Photosynthetic light requirements, metabolic carbon balance and zonation of sublittoral macroalgae from King George Island (Antarctica)

Photosynthesis, dark respiration, chlorophyll a contents and daily metabolic C balance were determined in 5 species of brown and red algae from Potter Cove (King George Island) during the Antarctic spring. In situ irradiance data were used to determine the light requirements of plants collected at 10, 20 and 30 m depth. Average daily maximum quantum irradiances measured in spring summer reached up to 23 mu mol photons m(-2) s(-1) at 30 m depth indicating that macroalgae can effectively be exposed to non-limiting quantum irradiances for photosynthesis. Net photosynthetic rates (P-max) were high in the brown alga Desmarestia anceps and the red algae Palmaria decipiens with values close to 33 and 36 mu mol O-2 g(-1) FW h(-1), respectively, at 20 m depth. With the exception of the brown alga Himantothallus grandifolius, all the species showed lower P-max in plants collected at 30 m than at 10 and 20 m depth. The photosynthetic efficiency (alpha) varied strongly among species, but no clear depth-dependent relations were found. Saturation (I-k) and compensation (I-c) points for photosynthesis were, in general, lower in plants growing at deep locations. In plants from 10 and 20 m, photosynthesis was saturated at significantly lower irradiances than in situ quantum irradiances. Values of I-k varied between 58 mu mol photons m(-2) s(-1) in D. anceps and 15 mu mol photons m(-2) s(-1) in the red alga Gigartina skottsbergii, while I-c ranged between 1 and 10 mu mol photons m(-2) s(-1) in most of the species. D. anceps exceptionally had I-c values close to 26 mu mol photons m(-2) s(-1) in plants from 10 m depth. Overall, photosynthetic performance in these species was comparable to rates measured in macroalgae from upper littoral zones and did not provide evidence for metabolic acclimation with depth. Apparently, the daily periods for which plants are exposed to saturation and compensation irradiances (H-sat and H-comp) and, consequently, the metabolic C balance account for the acclimation of macroalgae to deep sublittoral zones. At 10 m, H-sat for many species was between 12 and 14 h, while at 30 m these periods decreased to 7 h in D. anceps or 9 h in the red alga Kallymenia antarctica. The H-comp periods were longer, in the case of red algae up to 16 h. The daily carbon balance decreased with depth. At 30 m, algae exhibited C gains lower than 1 mg C g(-1) FW d(-1) and in D. anceps, due to its high respiration rates, carbon balance was negative at saturation and compensation irradiances. In general, greater C gains relative to losses were found in plants growing at 20 m depth. Although data on P-max, alpha, I-c and I-k indicate that Antarctic macroalgae are metabolically able to inhabit greater depths during spring-summer, the shortening of the daylengths for which algae are exposed to saturating or compensating irradiances impose a maximum depth limit at depths around 30 m.