Photosynthetic performance and pigment composition of Macrocystis pyrifera (Laminariales, Phaeophyceae) along a gradient of depth and seasonality in the ecoregion of Magellan, Chile

Macrocystis pyrifera (L.) C. Agardh is a species that forms extensive underwater forests along the coastline of the ecoregion of Magellan. There, this alga is exposed to marked variation in photoperiod, temperature, and irradiance, which are modulated by daily and seasonal climatic variations. This study aims to understand the ecophysiological behavior of M. pyrifera, in a natural forest localized in Puerto del Hambre, Magellan Region of Chile, along spatial (depth) and temporal (season) gradients of physical drivers by analyzing algal responses in terms of the photosynthetic pigments and fluorescence yield. In the apical, middle, and basal fronds, the following photosynthetic parameters were seasonally measured: electron transport efficiency (alpha), maximum relative rate of electron transport (rETR(max)), saturation point (E (k) ), and pigments such as chlorophyll a (Chl a), chlorophyll c (Chl c), and fucoxanthin. Both seasonal and stratified variations were observed. In autumn, alpha was decreased in the middle fronds (0.136 +/- 0.030 (mu mol photons m(-2) s(-1))(-1)) with respect to apical and basal fronds of autumn. For parameters such as E (k) , this decrease was observed relative to the depth gradient, with significant differences (p < 0.05) between distinct fronds. rETR(max) was high in the apical fronds in spring, autumn, and winter. High Chl a concentration was maintained in all seasons, while the concentration of Chl c in the apical fronds tended to be lower. The concentration of fucoxanthin remained stable without significant differences between dissimilar types of fronds, for seasons (spring, summer, autumn, and winter). The Chl a/Chl c ratio increased with depth, while the Chl a/fucoxanthin ratio varies seasonally. Variations of light intensity in a natural population in a depth gradient and pigment variations in M. pyrifera with the depth stratification reveal the behavior of these algae in the ecoregion of Magellan where Chl a, through the apical fronds, could be regulating the photosynthetic activity of the plants at stratification level. Furthermore, the increase in Chl c and fucoxanthin towards the middle and basal fronds showed similar trends as those measured for alpha, thus signifying higher photosynthetic efficiency at greater depth. Overall, our results indicate marked seasonal and depth acclimation to different environmental conditions. This study is the first of its kind for the ecoregion of Magellan in which M. pyrifera represents a keystone species of utmost ecological significance.