TEMPERATURE,PHOTOPERIOD AND LIGHT INTERACTIONS ON GROWTH AND FERTILITY OF GLOSSOPHORA KUNTHII (PHAEOPHYTA,DICTYOTALES) FROM CENTRAL CHILE1

Excised ligulae of Glossophora kunthii from central Chile were cultured of temperatures of 5–25° C, photoperiods of 16:8 and 8:16 h LD cycles, with photon irradiances of 10 and 50 μmol · m^?2· s^?1. Growth of the ligulae, number of fertile ligulae and number of tetrasporangia forming on the ligulae were assessed. Ligulae tolerated temperatures between 10 and 23°C. Temperature interacted with daylength and photon dose, determining quantitative responses in the growth and fertility of ligulae. Growth was least at 8:16 h LD and was not affected significantly by temperature. It was greatest at 16:8 h LD, 50 μmol · m^?2· s^?1 and increased with temperature up to 20°C. Percentage of fertile ligulae and number of tetrasporangia increased with temperature at the 8:16 h LD cycle, reaching a maximum at 20°C. Fertility was low at 16:8 h LD, except at 20° C (and low photon dose) suggesting that reproduction at 20° C is independent of daylength in this species. Ligulae grew larger at the long-day photoperiods and the proportions of fertile ligulae were higher at the short-day photoperiods, irrespective of the total photon dose received. These results suggest that some aspects of growth and fertility are controlled by photoperiod.     

CRITICAL IRRADIANCE LEVELS AND THE INTERACTIVE EFFECTS OF QUANTUM IRRADIANCE AND DOSE ON GAMETOGENESIS IN THE GIANT KELP,MACROCYSTIS PYRIFERA1

Gametophytes of Macrocystis pyrifera (L.) C. Ag. were cultured under a series of quantum irradiances in three photoperiod regimes. The quantum irradiances in each photoperiod were adjusted to provide equal daily irradiation dosages between photoperiods which allowed a critical examination of the interactions between quantum irradiance and quantum dose in determining gametophyte fertility. The lowest quantum irradiance which stimulated gametogenesis in more than 50% of the female gametophytes was 5 μE·m^?2·s^?1. The saturating irradiance was ca. 10 μE·m^?2·s^?1 at photoperiods of 12 h or greater. In terms of daily quantum dose, the lowest dose at which greater than 50% gametogenesis occurred was 0.2 E·m^?2·d^?1. However, this critical quantum dose was higher (0.4 E·m^?2·d^?1) when instantaneous irradiances were less than 5 μE·m^?2·s^?1. The saturation quantum dose was also affected by the rate at which the quantum dose was received and varied from 0.4 to 0.8 E·m^?2·d^?1. Gametophytes in all three photoperiods reached 100% fertility at quantum irradiances above 5 μE·m^?2·s^?1. Photoperiod effects were small and could be accounted for by quantum dosage effects.     

EFFECTS OF SUBSTRATE RELIEF ON THE DISTRIBUTION OF PERIPHYTON IN LABORATORY STREAMS. II. INTERACTIONS WITH IRRADIANCE1

Laboratory streams were used in a 42-day experiment designed to investigate how the spatial and temporal distribution of lotic periphyton created by current flow over cobble-size substrates is a affected by irradiance. The streams contained 22.5 × 22.5 × 4 cm substrate blocks and were exposed to either 385, 90 or 20 μE·m^?2·s^?1. We monitored periphyton succession in fast current regimes on top of blocks and in slower current regimes on surfaces recessed between blocks. The absolute differences in AFDW algal biomass between top and recessed substrates were significantly affected by irradiance and time. At the end of the experiment, biomass in streams exposed to 385 μE·m^?2·s^?1. was approximately 2 and 8 times greater than in streams exposed to 90 and 20 μE·m^?2·s^?1, respectively. Differences in biomass were greater between irradiance levels than between top and recessed substrates within an irradiance level. Irradiance also had a greater effect than current regime on the taxonomic composition of assemblages. Oscillatoria agardhii Gomont and Navicula minima Grun. characterized assemblages at 20 μE·m^?2·s^?1, whereas Fragilaria vaucheriae (Kütz.), Nitzschia oregona Sov., Navicula arvensis Hust. and Stigeoclonium tenue (Ag.) Kütz. were more abundant at the two higher irradiances. Detrended correspondence analysis indicated that the rate of succession was relatively high for assemblages at high irradiance and in the slow current regimes between blocks. The results suggested that in natural streams, periphyton patches produced by large differences in irradiance should have a greater effect on periphyton heterogeneity than substrate-induced patches. Moreover, the heterogeneity of algal patches produced by hydrologic differences over a substrate is constrained by irradiance level.     

A CIRCADIAN RHYTHM IN THE LONG-DAY PHOTOPERIODIC INDUCTION OF ERECT AXIS DEVELOPMENT IN THE MARINE RED ALGA NEMALION HELMINTHOIDES1,2

Two isolates of Nemalion helminthoides from the west coast of Ireland showed a heteromorphic life history in which tetraspores formed under short-day conditions on uniaxial, prostrate tetrasporophytes gave rise to uniaxial, prostrate growths similar in morphology to the tetrasporophytes. The induction of multiaxial, erect axes on tetraspore-derived plants was a long-day photoperiodic response. At 11°C, 16:8 h LD, nitrate concentration in the enriched seawater medium had little effect on the numbers of erect exes formed. Induction of erect axes occurred only in daylengths ≥ 12 h, mainly between 7–13° C, and the critical daylength, which generally lies between 14 and 16 h, changed with temperature. Night-breaks (NB) of 1 h light in the middle of 16 h night were ineffective in the promotion of erect axis formation, and day-breaks (DB) of 1 h darkness in the middle of a 16 h day did not inhibit axis formation. Neither continuous light nor NB of 1–1.5 h given at various times during the dark period of an 8:16 h LD cycle promoted the formation of erect axes. At 9° C, equivalent photon exposures (0.69 and 0.34 mol·m^?2) at two different instantaneous photon flux densities resulted in erect axis formation only in the 14 h, 16 h and DB regimes. Photoregimes of 16 h light in combination with dark cycles of various lengths resulted in the maximal promotion of the formation of erect axes in diurnal (22–24 h) and bi-diurnal (44–48 h) cycles, a diminishing response in non-diurnal cycles greater than 24 and 48 h, and a minimal response at 32 h. These data show that the formation of erect axes is a long-day photoperiodic response and provide further evidence for a connection between endogenous circadian rhythms and long-day photoperiodic responses.     

EFFECT OF GROWTH AND LIGHT/DARK CYCLES ON DIATOM LIPID CONTENT AND COMPOSITION1

Total extractable lipid (TEL) and lipid composition were studied throughout the growth cycle in three freshwater diatoms-Cyclotella meneghiniana Kütz., Melosira varians C. A. Ag., and Stephanodiscus binderanus (Kütz.) Krieg under three light regimes (16:8 h LD, 20:4 h LD, and 12:12 h LD) at 20°C. Two of the diatoms demonstrated strong daylength preferences for growth; C. meneghiniana grew best under long-day (20: 4-h LD) conditions, whereas S. binderanus grew best under short-day (12:12-h LD) conditions. The lipid composition of the diatoms was similar throughout the growth cycle. Aged (2-month-old) cells were high in total lipid and triacylglycerols. Before the onset of active growth and during the early part of active growth, there was a reduction in total neutral lipids, primarily triacylglycerols, and an increase in all polar lipids, including chlorophyll a, acetone-mobile polar lipids, and phospholipids. While cell numbers were still increasing, triacylglycerols increased and polar lipids decreased to levels near those found in aged cultures, Results suggest that increased triacylglycerol content of freshwater diatoms is not necessarily indicative of senescent populations.     

Growth rates of phytoplankton under fluctuating light

• 1 The effect of light fluctuations on the growth rates of four species of freshwater phytoplankton was investigated. Experimental light regimes included constant irradiance and fluctuations of a step function form, with equal proportion of high (maximum of 240 µmol photons m^‐2 s^‐1) and low light (minimum of 5 µmol photons m^‐2 s^‐1) (or dark) in a period. Fluctuations of 1, 8 and 24‐h periods were imposed over several average irradiances (25, 50, 100 and 120 µmol photons m^‐2 s^‐1).
• 2 Growth rate responses to fluctuations were species‐specific and depended on both the average irradiance and the period of fluctuations. Fluctuations at low average irradiances slightly increased growth rate of the diatom Nitzschia sp. and depressed growth of the cyanobacterium Phormidium luridum and the green alga Sphaerocystis schroeteri compared to a constant irradiance.
• 3 Fluctuations at higher average irradiance did not have a significant effect on the growth rates of Nitzschia sp. and Sphaerocystis schroeteri (fluctuations around saturating irradiances) and slightly increased the growth rates of the cyanobacteria Anabaena flos‐aquae and Phormidium luridum (when irradiance fluctuated between limiting and inhibiting levels).
• 4 In general, the effect of fluctuations tended to be greater when irradiance fluctuated between limiting and saturating or inhibiting levels of a species growth‐irradiance curve compared to fluctuations within a single region of the curve.
• 5 The growth rates of species under fluctuating light could not always be predicted from their growth‐irradiance curves obtained under constant irradiance. When fluctuations occur between limiting and saturating or inhibiting irradiances for the alga and when the period of fluctuations is long (greater than 8 h), steady‐state growth‐irradiance curves may be insufficient to predict growth rates adequately. Consequently, additional data on physiological acclimation, such as changes in photosynthetic parameters, may be required for predictions under non‐constant light supply in comparison to constant conditions.

     

Optimization of photosynthesis,growth, and biochemical composition of the microalga <Emphasis Type="Italic">Rhodomonas salina</Emphasis>—an established diet for live feed copepods in aquaculture

The cryptophyte Rhodomonas salina is widely used as feed for copepod cultures. However, culturing conditions to obtain high-quality algae have not yet been efficiently optimized. Therefore, we aimed to develop a cultivation protocol for R. salina to optimize its nutritional value and provide technical recommendations for later large-scale production in algal photobioreactors. We studied photosynthesis, growth, pigments, fatty acid (FA) and free amino acid (FAA) composition of R. salina cultured at different irradiances (10–300 μmol photons m^?2 s^?1) and nutrient availability (deficiency and excess). The optimal range of irradiance for photosynthesis and growth was 60–100 μmol photons m^?2 s^?1. The content of chlorophylls a and c decreased with increasing irradiance while phycoerythrin peaked at irradiances of 40–100 μmol photons m^?2 s^?1. The total FA content was maximal at optimal irradiances for growth, especially under nutrient deficiency. However, highly unsaturated fatty acids, desired components for copepods, were higher under nutrient excess. The total FAA content was highest at limiting irradiances (10–40 μmol photons m^?2 s^?1) but a better composition with a higher fraction of essential amino acids was obtained at saturated irradiances (60–140 μmol photons m^?2 s^?1). These results demonstrate that quality and quantity of FA and FAA of R. salina can be optimized by manipulating the irradiance and nutrient conditions. We suggest that R. salina should be cultivated in a range of irradiance 60–100 μmol photons m^?2 s^?1 and nutrient excess to obtain algae with high production and a balanced biochemical composition as feed for copepods.     

PHOTOSYNTHESIS AND CELL DIVISION BY ANTARCTIC MICROALGAE: COMPARISON OF BENTHIC,PLANKTONIC AND ICE ALGAE1

Irradiance-dependent rates of photosynthesis and cell division of six species of microalgae isolated from the benthos, plankton and sea ice microbial community in McMurdo Sound, Antarctica were compared. Microalgae isolated from different photic environments had distinct photosynthetic and growth characteristics. For benthic and ice algae, photosynthesis saturated at 6 to 20 μE.m^?2.s^?1 and was photoinhibited at 10 to 80 μE.m^?2.s^?1 while for the planktonic algae, saturation irradiances were up to 13 times higher and photoinhibition was not detected. The slope of the light-limited portion of the P-I relationship was up to 50 times greater for the benthic algae than for either the ice or planktonic algae suggesting that benthic algae used the low irradiances more efficiently for carbon uptake. Cell division was dependent on the incubation irradiance for all but one microalga examined. The dependence of division rates on irradiance was however much smaller than for carbon uptake, suggesting that cell division buffers the influence of short term variations of irradiance on cellular metabolism.     

Synechococcus sp. (PTCC 6021) cultivation under different light irradiances—Modeling of growth rate-light response

Synechococcus sp. (PTCC 6021), a cyanobacterium species, was cultivated in an internally illuminated photobioreactor. The reactor was designed to achieve a monoseptic cultivation of the species. The goal was to study the growth–irradiance behavior of Synechococcus sp. (PTCC 6021). To accomplish this, different initial light irradiances were implemented inside the photobioreactor and the growth of the cells was monitored. It was observed that cell growth increased with higher light intensity until the photoinhibition occurrence at light irradiance higher than 250?μE?m^?2?s^?1. The maximum OD₆₀₀, maximum growth rate, and biomass productivity increased, and hence the extinction coefficient decreased, with the increase in light irradiance before photoinhibition. The maximum optical density (OD₆₀₀) of 5.91 was obtained with irradiance below 250?μE?m^?2?s^?1 during a growth period of 80 days. The modified Monod function could model the growth–irradiance of cells with satisfactory agreement with the experimental data. The comparison of growth–irradiance of the studied species with other photosynthetic organisms showed the same trend as for cyanobacteria with photoinhibition.     

Photon irradiance required to support optimal growth and interrelations between irradiance and pigment composition in the green alga Haematococcus pluvialis

The aim of the work was to find the optimal photon irradiance for the growth of green cells of Haematococcus pluvialis and to study the interrelations between changes in photochemical parameters and pigment composition in cells exposed to photon irradiances between 50 and 600?µmol?m^?2?s^?1 and a light:dark cycle of 12:12?h. Productivity of cultures increased with irradiance. However, the rate of increase was higher in the range 50–200?µmol?^?2?s^?1. The carotenoid content increased with increasing irradiance, while the chlorophyll content decreased. The maximum quantum yield of PSII (F_v/F_m) gradually declined from 0.76 at the lowest irradiance of 50?µmol?^?2?s^?1 to 0.66 at 600?µmol?^?2?s^?1. Photosynthetic activity showed a drop at the end of the light period, but recovered fully during the following dark phase. A steep increase in non-photochemical quenching was observed when cultures were grown at irradiances above 200?µmol?^?2?s^?1. A sharp increase in the content of secondary carotenoids also occurred above 200?µmol?m^?2?s^?1. According to our results, with H. pluvialis green cells grown in a 5-cm light path device, 200?µmol?^?2?s^?1 was optimal for growth, and represented a threshold above which important changes in both photochemical parameters and pigment composition occurred.     

RESPONSE OF PROROCENTRUM MARIAE-LEBOURIAE (DINOPHYCEAE) TO LIGHT OF DIFFERENT SPECTRAL QUALITIES AND IRRADIANCES: GROWTH AND PIGMENTATION1

Growth and pigment concentrations of the, estuarine dinoflagellate, Prorocentrum mariae-lebouriae (Parke and Ballantine) comb. nov., were measured in cultures grown in white, blue, green and red radiation at three different irradiances. White irradiances (400–800 nm) were 13.4, 4.0 and 1.8 W · m^?2 with photon flux densities of 58.7 ± 3.5, 17.4 ± 0.6 and 7.8 ± 0.3 μM quanta · m^?2· s^?1, respectively. All other spectral qualities had the same photon flux densities. Concentrations of chlorophyll a and chlorophyll c were inversely related to irradiance. A decrease of 7- to 8-fold in photon flux density resulted in a 2-fold increase in chlorophyll a and c and a 1.6- to 2.4-fold increase in both peridinin and total carotenoid concentrations. Cells grown in green light contained 22 to 32% more peridinin per cell and exhibited 10 to 16% higher peridinin to chlorophyll a ratios than cells grown in white light. Growth decreased as a function of irradiance in white, green and red light grown cells but was the same at all blue light irradiances. Maximum growth rates occurred at 8 μM quanta · m^?2· s^?1 in blue light, while in red and white light maximum growth rates occurred at considerably higher photon flux densities (24 to 32 μM quanta · m^?2· s^?1). The fastest growth rates occurred in blue and red radiation. White radiation producing maximum growth was only as effective as red and blue light when the photon flux density in either the red or blue portion of the white light spectrum was equivalent to that of a red or of blue light treatment which produced maximum growth rates. These differences in growth and pigmentation indicate that P. mariae-lebouriae responds to the spectral quality under which it is grown.     

RESPONSE TO PHOTOPERIOD AND TEMPERATURE BY SPARTINA ALTERNIFLORA (POACEAE) FROM NORTH CAROLINA AND SPARTINA FOLIOSA FROM CALIFORNIA

Seedlings of Spartina alterniflora Loisel. from Oregon Inlet, North Carolina were generally taller and produced significantly more culms and total dry weight than those of S. foliosa Trin. from two California populations (Alameda Beach and Marin County) in eight photoperiod-temperature treatments over a 17-wk period. Seedlings of S. alterniflora produced maximal biomass in 30–26 C whereas those of S. foliosa produced maximal biomass in 22–18 C, both under long-day conditions. The average photosynthesis rate for S. alterniflora (3.0 mg C g dry wt^−- hr^−-1) was 1.6 times higher than that for S. foliosa (1.9 mg C g dry wt^−-1 hr^−-1), but the dark respiration rates (0.3 and 0.4 mg C g dry wt^−-1 hr^−-1, respectively) were not significantly different. The proportion of rhizomes was greater under short than long-day conditions for both species in most temperature regimes. The average shoot soluble carbohydrate and starch concentrations were higher for S. foliosa (8.4 and 0.9%, respectively) than those for S. alterniflora (6.0 and 0.4%, respectively). The average rhizome soluble carbohydrate concentration (18%) for S. alterniflora under long-day conditions was significantly lower than that under short-day conditions (28%) and also lower than those for S. foliosa under both long (26%) and short-day (25%) conditions. Rhizome starch concentrations of S. alterniflora were significantly higher in the short than in most long-day temperatures and were generally higher for S. alterniflora than for S. foliosa under short-day conditions. The root starch concentration of S. alterniflora under short-day conditions (1.3%) was higher than that under long-day conditions (0.2%) and also higher than those of S. foliosa under both long (0.2%) and short-day (0.7%) conditions. The two species exhibited similar patterns of carbohydrate storage in belowground organs, similar flower initiation processes not under strict photoperiod control, and similar respiration rates, but significantly different photosynthetic rates and growth responses with S. alterniflora having the potential to be the more productive species.     

Understanding the intertidal zonation of two estuarine red macroalgae from ex situ photoacclimative responses

This study investigated the influence of light on the distributional limits of Bostrychia scorpioides and Catenella caespitosa within a low amplitude intertidal zonation in the Palmones estuary, Spain. Changes in photosynthesis- and growth-irradiance curves and pigment content were examined ex situ at irradiances found in their natural habitat (10–230 µmol photons m^?2 s^?1). The highest maximum photosynthesis rate (P_m) and photosynthetic efficiency (α) were found between 20–40 µmol photons m^?2 s^?1 in both species. Bostrychia scorpioides, the uppermost intertidal species, had a higher P_m, α, dark respiration rate and light compensation point (E_c) than C. caespitosa at all acclimation irradiances. Net photosynthetic rates measured at their respective acclimation irradiances showed photosynthetic responses of B. scorpioides to be maximized at high irradiances, while in C. caespitosa they did not change. Higher growth rates were obtained in C. caespitosa than in B. scorpioides, which may be related to its lower thallus specific carbon content. When irradiance decreased, the chlorophyll a content of B. scorpioides increased whereas in C. caespitosa R-phycoerythrin increased. E_c for growth of B. scorpioides coincided with the irradiance at its lower distributional limit in the estuary, below which this species showed losses in biomass. However, in C. caespitosa the sustained growth ex situ at saturating irradiances contrasts with its absence from the upper intertidal zone, where similar light regimes occur. We demonstrated that while light clearly restricts the growth of B. scorpioides to the uppermost intertidal zone, this environmental factor would not prevent C. caespitosa from growing at higher levels.     

Adventitious Root Formation Induced by Gibberellic Acid and Regulated by the Irradiance to the Stock Plants

Pea plants (Pisum sativum L. cv. Alaska) were grown from seeds for eleven days at different irradiances. Cuttings were then excised and rooted at 16 W × m^?2. Gibberellic acid (GA₃, 10^?11 to 10^?3M) was applied to the cuttings immediately after excision. Cuttings from stock plants grown at the highest level of irradiance (38 W × m^?2) formed the lowest number of roots. An increasing number of roots per cutting was obtained by decreasing the irradiance to the stock plants. In cuttings from stock plants grown at low irradiances, low concentrations of GA₃ (10^?8 and 10^?7M) promoted root formation further. No effect on rooting by these GA₃ concentrations was observed when applied to cuttings originating from stock plants grown at the high irradiances. Root formation in all cuttings was inhibited by GA₃ at concentrations higher than 10^?6M. The degree of inhibition by GA₃ was influenced by the irradiance pretreatment and was increased with an increase in the irradiance during the stock plant growth. Seeds from different years produced cuttings with different response patterns regarding the irradiance and GA₃ effects on rooting.     

THF INFLUENCE OF IRRADIANCE ON THF BIOCHEMICAL COMPOSITION OF THE PRYMNESIOPHYTE ISOCHRYSIS SP. (CLONE T-ISO)1

The effects of irradiance on the biochemical composition of the prymnesiophyte microalga, Isochrysis sp. (Parke; clone T-ISO), a popular species for mariculture, were examined. Cultures were grown under a 12:12 h light: dark (L:D) regime at five irradiances ranging from 50 to 1000 μE·m ²·s^?1 and harvested at late-logarithmic phase for analysis of biochemical composition. Gross composition varied aver the range of irradiances. The highest levels of protein were present in cells from cultures grown at 100 and 250 μE·m ³·s¹, and minimum levels of carbohydrate and lipid occurred at 50 μE·m^?2·s^?1. Because the cell dry weight was reduced at lower irradiances, different trends were evident when results were expressed as percentage of dry weights. Protein percentages were highest at Wand 100 μE·m^?2·s^?1 and carbohydrate at 100 μE·m^?2·s^?1. The composition of amino acids did not differ over the range of irradiances. Glutamate and aspartate were always present in high proportions (9.0–13.5%); histidine. methionine, tryptophan, cystine, and hydroxy-proline were minor constituents (0.0–2.6%). Glucose was the predominant sugar in all cultures, ranging from 23.0% (50 μE·m^?2·s^?1) to 45.0% (100 μE·m^?2·s^?1) of total polysaccharide. No correlation was found between the proportion of any of the sugars and irradiance. The proportions of the lipid class components and fatty acids showed little change with irradiance. The main fatty acids were 14:0, 16:0, 16:1(n-7), 18:1(n-9), 18:3(n-3). 18:4(n-3), 18:5(n-3), and 22:6(n-3). Proportions of 22: 6(n-3) increased, whereas l8:3(n-3). 18:3(n-6). and 18:4(n-3) decreased, with increasing irradiance. Pigment concentrations were highest in cultures grown at 50 μE·m^?2·s^?1, except for fucoxanthin and diadinoxanthin (100 μE·m^?2·s^?1). The concentrations of accessory pigments correlated with chlorophyll a, which decreased in concentration with increasing irradiance. On the basts of biochemical composition, an irradiance of 100 μE·m^?1·s^?1 (12:12 h L:D cycle)for the culture of Isochrysis sp. (clone T-ISO) may provide optimal nutritional value for maricultured animals, although feeding trials are now necessary to substantiate this.     

Photoperiodic history affects the critical daylength of the short-day plant Acrosymphyton purpuriferum (Rhodophyta)

The crustose tetrasporophyte of the red alga Acrosymphyton purpuriferum is a qualitative short-day plant in the formation of its tetrasporangia. The critical daylength for the response was determined in plants precultured in various long-day regimes [20:4, 18:6, 16:8 and 14:10 (L:D, h)]. There was a strong influence of photoperiodic history. The sharper the decrease in daylength the stronger and faster the plants responded. The critical daylength (daylength inducing 50% response) increased from 9·5 h for plants precultured in 14 h days to 12·5 h for plants precultured in 20 h days. Acrosymphyton thus responds to a change in daylength, rather than to a fixed critical value. This is of adaptive significance in synchronising the onset of reproduction throughout its broad depth range in the subtidal region.     

Narrow range of temperature and irradiance supports optimal development of Lessonia corrugata (Ochrophyta) gametophytes: implications for kelp aquaculture and responses to climate change

The kelp Lessonia corrugata (Ochrophyta, Laminariales) is being developed for integrated multi-trophic aquaculture (IMTA) trials in the vicinity of salmon cages in Tasmania, Australia. Gametophytes are vegetally maintained before seeding on hatchery twine; however, the optimal temperature and light conditions for growth and sexual development are unknown. We measured vegetative size of female and male gametophytes and sexual development of females over a range of temperatures and irradiances using a temperature gradient table and neutral density light filters. Over a 4-week experiment, gametophytes were exposed to a combination of thermal (5.7–24.9 °C) and irradiance (10–100 μmol photons m^?2 s^?1) gradients, to assess biological performance. At the temperature extremes (hottest = 24.9 °C, coldest = 5.7 °C), we observed the critical thermal limits for this species and the results reveal a narrow optimal temperature range for growth and sexual development between 15.7 and 17.9 °C, with irradiances between 40 and 100 μmol photons m^?2 s^?1 resulting in fertile female gametophytes. Lessonia corrugata inhabits a small geographic range, found only around Tasmania, south of the Australian mainland, hence oceanic changes such as ongoing increases in sea surface temperatures (SSTs), and altered irradiance regimes may limit recruitment of the early microscopic life stages in the future. Our findings provide optimised culture conditions for aquaculture and information to predict the future geographic range of L. corrugata under ocean global change.

     

PRELIMINARY EVALUATION OF THE BIOREMEDIATION POTENTIAL OF PORPHYRA: PHOTOSYNTHETIC PRODUCTION BY BLADES AND CONCHOCELIS

Given their rapid growth and nutrient assimilation rates, Porphyra spp. are good candidates for bioremediation. The production potential of two northeast U.S. Porphyra species currently in culture (P. purpurea and P. umbilicalis) was evaluated by measuring rates of photosynthesis (as O₂ evolution) of samples grown at 20° C. Gametophytes of P. umbilicalis photosynthesized at rates that were 80% higher than those of P. purpurea over 5–20° C at both sub‐saturating and saturating irradiances (37 and 289 μmol photons m^?2 s^?1). Porphyra umbilicalis was both more efficient at low irradiances (higher alpha) and had a higher P_max than did P. purpurea (23.0 vs. 15.6 μmol O₂ g^?1 DW min^?1), suggesting that P. umbilicalis is a better choice for mass culture where self‐shading may be severe. The photosynthesis‐irradiance relationship for the Conchocelis stage of P. purpurea was also examined. Tufts of filaments, grown at 10, 15, and 20° C, were assayed at growth temperatures at irradiances ranging from 0–315 μmol photons m^?2 s^?1. Tufts were slightly more productive at 15° than at 10° C, but only ca. 4–6% as productive as gametophytes. Maximum rates of net photosynthesis were reduced by 66–74% in tufts grown at 20° C (only about 2% of gametophytes). The Conchocelis stage, however, need not limit mariculture operations; once Conchocelis cultures are established, they can be maintained over the long‐term as ready sources of spores for net seeding.     

THE LIFE CYCLE OE SARGASSUM HORNERI (PHAEOPHYTA) IN LABORATORY CULTURE1

The life cycle of the large dioecious alga Sargassum horneri (Turner) C. Agardh was completed in unialgal culture by controlling photoperiod in relation to the phase of growth. Embryos isolated from a naturally grown female thallus gave rise to early germlings that rapidly formed blades under both short-day (9 h L) and long-day (15 h L) conditions at 20° C Shoot elongation, which followed early blade formation, occurred under the short-day conditions hut not under the long-day conditions. Functional female and male receptacles developed when thalli 8–14 cm long grown under the short-day conditions were transferred to the long-day conditions; gamete fusion occurred when male and female thalli were grown together. Fertilized oospores gave rise to normal thalli in a manner similar to that for in situ plants. Thus, the life cycle of S. horneri was completed in laboratory culture.     

THE EFFECTS OF HIGH IRRADIANCE ON THE SETTLEMENT COMPETENCY AND VIABILITY OF KELP ZOOSPORES1

Elevated irradiance has a profound effect on the successful dispersal and establishment of kelp zoospores, affecting their physiology and viability. The research to date, however, has been on zoospores localized near the benthos, with little attention on the importance of vertical transportation and subsequent exposure to increased irradiance. Therefore, we wanted to investigate the effects of exposure to high irradiance on the reproductive planktonic life‐history stages of kelps Macrocystis pyrifera (L.) C. Agardh and Pterygophora californica Rupr. Zoospores of both species were exposed to different irradiances (75, 275, 575, 1,025 μmol photons · m^?2 · s^?1) over varying durations (1, 2, 4, 8, 12 h) and subsequently monitored for settlement competency, gametophyte development, and reproductive viability. Settlement success for M. pyrifera was uniform throughout all irradiance × time treatments, while settlement for P. californica decreased with increasing exposure time but not irradiance, although settlement was generally reduced at the highest irradiance level. Following zoospore settlement, germ tube development was visible in the gametophytes of both species within 1 week, although a significant decline of germ tube density in P. californica was observed with increasing irradiance. Similarly, a decrease in germ tube development with increasing exposure was observed across all irradiance levels for M. pyrifera, but irradiance itself was not significant. Further development into embryonic sporophytes was remarkably similar to gametophyte development, suggesting that the effect of exposure of kelp zoospores to high irradiance on subsequent sporophyte production is mediated through gametophyte development as well as zoospore survival.