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 sitii irradiance data were used to d e t e r m~n e the light requirements of plants collected at 10, 20 and 30 m depth. Average daily maximum quantum irradiances measured in springsummer reached up to 23 pmol photons m-2 s-' a t 30 m depth indicating that macroalgae can effectively be exposed to non-limiting quantum irradiances for photosynthesis. Net photosynthetic rates (P,,,,,) were high in the brown alga Desmarestia anceps and the red algae Palmaria decipiens with values close to 33 and 36 pmol O2 g-l FW h-', respectively, at 20 m depth. With the exception of the brown alga Himantothallus grandifolius, all the species showed lower P,,,,, in plants collected at 30 m than at 10 and 20 m depth. The photosynthetic efficiency (a) varied strongly among species, but no clear depthdependent relations were found. Saturation (&) and compensation (L) 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, varied between 58 pm01 photons m-' S ' in D. anceps and 15 pm01 photons m-2 S-' in the red alga Gigartina skottsbergii, while I, ranged between 1 and 10 pmol photons m-2 S-' in most of the species. D. anceps exceptionally had I, values close to 26 pm01 photons m-2 S-' 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,,, and H,,,,,) and, consequently, the metabolic C balance account for the acclimation of macroalgae to d e e p sublittoral zones. At 10 m, H,,, 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 KaUymenia antarctica. The H,,,,,, periods were longer, in the case of the 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-l FW d-' 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,,.,,, a. I, and I, indicate that Antarctic macroalgae are metabolically able to inhabit greater depths during spring-summer, the shortening oI the daylengths for which algae are exposed to saturating or compensating irradiances Impose a maximum depth limit at depths around 30 m.

KeywordsAntarctica. dark respiration. light. marine macroalgae, photosynthesis. temperature. 0.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here. AbstractThe photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae HimontothalIus qrandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsberqii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta. Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at O°C. the green algae were cultivated at 10°C. I K values were between 18 and 53 pmol m -2 S-I characteristic for low light adapted algae. Only the two Ulothrix species showed higher I K values between 70 and 74pmol m-2 S-I. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 pmol m-2 S-I. Values of a were high: between 0.4 and L'l umol O 2 g-I FW h-I (pmol m-2 S-I)-I in the brown and red algae and between 2.1 and 4.9 pmol O 2 s' FW h-I (pmol m-2 S-I)-1 in the green algal species. At O°C Pmax values of the brown and red algae ranged from 6.8 to 19.1pmol O 2 g-I FW h-I and were similarly high or higher than those of comparable Arcticcold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis. 10°C in H. qrondifolius. 15°C in G. skottsberqii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q IO values for photosynthesis a:Chi: OW: FW: Ie:P m ax : P: R ratio: P us I curve: QIO: Abbreviations and Symbols initial slope in the linear range of the P us I curve (photosynthetic efficiency) chlorophyll dry weight fresh weight photosynthetic light compensation point photon fluence rate. at which the linear branch of the P us I curve and saturation plateau intersect (initial saturation of photosynthesis) light saturated net photosynthesis photosynthesis: respiration ratio photosynthesis us irrandiance curve ratio of reaction rates at (t + 10)°C and t°C

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A male gametophyte of the endemic Antarctic red macroalga Palmaria decipiens (Reinsch) Ricker was ctlltivated under fluctuating daylengths, simulating the s~:,asonal changes at the site of collection (King George Island, Antarctica). The plant was maintained at 0 :F I~ an irradiance of 25gmolm-2s -t and under growth-saturating nutrient conditions. Samples were ~:aken at intervals of 3-6 weeks to measure growth, photosynthesis, dark respiration and pigment content. The growth optimum in spring coincided with a higher photosynthetic activity. Whereas dark respiration was constantly low over the year, there was a rapid increase in maximum photosynthetic rate (Pmax) in conditions corresponding to September and October. This was correlated with a change in the initial slope (e) of the photosynthesis versus irradiance (P vs I) curve. Higher activity in photosynthesis mainly resulted from highe~ Chl a and phycobilin concentrations during Antarctic spring, an indication of an increase in absorption <:ross-section areas of photosynthetic reaction centres. These changes in physiology are discussed in relation to the seasonal growth "strategy" of the specie:;, which is controlled by seasonal variation in daylel:gth.

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