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summary 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.

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In the coarse of investigations on the nitrogen-fixing alga Anabaena cylindrica Lemm., observations on the growth of pure cultures have yielded information which appears to shed some light on the nature of the My.xophycean heterocyst. The more important results obtained are described and discussed in the following account.

Picoplankton consists of those organisms found in the open waters of seas and lakes which are capable of passing through a filter with 2 μm pores but not through one with 0.2 μm pores. Cells in this size range are well adapted to planktonic life in that they sink extremely slowly and are more efficient than larger forms in taking up nutrients and absorbing radiant energy. Picophytoplankton includes coccoid cyanobacteria and a variety of eukaryotic algal forms. Strains studied in the laboratory have all been found to show maximum growth at relatively low irradiances, the eukaryotic forms being more efficient than the cyanobacteria in utilizing the blue light which predominates at the bottom of the photic zone in clear oceanic waters. Oceanic strains of coccoid cyanobacteria, however, are characterized by high concentrations of phycoerythrin, which appears to function as a nitrogenous reserve as well as an accessory pigment in photosynthesis. The seasonal and spatial distribution of picophytoplankton seems explicable in terms of these physiological characteristics. Numbers of coccoid cyanobacteria have shown a striking correlation with temperature in a number of different situations. Heterotrophic bacteria are also included in the picoplankton, and a review of the information concerning them suggests that they form a highly dynamic population subsisting on dissolved organic matter liberated by living phytoplankton and zooplankton and by decomposition of dead matter. The productivity of this population in the euphotic zone approaches that of the phytoplankton. Both the picophytoplankton and the bacterioplankton are preyed on by phagotrophic flagellates. Both bacteria and flagellates are active in regeneration of mineral nutrients. Regardless of the salinity, temperature or nutrient status of the water, the numbers of heterotrophic bacteria, picophytoplankton and flagellates tend to be around 10 6 , 10 4 and 10 3 organisms per millilitre respectively. It is suggested that these populations form a basic, self-sustaining and self-regulating community in all natural waters. From present information, it seems that little of the energy which passes through this community finds its way into the larger planktonic organisms, but the role of picoplankton in recycling nutrient elements is of great importance in the marine ecosystem.

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