Constant short-day treatment of outdoor-cultivated Laminaria digitata prevents summer drop in growth rate

Abstract

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.