The seasonal development of the endemic Antarctic DesmarestialesHimantothallus grandifolius, Phaeurus antarcticus, Desmarestia anceps, of a ligulateDesmarestiasp., of the Antarctic cold-temperateAdenocystis utricularis(Dictyosiphonales) and of the endemic AntarcticAscoseira mirabilis(Ascoseirales) was monitored in a 2-year culture study under fluctuating daylengths mimicking the daylength conditions on King George Island (Antarctica). Temperature was kept constant at 0° C and nutrient levels were maintained at 0.6 moles m−3nitrate and 0.025 moles m−3phosphate. Sporophytes were initiated between (April-) June and July in all Desmarestiales. This event was controlled either by induction of gametophyte fertility (inH. grandifoliusandD. anceps) or by induction of spore formation (inDesmarestiasp. andP. antarcticus). Young sporophytes of all species showed a growth optimum from September to December (-February).Desmarestiasp. andP. antarcticusproduced spores and degenerated subsequently after one year of culture at ≥3μmol photons m−2s−1or after 22 months of culture at 2μmol m−2s−1. InD. ancepsspores were released without degeneration of the mother plants after 20 and 19 months of culture at 3 and 10μolm−2s−1, respectively. InH. grandifoliusspore formation was not observed. Adult one year old plants of the latter two perennial species showed growth optima between September and November. Microthalli ofA. utriculariswere the dominant life phase of this alga in winter. Macrothalli started to develop from June onwards at ≥3μmol m−2s−1or from August to September at 2μmol m−2s−1. Growth rates of macrothalli cultivated at ≥9μmol m−2s−1showed a growth optimum from September to November. The macrothalli released spores from January to February. Macrothalli cultivated at ≥3μmol m−2s−1maximally grew in January. They became fertile after almost 2 years of culture at 3μmol m−2s−1and remained vegetative at 2μmol m−2s−1.A. mirabilisexhibited a prominent growth optimum from August to October, at photon fluence rates between 2 and 47μmol m−2s−1. A second optimum was evident from January to March in plants cultivated at ≥9μmol m−2s−1. The results closely correspond to available field data and indicate that the phenology of the studied species can be controlled in the laboratory solely by simulating Antarctic daylengths conditions. The light requirements for growth were very low in microthalli and in juvenile macrothalli and growth was mostly light saturated at 4–12μmol m−2s−1. Few-celled sporophytes ofH. grandifoliusandD. ancepstolerated at least 8 and 11 months of darkness. The minimum light demands for completion of the life cycle are 31.4 mol m−2year−1inDesmarestiasp.,P. antarcticusand probably also in the 2 perennial Desmarestiales; 47.1 mol m−2year−1are needed inA. utricularisand probably also inA. mirabilis. These values predict a lower distribution limit of the investigated species at 53±23 m or 48±21 m in clear offshore waters and
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