Seasonality of brown macroalgae from Antarctica—a long-term culture study under fluctuating Antarctic daylengths

Summary

The seasonal development of the endemic Antarctic Desmarestiales Himantothallus grandifolius, Phaeurus antarcticus, Desmarestia anceps, of a ligulate Desmarestia sp., of the Antarctic cold-temperate Adenocystis utricularis (Dictyosiphonales) and of the endemic Antarctic Ascoseira 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⁻³ nitrate and 0.025 moles m ⁻³ phosphate. Sporophytes were initiated between (April-) June and July in all Desmarestiales. This event was controlled either by induction of gametophyte fertility (in H. grandifolius and D. anceps) or by induction of spore formation (in Desmarestia sp. and P. antarcticus). Young sporophytes of all species showed a growth optimum from September to December (-February). Desmarestia sp. and P. antarcticus produced spores and degenerated subsequently after one year of culture at ≥3 μmol photons m⁻² s⁻¹ or after 22 months of culture at 2 μmol m⁻² s⁻¹. In D. anceps spores were released without degeneration of the mother plants after 20 and 19 months of culture at 3 and 10 μolm⁻² s⁻¹, respectively. In H. grandifolius spore formation was not observed. Adult one year old plants of the latter two perennial species showed growth optima between September and November. Microthalli of A. utricularis were the dominant life phase of this alga in winter. Macrothalli started to develop from June onwards at ≥3 μmol m⁻² s⁻¹ or from August to September at 2 μmol m⁻² s⁻¹. Growth rates of macrothalli cultivated at ≥9 μmol m⁻² s⁻¹ showed a growth optimum from September to November. The macrothalli released spores from January to February. Macrothalli cultivated at ≥3 μmol m⁻² s⁻¹ maximally grew in January. They became fertile after almost 2 years of culture at 3 μmol m⁻² s⁻¹ and remained vegetative at 2 μmol m⁻² s⁻¹. A. mirabilis exhibited a prominent growth optimum from August to October, at photon fluence rates between 2 and 47 μmol m⁻² s⁻¹. A second optimum was evident from January to March in plants cultivated at ≥9 μmol m⁻² s⁻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 4–12 μmol m⁻² s⁻¹. Few-celled sporophytes of H. grandifolius and D. anceps tolerated at least 8 and 11 months of darkness. The minimum light demands for completion of the life cycle are 31.4 mol m⁻² year⁻¹ in Desmarestia sp., P. antarcticus and probably also in the 2 perennial Desmarestiales; 47.1 mol m⁻² year⁻¹ are needed in A. utricularis and probably also in A. 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 at 28±5 m or 26±5 m, respectively, in inshore fjords of the Antarctic Peninsula region.