of the organisms with the periodicities in the environment. In contrast to the ultraplankton microplankton species show distinct differences in biogeographical distribution. Throughout the discussion attention is drawn to the intimate relationships between the activities of phytoplankton and those of viruses, bacteria and zooplankton and the impossibility of getting a proper understanding of the physiology of the phytoplankton if they are considered in isolation. CONTENTS Summary 191 I. Introduction 192 II. Some reminders about the aqueous medium 193 III. The ideal phytoplankton organism 194 IV. Some aspects of the physiological ecology of the picoplankton 195 V. The ultraplanktonic community 197 VI. A divide in the phytoplankton 201 VII. Some aspects of the physiological ecology of the microphytoplankton 203 VIII. Small scale turbulence in the environment of phytoplankton 205 IX. Intermediate scale turbulence in the environment of phytoplankton 207 X. The effects of large scale rotational stirring 220 XI. Phytoplankton as part of a community 221 XII. Conclusions 222 References 223.