MICROALGAL LIGHT-HARVESTING IN EXTREME LOW-LIGHT ENVIRONMENTS IN MCMURDO SOUND, ANTARCTICA

Microalgal pigment composition, photosynthetic characteristics, single -cell absorption efficiency (Qa((lambda))) spectra, and fluorescence-e xcitation (FE) spectra were determined for platelet ice and benthic co mmunities underlying fast ice in McMurdo Sound, Antarctica, during aus tral spring 1988. Measurements for spectral irradiance (E((lambda))) a nd photosynthetically active radiation (PAR) as well as samples for pa rticulate absorption measurements were taken directly under the congel ation ice, within the platelet layer, as profiles vertically through t he water column, and at the benthic surface. Light attenuation by sea ice, algal pigments, and particulates reduced PAR reaching the platele t ice layer to 3% (9-33 mu mol photons . m(-2). s(-1)) of surface valu es and narrowed its spectral distribution to a band between 400 and 58 0 nm. Attenuation by the water column further reduced PAR reaching the sea floor (28-m depth) to 0.05% of surface levels (<1 mu mol photons . m(-2). s(-1)), with a spectral distribution dominated by 470-580-nm wavelengths. The photoadaptive index (I-k) for platelet ice algae (5.9 -12.6 mu mol photons . m(-2). s(-1)) was similar to ambient PAR, indic ating that algae had acclimated to their light environment (i.e. the a lgae were light-replete). Maximum Qa(lambda) at the blue absorption pe ak (440 nm) was 0.63, and enhanced absorption was observed from 460-50 0 nm and was consistent with observed high cellular chlorophyll (chl) c : chl a and fucoxanthin : chl a molar ratios (0.4 and 1.2, respectiv ely). Benthic algae were light-limited despite the maintenance of very low I-k values (4-11 mu mol photons . m(-2). s(-1)). Extremely high f ucoxanthin : chl a ratios (1.6) in benthic algae produced enhanced gre en light absorption, resulting in a high degree of complementation bet ween algal absorption and ambient spectral irradiance. Qa((lambda)) va lues for benthic algae were maximal (0.9) between 400 and 510 nm bet r emained > 0.35 even at absorption minima. Strong spectral flattening, a characteristic of intense pigment packaging, was also apparent in th e Qa(lambda) spectra for benthic algae. FE and Qa((lambda)) spectra we re similar in shape for platelet ice algae, indicating that the effici ency at which absorbed energy was transferred to photosystem II (PSII) was independent of wavelength. fluorescence emission by benthic algae was greatest for 500-560-nm excitation wavelengths, suggesting that m ost energy absorbed by accessory pigments was transferred to PSII. The se results suggest that under-ice algae employ complementary pigmentat ion and maximize absorption efficiency as adaptive strategies to low-l ight stress. Regulating the distribution of absorbed energy between PS I and PSII may be an adaptive response to the restricted spectral dist ribution of irradiance.