Fine structure of vegetative cells, development and morphology of sporangia and general morphology of zoospores were studied for the first time in the genus Endophyton Gardner. Vegetative cells contained one parietal perforate chloroplast with 2–4 ulvophycean pyrenoids and transverse cell walls without plasmodesmata. Sporangial walls were formed by two distinct electron-dense and fuzzy layers which contained a contiguous electron-translucent, fibrillar material. Zoospores were naked, with an anterior dense aggregation of vesicles and a cup-shaped chloroplast with one eyespot. The flagellar apparatus showed 180° rotational symmetry and counterclockwise absolute orientation of its components. Microtubular roots had a cruciate pattern, in which d=2 and s=4 with 3/1 arrangement. Electron-dense bilobed terminal caps covered the proximal end of each basal body. These observations support the suggested close relationship of Endophyton to the allied genus Entocladia and other ulvalean algae, and reinforce the criteria for including the genus in the class Ulvophyceae, order Ulvales, family Ulvellaceae.

In both field samples and cultures the total amount of sugar and of carbon in colonies of Phaeocystis globosa was correlated with colony surface area, suggesting a hollow structure. A conceptual model based on biochemical data and on the assumption that the mucus occurs as a layer of a fixed thickness, irrespective of colony size, predicts that the thickness of the layer is 7 μm. A confocal laser scanning microscope image of fluorescently labelled mucus confirmed this view of the colony structure. The measured contents of carbon and sugar per cell (including mucus) were constant for all colony sizes. Cells in laboratory cultures contained 122 pg C, which is twice the value for cells obtained at a field station in the North Sea (57 pg). In contrast, sugar per cell was higher in the field than in exponentially growing cultures. Therefore the percentage of sugar carbon relative to total carbon of colonies in the field was higher (19–35%) than that in cultures (10%). Highest values were found at low ambient nutrient concentrations, probably due to the presence of storage glucans in the cells that are produced under nutrient limitation. The functional role of Phaeocystis colonies in the pelagic system has to be re-evaluated because (i) biomass estimates have previously often been based on the assumption of a colony filled with mucus while in reality it is a hollow structure with an aqueous lumen, and (ii) in contrast to earlier reports it is shown that only a small amount of carbohydrate is needed to build the colony structure.

Morphological characters and growth rates of Ascophyllum nodosum were compared on fronds collected from different shore levels at one site in Strangford Lough, Northern Ireland. Both the bladders and axes of thallus tips were smaller and less variable on the upper shore than on the middle and lower shores, and the proportion of biomass invested in the production of bladders was smaller. The axes of upper shore plants were wider and flatter than on the lower shore, and the bladders were also relatively flat on the upper shore but more rounded on the lower shore. The maximum age of unbroken fronds, as estimated from the number of air bladders, was 6 years on the upper shore and 17 years on the lower shore, indicating the higher probability of breakage of plants caused by stress on the upper shore. In situ growth rates of individually marked plants along a vertical shore transect were monitored at 2- to 4-week intervals for 31 months. Length increase was highest (16 cm yr−1) in the middle of the Ascophyllum nodosum zone, and lowest (10 cm yr−1) on the upper shore. Growth was strongly seasonal with low (but not zero) growth rates during November and December, and highest growth rates in late spring and early summer. A decline in growth in mid-summer was observed at all shore levels. Growth was positively correlated with sunshine hours and total solar irradiance in winter and spring but there was no correlation with these factors during summer months. Maximum daily temperatures had a positive influence on growth in late spring, but no effect in the rest of the year. In transplantation experiments conducted in summer and winter, 80% of plants moved from the lower to the upper shore died within 3 months, whereas all transplants from the upper to the lower shore and all controls survived. Growth rates of surviving transplants at both shore levels were similar to those of control plants at the same shore level in the summer experiment. The pigment content and photosynthetic capacity of surviving plants acclimated to ambient conditions and, after 3 months, were not significantly different from those of control plants at the same shore level. It is suggested that the phenotypic acclimation of physiological characteristics and the genetic adaptation of morphological features contribute to the ability of this species to dominate such a wide band in the intertidal zone of sheltered rocky shores in the North Atlantic.