Previous laboratory studies in species of the Laminariales have revealed that both the onset of growth in early winter and the summer drop in growth rate are controlled by the annual course of daylength synchronizing endogenous, circannual clocks within the thallus. Moreover, it is known for some laminarian species that cultivation in the laboratory in constant short days (SD) leads to arhythmic, continuous growth activity of the blade throughout the year. Such a prolonged SD treatment has now been performed for the first time in outdoor-cultivated Laminaria digitata. Field-grown sporophytes were collected in May from the sea near Helgoland (North Sea) and cultivated on the island of Sylt (North Sea) in temperature-controlled outdoor tanks (300 l) at 10 °C for 1·5 years either in a constant 8 h of light per day, controlled by an automatic blind on top of the tank, or in ambient daylengths. In constant SD, the growth rate remained at a steady-state level of approximately 0·4 cm day−1 from the end of June until mid-October, whereas growth rate in ambient daylengths declined steadily after June to half the rate in SD in October. Growth became light-limited between October and February in both treatments and, in the second year from July onwards, higher growth rates were again observed in constant SD than in ambient daylengths. Further work is required to find out whether SD treatment in summer would also prevent the summer drop in growth rate in other perennial seaweed species, e.g. commercially valuable red algae. This would potentially increase the chance of more constant biomass production throughout the year, and decrease the danger of the cultivated perennial algae being overgrown by annual epiphytes in summer.

Turbidostat cultures of Dunaliella salina (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) were grown at fluctuating concentrations of nitrate, phosphate and silicate. In-line measurements of PAM fluorescence were used to monitor the effects of fluctuating nutrient supply on the photochemical efficiency of photosystem II reaction centres of light-adapted cells (ΔF/F'm). Besides the maximal photochemical efficiency of photosystem II reaction centres of dark-adapted cells (Fv/Fm), chlorophyll a, particulate organic carbon, nitrogen and phosphorus, and the cell number were measured frequently during the experiments. Following nutrient-replete growth, the cells were supplied with medium from which either nitrate, phosphate or silicate was omitted. When significant effects of nutrient starvation were indicated by the fluorescence parameters, a pulse of the deficient nutrient was added to the cultures. Our experimental set-up for in-line fluorescence measurements provided sensitive and reproducible detection of the various fluorescence signals, revealing strong influences of nutrient supply on the photochemical efficiency of photosystem II. In general the fluorescence values changed substantially within 1–30 min after re-addition of the deficient nutrient. Addition of phosphate and silicate induced an immediate characteristic decrease in fluorescence, whereas nitrate addition was characterized by a strong, delayed increase in fluorescence. Complete recovery to pre-starvation fluorescence values took about 48 h in all experiments. The physiological background of nutrient uptake is used to explain the observed tight couplings between fluorescence responses and nutrient re-addition. Our study clearly demonstrates that in-line fluorescence measurements provide a new tool for the investigation of phytoplankton reactions to fluctuating nutrients and offer the possibility to detect nutrient starvation in the field.

Branch structure and growth in Amphiroa generated by primarily dividing apical medullary cells was documented by light and scanning electron microscopy. Specific numbers of long- and short-celled tiers are formed when intergenicular and genicular medullary cells periodically divide, leading to the unit pattern concept. Zonations on the intergenicular surface, grooves on the decalcified intergenicular surface and spaces in longitudinal section of intergenicula underlie units. Three unit pattern types are recognized: angled medullary tiers arranged in a straight horizontal row with lateral initials cut off abruptly from peripheral medullary cells in A. annulata and A. anastomosans (type A), medullary tiers whose length gradually reduces as they curve into the cortex in A. fragilissima var. debilis, A. rigida var. antillana, A. spina and A. verruculosa (type B), and rounded medullary tiers arranged in slightly irregular horizontal rows with lateral initials or cortical cells surrounding circularly peripheral medullary cells that are cut off from the medulla in A. cuspidata (type C). These patterns were statistically analysed in four Bermudan species: A. annulata, A. cuspidata, A. fragilissima var. debilis and A. rigida var. antillana. Unit patterns can usefully distinguish species. The association of intergenicula and genicula at the branch apex during the initial stage of branch formation is explained for the following four species: A. annobonensis, A. crustiformis, A. fragilissima var. debilis, and A. itonoi.