PHOTOSYNTHETIC QUANTUM EFFICIENCIES OF PHYTOPLANKTON FROM PERENNIALLY ICE COVERED ANTARCTIC LAKES1

A recently introduced approach far estimating the photosynthetic quantum efficiency (φ) of a freshwater or marine phytoplankton community has been applied for the first time to high latitude polar ecosystems, namely four lakes of southern Victoria Land, Antarctica. Values for φ at various depths ranged from 0.0022–0.1560 when calculated using a recommended mean extinction coefficient for phytoplankton (i.e. k?_c= 0.016). By contrast, φ ranged from 0.0037–0.0760 when calculated using an empirically estimated value for k?_c of 0.0328. If the recommended k?_c= 0.016 more closely approaches an accurate estimate, then the φ valves indicate that the phytoplankton convert light to organic carbon more efficiently than elsewhere. However, if the empirically derived k?_c= 0.0328 more closely approaches an accurate estimate, then the φ values indicate the phytoplankton trap light more efficiently than elsewhere. Although we have not resolved whether light conversion (φ) or light trapping are more efficient, the results show that the phytoplankton of these Antarctic lakes are well adapted to performing photosynthesis under extremely low light conditions.     

PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIPS IN SEA ICE MICROALGAE FROM McMURDO SOUND,ANTARCTICA1

Sea ice microalgae in McMurdo Sound, Antarctica were examined for photosynthesis-irradiance relationships and for the extent and time course of their photoadaptation to a reduction in in situ irradiance. Algae were collected from the bottom centimeter of coarse-grained congelation ice in an area free of natural snow cover. Photosynthetic rate was determined in short term (1 h) incubations at ?2° C over a range of irradiance from 0 to 286 μE·m^?2·s^?1. Assimilation numbers were consistently below 0.1 mg C·mg chl a^?1·h^?1. The I_k's³ averaged only 7 μE·m^?2·s^?1, and photosynthesis was inhibited at irradiances above 25 μE·m^?2·s^?1. Photosynthetic parameters of the ice algal community were examined over a nine day period following the addition of 4 cm of surface snow while a control area remained snow-free. A reduction of 40% in P_m^B relative to the control occurred after two days of snow cover; α, β, I_k, and I_m were not significantly altered. Low assimilation numbers and constant standing crop size, however, suggested that the algal bloom may have already reached stationary growth phase, possibly minimizing their photoadaptive response.     

PHOTOSYNTHESIS IN AN EXTREME SHADE ENVIRONMENT: BENTHIC MICROBIAL MATS FROM LAKE HOARE, A PERMANENTLY ICE-COVERED ANTARCTIC LAKE

We investigated the composition of benthic microbial mats in permanently ice-covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic oxygen exchange relationships. Mats could be subdivided into three distinct depth zones: a seasonally ice-free “moat” zone and two under-ice zones. The upper under-ice zone extended from below the 3.5 m thick ice to approximately 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated to the high irradiance they experienced during summer. They contained photoprotective pigments, predominantly those characteristic of cyanobacteria, and had high compensation and saturating irradiances (E_c and E_k) of 75 and 130 μmol photons·m⁻²·s⁻¹, respectively. The moat mats used light inefficiently. The upper under-ice community contained both cyanobacteria and diatoms. Within this zone, biomass (as pigments) increased with increasing depth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident light increasing with depth to a maximum of 0.06 mol C fixed·mol⁻¹ incident photons under light-limiting conditions. Precipitation of inorganic carbon as calcite was associated with this community, representing up to 50% of the carbon sequestered into the sediment. The lower under-ice zone was characterized by a decline in pigment concentrations with depth and an increasing prevalence of diatoms. Photosynthesis in this community was highly shade acclimated and efficient, with E_c and E_k below 0.5 μmol·m⁻²·s⁻¹ and 2 μmol·m⁻²·s⁻¹, respectively, and maximum yields of 0.04 mol C fixed·mol⁻¹ incident quanta. Carbon uptake in situ by both under-ice and moat mats was estimated at up to 100 and 140 mg·m⁻²·day⁻¹, based on the photosynthesis–irradiance curves, incident irradiance, and light attenuation by ice and the water column.     

RATES OF PHOTOADAPTATION IN SEA ICE DIATOMS FROM MCMURDO SOUND,ANTARCTICA1

Sea ice microalgae are released from their relatively stable light environment to the water column seasonally, and any subsequent growth in a vertically mixed water column may depend, in part, on their photoadaptation rates. In this study we followed the time course of photoadaptation in natural sea ice algal communities from bottom ice and surface ice by measuring their photophysiological response to an artificial shift in the ambient irradiance field. Microalgae from under-ice habitats, were incubated under full sunlight (LL-HL) and microalgae from surface ice habitats were incubated under artificial light to mimic under-ice irradiance (HL-LL). During 3- to 4-day time course studies, opposite shifts in chlorophyll: carbon, α, P^B_m, and I_k were observed, depending on the direction of the irradiance change. First-order rate constants (k) ranged from 0.0067 to 0.29 h^?1 for photosynthetic parameters, although P^B_m did not always show a clear change over time. Rates of photoadaptation for ice algae are comparable to k values reported for temperate phytoplankton, suggesting that sea ice algae may be equally capable of adapting to the light conditions experienced in a vertically mixed water column. This study presents the first evidence that sea ice microalgae are physiologically capable of adapting to a planktonic life and thus could serve as a seed population for polar marine phytoplankton blooms.     

PHOTOINHIBITION OF TEMPERATE LAKE PHYTOPLANKTON BY NEAR-SURFACE IRRADIANCE: EVIDENCE FROM VERTICAL PROFILES AND FIELD EXPERIMENTS1

To evaluate the in situ occurrence of phytoplankton photoinhibition, the light-mediated depression of chlorophyll in vivo fluorescence (IVF) and of the cellular fluorescence capacity (CFC) of phytoplankton was determined in three southeastern United States reservoirs. Vertical profiles of a fluorescence depression index (FDI) and of the CFC for reservoir phytoplankton showed that near-surface photoinhibition of fluorescence properties occurred in association with high surface irradiance and weak vertical mixing of the water column. To characterize the time scales of photochemical and photosynthetic responses to and recovery from exposure to supraoptimal light intensity, phytoplankton IVF responses and ¹⁴C-fixation rates were measured infield experiments. Phytoplankton chlorophyll IVF, CFC, and photosynthetic ¹⁴C fixation were rapidly (20–40 min) depressed when reservoir phytoplankton were exposed to surface irradiances (1700–2000 μE·m^?2·s^?1). Light-mediated increases in the FDI declined rapidly (20–40 min) to pre-exposure levels during a subsequent low-light (75–200 μE·m^?2·s^?1) period, but CFC and ¹⁴C fixation recovered more slowly (>40 min). Exposure of reservoir phytoplankton to a light-intensity gradient revealed both intensity and time thresholds for IVF and CFC depression. Phytoplankton photochemical responses to bright light operate on time scales that, in conjunction with vertical mixing, should limit the occurrence of photoinhibition to extreme irradiance environments. Our results support the hypothesis that the photoinhibition of phytoplankton productivity occurs less commonly than is indicated by fixed-depth incubation measurements.     

ENVIRONMENTAL FACTORS CONTROLLING PHYTOPLANKTON PROCESSES IN THE SOUTHERN OCEAN1

The factors controlling the distribution of phytoplankton stocks, species composition, and their physiological status in the Southern Ocean are reviewed. In the last decade, the key data sources have been observational and experimental. Together, they provide a framework to understand the complex temporal and spatial patterns of environmental control within the distinct basins and ecological provinces. High resolution remotely sensed observational data have overcome the issue of geographical remoteness. Furthermore, by exploiting seasonal and spatial differences in algal distributions, observational data have enabled the cross‐correlation of such trends with patterns in other environmental properties. Perturbation experiments have offered a mechanistic understanding to help interpret observational data by altering environmental properties under carefully controlled conditions. A consistent set of trends, on the modes of environmental control of phytoplankton processes, is now emerging across the different basins and provinces. The key determinants are light, iron, and silicic acid supply (top‐down control was not considered). However, their interplay in time and space (i.e. simultaneous limitation of phytoplankton processes) is less clear, requires further study, and is discussed. Future challenges include the need to understand better the mode(s) of environmental control on key algal functional groups via more taxon‐ and species‐specific studies. The initiation of more time‐series moorings with “smart” bio‐optical and sampling sensors are needed to define the seasonal distributions of algal taxa. Moreover, new perturbation experiments are required to investigate the influence on phytoplankton processes of projected climate‐mediated alteration of mixed layer depth and nutrient supply as widely predicted by modelers.     

PHYTOPLANKTON SELENIUM REQUIREMENTS: THE CASE FOR SPECIES ISOLATED FROM TEMPERATE AND POLAR REGIONS OF THE SOUTHERN HEMISPHERE1

A series of laboratory culture experiments was used to investigate the effect of selenium (Se, 0–10 nM) on the growth, cellular volume, photophysiology, and pigments of two temperate and four polar oceanic phytoplankton species [coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. P. Mohler, cyanobacterium Synechococcus sp., prymnesiophyte Phaeocystis sp., and three diatoms—Fragilariopsis cylindrus (Grunow) Kriegar, Chaetoceros sp., and Thalassiosira antarctica G. Karst.]. Only Synechoccocus sp. and Phaeocystis sp. did not show any requirement for Se. Under Se‐deficient conditions, the growth rate of E. huxleyi was decreased by 1.6‐fold, whereas cellular volume was increased by 1.9‐fold. Se limitation also decreased chl a (2.5‐fold), maximum relative electron transport rate (1.9‐fold), and saturating light intensity (2.8‐fold), suggesting that Se plays a role in photosynthesis or high‐light acclimation. Pigment analysis for Antarctic taxa provided an interesting counterpoint to the physiology of E. huxleyi. For all Se‐dependent Antarctic diatoms, Se limitation decreased growth rate and chl a content, whereas cellular volume was not affected. Pigment analysis revealed that other pigments were affected under Se deficiency. Photoprotective pigments increased by 1.4‐fold, while diadinoxanthin:diatoxanthin ratios decreased by 1.5‐ to 4.9‐fold under Se limitation, supporting a role for Se in photoprotection. Our results demonstrate an Se growth requirement for polar diatoms and indicate that Se could play a role in the biogeochemical cycles of other nutrients, such as silicic acid in the Southern Ocean. Se measurements made during the austral summer in the Southern Ocean and Se biological requirement were used to discuss possible Se limitation in phytoplankton from contrasting oceanographic regions.     

IRON-MEDIATED CHANGES IN THE GROWTH OF LAKE ERIE PHYTOPLANKTON AND AXENIC ALGAL CULTURES1

The effect of iron enrichment on algal growth and photosynthesis was investigated using natural assemblages of Lake Erie phytoplankton and axenic cultures of Anabaena, Scenedesmus and Selenastrum. Cell yield and photosynthesis were frequently inhibited in the presence of unchelated iron over the range of 3.6 to 53.7 μM iron as FeCl₃. In lake water and in a defined medium with low nutrient concentrations, the degree of inhibition by iron could be reduced by chelating the iron with EDTA or by enriching the cultures with phosphorus. Chemical analyses revealed that the EDTA efectively reduced the ability of the ferric iron to remove soluble phosphorus from the media. EDTA was also observed to reduce rather than enhance iron uptake by axenic cultures of A. flos-aquae. These data support the hypothesis that additions of EDTA to low-nutrient media may serve to stimulate algal growth in the presence of iron by preventing the iron from altering extracellular concentrations of soluble ions essential for algal metabolism. In medium with high nutrient concentrations, the soluble phosphorus concentration was not appreciably altered by either EDTA-chelated or unchelated iron enrichment (0.9 to 53.7 μM). Instead, the observed enhancement of cell yield by EDTA-chelated iron in nutrient-rich media appeared to be due to the direct effect of iron on intracellular metabolic processes.     

IDENTIFICATION OF A PSYCHROPHILIC GREEN ALGA FROM LAKE BONNEY ANTARCTICA: CHLAMYDOMONAS RAUDENSIS ETTL. (UWO 241) CHLOROPHYCEAE1

An unusual psychrophilic green alga was isolated from the deepest portion of the photic zone (<0.1% of incident PAR) at a depth of 17 m in the permanently ice‐covered lake, Lake Bonney, Antarctica. Here we identify and report the first detailed morphological and molecular examination of this Antarctic green alga, which we refer to as strain UWO 241. To determine the taxonomic identity, UWO 241 was examined using LM and TEM and partial sequences of the small subunit (SSU), internal transcribed spacer (ITS) 1 and ITS2 regions (including the 5.8S) of the ribosomal operon. These data were compared with those of previously described taxa. We identified UWO 241 as a strain of Chlamydomonas raudensis Ettl (SAG 49.72). Chlamydomonas raudensis is closely related to C. noctigama Korshikov (UTEX 2289) as well as foraminifer symbionts such as C. hedleyi Lee, Crockett, Hagen et Stone (ATCC 50216). In addition, its morphology, pigment complement, and phototactic response to temperature are reported. Chlamydomonas raudensis (UWO 241) contains relatively high levels of lutein and low chl a/b ratios (1.6±0.15), and the phototactic response was temperature dependent. The Antarctic isolate (UWO 241) included the typical photosynthetic pigments found in all chl a/b containing green algae. It possesses a small eyespot and, interestingly, was positively phototactic only at higher nonpermissive growth temperatures. Comparison of SSU and ITS rDNA sequences confirms the identification of the strain UWO 241 as C. raudensis Ettl and contradicts the previous designation as C. subcaudata Wille.     

DISTRIBUTION OF CARBON AMONG PHOTOSYNTHETIC END-PRODUCTS IN PHYTOPLANKTON OF THE EASTERN CANADIAN ARCTIC1

The distribution of ¹⁴C among photosynthetic end-products was examined in eastern Canadian arctic phytoplankton, with particular emphasis on the synthesis of lipids. The pattern of ¹⁴C distribution for phytoplankton at each of three depths was generally similar among populations from 12 stations. About 18% of the total ¹⁴C fixed was incorporated into lipids. At one station, phytoplankton were experimentally subjected to temperature and light conditions different from those in situ: lipid-¹⁴C did not exceed 30% of total ¹⁴C fixed within the temperature range -1.0 to 6.0° C and irradiance range 1 to 700 W · m^?2. It is suggested that low temperatures and low light intensities, even when, maintained for prolonged periods, are not fully sufficient conditions for eliciting high relative rates of ¹⁴C incorporation into lipids. It is possible that differences in species composition may be a factor accounting for different patterns of ¹⁴C distribution between north and south polar phytoplankton under apparently similar environmental conditions.     

PHOTOPHYSIOLOGY IN TWO SOUTHERN OCEAN PHYTOPLANKTON TAXA: PHOTOSYNTHESIS OF PHAEOCYSTIS ANTARCTICA (PRYMNESIOPHYCEAE) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE) UNDER SIMULATED MIXED‐LAYER IRRADIANCE1

In the Ross Sea, the prymnesiophyte Phaeocystis antarctica G. Karst. dominates deeply mixed water columns, while diatoms dominate shallower mixed layers. Understanding what controls the dynamics of these two phytoplankton taxa is essential because they dominate virtually all coastal polar waters, have different nutrient utilization characteristics, and support dissimilar food webs. We cultured two strains of P. antarctica and one strain of the diatom Fragilariopsis cylindrus (Grunow) Willi Krieg under three dynamic irradiance regimes that simulated different mixed‐layer depths and measured their photosynthetic characteristics, cellular pigment concentrations, and cellular carbon and nitrogen content. In both species, chl a–normalized maximum carbon uptake rate (P_m^* ) and specific growth rate were highest in the deeply mixed treatment that had a dark period. In all irradiance treatments, both (P_m^* ) and photosynthetic efficiency (α*) were greater for the two P. antarctica strains than for the F. cylindrus strain. In contrast, P. antarctica strains were more susceptible to photoinhibition (β*) than the F. cylindrus strain. When photosynthetic rates of each phytoplankton taxon were normalized by cellular particulate organic carbon (POC), the difference in the maximal photosynthetic rate () was generally reduced. In the dynamic irradiance treatment that simulated the shallowest mixed‐layer irradiance, all three phytoplankton had similar ; however, the diatom had a 2‐fold higher POC‐normalized photosynthetic efficiency (α^C). Finally, we performed calculations using the measured POC‐normalized photosynthetic parameters to show that α^C and can play a greater role than β^C in determining the competitive outcome between P. antarctica and F. cylindrus in both shallow and deep mixed‐layer environments of the Ross Sea.     

以浮游植物评价巢湖水质污染及富营养化

对巢湖浮游植物在22个样点,四季采样,进行了种类组成、种群数量、优势种、污染指示种,多样性指数及硅藻指数等群落生态学的初步研究。选用其中种群数量、优势种、污染指示种、硅藻指数对巢湖水质污染及营养水平进行了总评与分区评价。就巢湖总体而论,藻量年均值达111×10~6个/升(细胞数,或个体数为14.7×10~6个/升),硅藻指数96.7,群落组成种类中,污染指示种占64.2％,除冬季外,各样点多以微囊藻、鱼腥藻为主的蓝藻占优势,其分布频度年平均达98.8％,多度年平均达79.7％,而在夏、秋季,两者则分别为100和99.7％,表明巢湖已受到中等程度的污染,属蓝藻型富营养湖泊;与过去巢湖浮游植物的调查资料对比,表明最近几年来,人为富营养化进程在加速。     

DIEL OSCILLATIONS IN THE PHOTOSYNTHESIS-IRRADIANCE RELATIONSHIP OF A PLANKTONIC MARINE DIATOM1

The amplitude of diel oscillations in photosynthesis as a function of irradiance varied with the growth phase in a marine phytoplankton species. The common centric diatom (Bacillariophyta), Ditylum brightwellii (West) Grun., showed strong periodicity in the photosynthesis-irradiance (P-I) relationship, which damped progressively from early to late exponential and stationary phase. These findings suggest that short-term temporal characteristics of phytoplankton production depend on factors which affect growth, and that the amplitude is most enhanced at maximal growth rates likely to be encountered in the natural environment.     

EFFECTS OF TEMPERATURE AND IRRADIANCE ON THE SEASONAL VARIATION OF A SPIROGYRA (CHLOROPHYTA) POPULATION IN A MIDWESTERN LAKE (U.S.A.)

Although Spirogyra Link (1820) is a common mat‐forming filamentous alga in fresh waters, little is known of its ecology. A 2‐year field study in Surrey Lake, Indiana, showed that it grew primarily in the spring of each year. The population consisted of four morphologically distinct filamentous forms, each exhibiting its own seasonal distribution. A 45‐μm‐wide filament was present from February to late April or early May, a 70‐μm‐wide form was present from late April to mid‐June, a 100‐μm‐wide form was present from February to mid‐June, and a 130‐μm‐wide form appeared only in February of 1 of 2 study years. The 70‐ and 100‐μm‐wide forms contributed to the peak amount of biomass observed in late May and early June. Multiple regression analysis indicated that the presence of the 45‐, 70‐, and 100‐μm‐wide forms was negatively correlated with temperature. Presence of the 130‐μm‐wide form was negatively correlated with irradiance. Isolates of these filament forms were exposed to temperature (15, 25, and 35° C)/irradiance (0, 60, 200, 400, 900, and 1500 μmol·m^?2·s^?1) combinations in the laboratory. Growth rates of the 45‐μm‐wide form were negative at all irradiances at 35° C, suggesting that this form is susceptible to high water temperatures. However, growth rates of the other forms did not vary at the different temperatures or at irradiances of 60 μmol·m^?2·s^?1 or above. Net photosynthesis was negative at 35° C and 1500 μmol·m^?2·s^?1 for the 100‐ and 130‐μm‐wide forms but positive for the 70‐μm‐wide form. All forms lost mat cohesiveness in the dark, and the 100‐ and 130‐μm‐wide forms lost mat cohesiveness under high irradiances and temperature. Thus, the morphological forms differed in their responses to irradiance and temperature. We hypothesize that the rapid disappearance of Spirogyra populations in the field is due to loss of mat cohesiveness under conditions of reduced net photosynthesis, for example, at no to low light for all forms or at high light and high temperatures for the 100‐ and 130‐μm‐wide forms. Low light conditions can occur in the interior of mats as they grow and thicken or under shade produced by other algae.     

IN SEARCH OF A PHYSIOLOGICAL BASIS FOR COVARIATIONS IN LIGHT‐LIMITED AND LIGHT‐SATURATED PHOTOSYNTHESIS1

The photosynthesis‐irradiance (PE) relationship links indices of phytoplankton biomass (e.g. chl) to rates of primary production. The PE curve can be characterized by two variables: the light‐limited slope (α^b) and the light‐saturated rate (P^b_max) of photosynthesis. Variability in PE curves can be separated into two categories: that associated with changes in the light saturation index, E_k (=P^b_max/α^b) and that associated with parallel changes in α^band P^b_max (i.e. no change in E_k). The former group we refer to as “E_k‐dependent” variability, and it results predominantly from photoacclimation (i.e. physiological adjustments in response to changing light). The latter group we refer to as “E_k‐independent” variability, and its physiological basis is unknown. Here, we provide the first review of the sporadic field and laboratory reports of E_k‐independent variability, and then from a stepwise analysis of potential mechanisms we propose that this important yet largely neglected phenomenon results from growth rate–dependent variability in the metabolic processing of photosynthetically generated reductants (and generally not from changes in the oxygen‐evolving PSII complexes). Specifically, we suggest that as growth rates decrease (e.g. due to nutrient stress), reductants are increasingly used for simple ATP generation through a fast (<1s) respiratory pathway that skips the carbon reduction cycle altogether and is undetected by standard PE methodologies. The proposed mechanism is consistent with the field and laboratory data and involves a simple new “twist” on established metabolic pathways. Our conclusions emphasize that simple reductants, not reduced carbon compounds, are the central currency of photoautotrophs.     

IN SITU ALLELOPATHIC POTENTIAL OF MYRIOPHYLLUM VERTICILLATUM (HALORAGACEAE) AGAINST SELECTED PHYTOPLANKTON SPECIES1

The potential allelopathic impact of Myriophyllum verticillatum L. under in situ conditions was determined in a series of field and laboratory experiments. Coexistence experiments were performed in a lake dominated by M. verticillatum (Van Goor) Meffert where we exposed three unialgal phytoplankton cultures in dialysis tubes to macrophyte exudates regularly during the vegetated period. Plant content and exudation of polyphenolic compounds were determined, and the inhibitory activity of polyphenol‐containing extracts was tested in bioassays with cyanobacteria. To account for possible resource interference, we monitored growth and photosynthesis of phosphorus‐limited and unlimited cyanobacterium Limnothrix redekei in dialysis tubes exposed to M. verticillatum in aquaria. A high allelopathic potential of M. verticillatum was concluded from high tissue concentrations of total phenolic compounds of 6%–12% of dry matter, the demonstrated release of bioactive polyphenols into the surrounding medium, and the inhibition of cyanobacteria by extracts. We could not unambiguously demonstrate the exudation of polyphenols by M. verticillatum in situ due to interference with allochthonous humic compounds. In laboratory experiments, L. redekei exhibited significantly reduced maximum relative electron transport rates when co‐cultivated in dialysis tubes with M. verticillatum. The field dialysis tube experiment confirmed this result, accompanied by a decline of chl a and PSII activity for L. redekei and the diatom Stephanodiscus minutulus (Kütz.) Greve et Möller, but not for the green alga Scenedesmus armatus Chodat in August. At other times, either no effects or stimulatory effects were observed with all species. Nutrient limitation of the target species may have masked allelopathic effects, and M. verticillatum may have enhanced phytoplankton growth due to phosphorus leakage.     

STRATEGIES AND RATES OF PHOTOACCLIMATION IN TWO MAJOR SOUTHERN OCEAN PHYTOPLANKTON TAXA: PHAEOCYSTIS ANTARCTICA (HAPTOPHYTA) AND FRAGILARIOPSIS CYLINDRUS (BACILLARIOPHYCEAE)1

We investigated rates and mechanisms of photoacclimation in cultures of Phaeocystis antarctica G. Karst. and Fragilariopsis cylindrus (Grunow) Willi Krieg, phytoplankton taxa that each dominate distinct areas of the Ross Sea, Antarctica. Both P. antarctica and F. cylindrus acclimated to increases in irradiance by reducing the effective size of the pigment antenna (σPSII) via xanthophyll‐cycle activity and reductions in chl. While enhanced photoprotection facilitated increases in specific growth rate and eventually led to higher light‐saturated photosynthetic rates (P^cell_m) in P. antarctica, increases in those variables were much smaller in F. cylindrus. In response to a lower irradiance, relaxation of xanthophyll‐cycle activity led to an increase in σPSII in both taxa, which occurred much more slowly in F. cylindrus. A surprising increase in specific growth rate over the first 36 h of acclimation in P. antarctica may have facilitated the significant reductions in P^cell_m observed in that taxon. In general, P. antarctica acclimated more quickly to changes in irradiance than F. cylindrus, exhibited a wider range in photosynthetic rates, but was more susceptible to photoinhibition. This acclimation strategy is consistent with growth in deeply mixed water columns with variations in irradiance that allow time for repair. In contrast, the slower acclimation rates, extensive photoprotection, and low photoinhibition exhibited by F. cylindrus suggest that it does not require the same period for repair as P. antarctica and is best suited for growth in habitats with relatively uniform irradiance, such as shallow mixed layers or sea ice.     

UV RADIATION,PHOSPHORUS, AND THEIR COMBINED EFFECTS ON THE TAXONOMIC COMPOSITION OF PHYTOPLANKTON IN A BOREAL LAKE1

We examined how UV radiation and phosphorus (P) affect the taxonomic composition, abundance, and biomass of phytoplankton in an oligotrophic boreal lake. We exposed phytoplankton to three different solar radiation regimes (PAR + UV‐A radiation [UVAR]+ UV‐B radiation [UVBR], PAR + UVAR, and PAR only) and to five levels of P. The biomass of small chrysophytes was reduced by 350% after exposure to PAR + UVAR + UVBR compared with PAR only. No other taxa were found to be negatively affected by exposure to UVBR. Several taxa (e.g. Chry‐ sochromulina laurentiana Kling) were sensitive to UVAR, whereas others (e.g. Tabellaria flocculosa (Roth) Kutzing) were not affected by UV radiation exposure. Principal components analysis ordination separated phytoplankton that were negatively affected by UV radiation and/or positively affected by P treatments (e.g. small chrysophytes, Cryptomonas rostratiformis, T. flocculosa) from those that generally were unaffected by either treatment (e.g. desmids, some Cyanobacteria). Richness, Shannon‐Weaver diversity, and evenness were significantly higher in phytoplankton communities shielded from UVAR and UVBR. The relationship between diversity and richness was positive in all phytoplankton samples except in those exposed to UVBR. Thus, UVBR‐exposed phytoplankton communities were dominated by a few species even though the number of taxa remained relatively unchanged. Consequently, alterations in the UV environments of lakes resulting from climate warming (e.g. drought) and land‐use change (e.g. increased P export) will likely promote shifts in the community composition of lake phytoplankton.     

A STUDY OF IN VITRO ELECTRON TRANSPORT ACTIVITY IN MARINE PHYTOPLANKTON AS A FUNCTION OF TEMPERATURE1

The in vitro temperature inactivation of the electron transport system (ETS) activity was measured for 11 phytoplankton species. The average inactivation temperature is between 18–23 C, but exceptions with inactivation temperatures as high as 43 C were noted for a tide pool phytoplankton, Dunaliella tertiolecta Butcher. Despite differences in temperatures at which the ETS activity begins to decrease, experimental values for the energies of activation (E) of electron transport are very similar and average ca. 12 kcal/mol. The results show the necessity of applying temperature corrections to in vitro ETS activity measurements when oceanic in situ oxygen consumption is computed from ETS activity measurements. The progress of thermal inactivation of the ETS activity in phytoplankton species studied yields biphasic curves. The biphasic nature of the curves is expressed when using both relatively high and low temperature of inactivation and in more or less heat sensitive species. These curves are described in terms of microsomal and mitochondrial substrate dependence. Based on the obtained temperature response of the ETS activity, the adaption of phytoplankton species to growth temperature is discussed.     

THE BOTTOM-ICE MICROALGAL COMMUNITY FROM ANNUAL ICE IN THE INSHORE WATERS OF EAST ANTARCTICA1

The structure, productivity and heterotrophic potential of an extensive microalgal community growing on the underside of sea ice near the Australian Antarctic Station of Casey, are described. Underwater observations made near the Australian Antarctic stations of Davis and Mawson are also reported. This community develops during September, is largely suspended from the bottom surface of annual sea ice and often extends into the underlying water column as conspicuous strands up to 15 cm long. The algal community structure in the strands is dominated by an unidentified tube diatom belonging to the Amphipleura/Berkeleya group and chains of a species of Entomoneis cf. Amphiprora paludosa var. hyperborea (Grunow) Cleve. Unlike previously described bottom ice environments, a brash ice layer under the hard sea ice is absent. Living cells, predominantly Nitzschia frigida Grunow, also occur in microbrine channels in the bottom 3 cm of the ice. Maximal primary production rates of 81 μg C · L^-1· h^-1 occurred during November, then began declining near the end of December. Minimal rates (2.8 μg C · L^-1· h^-1) were reached in mid-January and coincided with changes in the physical structure of the sea ice and in the stability of the water column. An abundant epibacterial community associated with the microalgal strands assimilated ³H-labelled amino acids suggesting significant heterotrophic recycling of dissolved organic matter. Turnover times of assimilated amino acids in the bottom ice community averaged 55 h during November while negligible turnover of these substrates occurred in the water column 1.5 m below the ice. These bottom ice communities have higher primary productivity than typical brash ice communities; they are also accessible to marine herbivores and so may be more important to the Antarctic marine food chain than previously supposed.