Photosynthetic performance of freshwater Rhodophyta in response to temperature, irradiance, pH and diurnal rhythm

Responses of net photosynthetic rates to temperature, irradiance, pH/inorganic carbon and diurnal rhythm were analyzed in 15 populations of eight freshwater red algal species in culture and natural conditions. Photosynthetic rates were determined by oxygen concentration using the light and dark bottles technique. Parameters derived from the photosynthesis–irradiance curves indicated adaptation to low irradiance for all freshwater red algae tested, confirming that they tend to occur under low light regimes. Some degree of photo‐inhibition (β= ‐0.33–0.01 mg O₂ g^?1 DW h^?1 (μmol photons m^?2 s^?1)^?1) was found for all species/populations analyzed, whereas light compensation points (Ic) were very low (≤ 2 μmol photons m‐ photons s^?1) for most algae tested. Saturation points were low for all algae tested (Ik = 6–54 μmol photons m^?2 s^?1; Is = 20–170 umol photons m^?2 s^?1). Rates of net photosynthesis and dark respiration responded to the variation in temperature. Optimum temperature values for net photosynthesis were variable among species and populations so that best performances were observed under distinct temperature conditions (10, 15, 20 or 25°C). Rates of dark respiration exhibited an increasing trend with temperature, with highest values under 20–25°C. Results from pH experiments showed best photosynthetic performances under pH 8.5 or 6.5 for all but one species, indicating higher affinity for inorganic carbon as bicarbonate or indistinct use of bicarbonate and free carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for all algae tested, which was characterized by two relatively clear peaks, with some variations around it: a first (higher) during the morning (07.00–11.00 hours.) and a second (lower) in the afternoon (14.00–18.00 hours). Comparative data between the ‘Chantransia’ stage and the respective gametophyte for one Batrachospermum population revealed higher values (ca 2‐times) in the latter, much lower than previously reported. The physiological role of the ‘Chantransia’ stage needs to be better analyzed.     

Effect of blue and red-orange LEDs on the growth and biochemical profile of Chlamydomonas reinhardtii

Light management methods are considered effective to enhance the quantum yield and photosynthetic efficiency and promote the biomass and nutrient production; however, light saturation and inhibition restrain further improvement. This work studies the effect of light mixing on algal light saturation/inhibition, growth kinetics, and biochemical profile. The green alga Chlamydomonas reinhardtii was cultivated with batch culture under an LED light panel with multiple spectra options. Different combinations of blue (B) and red-orange (RO) light intensities were tested with blue light ranging from 45 to 65 μmol photons m^?2 s^?1 and red-orange light ranging from 45 to 205 μmol photons m^?2 s^?1. Results reveal that the mixed blue and red-orange light significantly improved the growth kinetics and relieved the light saturation under blue light and the light inhibition under the red-orange light. The maximum specific growth rate, biomass concentration, and productivity increased by 22, 50, and 57%, respectively, compared with the results under the red-orange light. The lipid and protein synthesis were observed to be promoted under mixed light with relatively low red-orange light intensities (45 and 105 μmol photons m^?2 s^?1) and repressed at high red-orange light intensities (155 and 205 μmol photons m^?2 s^?1). The carbohydrate content did not change.

     

THE INTERACTION BETWEEN INORGANIC CARBON ACQUISITION AND LIGHT SUPPLY IN PALMARIA PALMATA (RHODOPHYTA)1

The dependence of the carbon concentrating mechanism of Palmaria palmata (L.) Kuntze on the growth light level was examined 1) to determine whether or not there is a threshold photon flux density (PFD) at which the inorganic carbon uptake mechanism can operate and 2) to attempt to quantify the relative energetic costs of acclimation to the two different limiting factors, PFD and dissolved inorganic carbon (DIC) concentration. Plants were grown at six PFDs: 5, 25, 50, 75, 95, and 125 μmol photons. m^?2.s^?1. Growth rates increased with increasing PFD from 5 to 50 μmol photons. m^?2. s^?1 and were light-saturated at 75, 95, and 125 μmol photons. m^?2. s^?1 Values of δ¹³C increased continuously with increasing growth PFD and did not saturate over the range of light levels tested. Time-resolved fluorescence characteristics indicated a progressive photoacclimation below 50 μmol photons. m^?2. s^?1. Analysis of chlorophyll fluorescence induction showed three levels of light use efficirncy associated with growth at 5 or 25, 50, and >75 μmol photons. m^?2. s^?1. The light-haruesting efficiency was inversely proportional to the effectiveness of DIC acquisition in plants grown at the six PFDs. These data were interpreted to indicate that there is a physiological tradeoff between photosynthetic efficiency and bicarbonate use in this species.     

In situ measurements of bioluminescence response of Gonyaulax spinifera to various mechanical stimuli

A conspicuous bioluminescence during nighttime was reported in an aquaculture farm in the Cochin estuary due to Gonyaulax spinifera bloom on March 20, 2020. In situ measurements on bioluminescence was carried out during nighttime to quantify the response of G. spinifera to various mechanical stimuli. The bioluminescence intensity (BI) was measured using Glowtracka, an advanced single channel sensor, attached to a Conductivity–Temperature–Depth Profiler. In steady environment, without any external stimuli, the bioluminescence generated due to the movement of fishes and shrimps in the water column was not detected by the sensor. However, stimuli such as a hand splash, oar and swimming movements, and a mixer could generate measurable bioluminescence responses. An abundance of?~?2.7?×?10⁶ cells L^?1 of G. spinifera with exceptionally high chlorophyll a of 25 mg m^?3 was recorded. The BI in response to hand splash was recorded as high as 1.6?×?10¹¹ photons cm^?2 s^?1. Similarly, BI of?~?1–6?×?10¹⁰ photons cm^?2 s^?1 with a cumulative bioluminescence of?~?2.51?×?10¹² photons cm^?2 (for 35 s) was recorded when there is a mixer with a constant force of 494 N/800 rpm min^?1. The response of G. spinifera was spontaneous with no time lapse between application of stimuli and the bioluminescence response. Interestingly, in natural environment, application of stimulus for longer time periods (10 min) does not lower the bioluminescence intensity due to the replenishment of water thrusted in by the mixer from surrounding areas. We also demonstrated that the bioluminescence intensity decreases with increase in distance from the source of stimuli (mixer) (av. 1.84?×?10¹⁰ photons cm^?2 s^?1 at 0.2 m to av. 0.05?×?10¹⁰ photons cm^?2 s^?1 at 1 m). The BI was highest in the periphery of the turbulent wake generated by the stimuli (av. 3.1?×?10¹⁰ photons cm^?2 s^?1) compared to the center (av. 1.8?×?10¹⁰ photons cm^?2 s^?1). When the stimuli was applied vertically down, the BI decreased from 0.2 m (0.3?×?10¹⁰ photons cm^?2 s^?1) to 0.5 m (0.10?×?10¹⁰ photons cm^?2 s^?1). Our study demonstrates that the BI of G. spinifera increases with increase in mechanical stimuli and decreases with increase in distance from the stimuli.

     

Responses of vegetative gametophytes of Undaria pinnatifida to high irradiance in the process of gametogenesis

The population of Undaria pinnatifida in its ecologic niche sustains itself in high temperature summer in the form of vegetative gametophytes, the haploid stage in its heteromorphic life cycle. Gametogenesis initiates when seawater temperature drops below the threshold levels in autumn in the northern hemisphere. Given that the temperature may fall into the appropriate range for gametogenesis, the level of irradiance determines the final destiny of a gametophytic cell, either undergoing vegetative cell division or initiating gametogenesis. In elucidating how vegetatively propagated gametophytes cope with changes of irradiance in gametogenesis, we carried out a series of culture experiments and found that a direct exposure to irradiance as high as 270 μmol photons m^?2 s^?1 was lethal to dim‐light (7–10 μmol photons m^?2 s^?1) adapted male and female gametophytes. This lethal effect was linearly corelated with the exposure time. However, dim‐light adapted vegetative gametophytes were shown to be able tolerate as high as 420 μmol photons m^?2 s^?1 if the irradiance was steadily increased from dim light levels (7–10 μmol photons m^?2 s^?1) to 90, 180 and finally 420 μmol photons m^?2 s^?1, respectively, at a minimum of 1–3 h intervals. Percentage of female gametophytic cells that turned into oogonia and were eventually fertilized was significantly higher if cultured at higher but not lethal irradiances. Findings of this investigation help to understand the dynamic changes of population size of sporophytic plants under different light climates at different site‐specific ecologic niches. It may help to establish specific technical details of manipulation of light during mass production of seedlings by use of vegetatively propagated gametophytes.     

A SIMPLE METHOD FOR ISOLATING FILAMENTS AS “ALGAL SEED STOCK” FROM MONOSTROMA LATISSIMUM (CHLOROPHYTA) GERMLINGS,AND APPLICATION FOR MASS CULTIVATION1

Germlings were grown from Monostroma latissimum Wittr. reproductive cells on nylon ropes. Holdfast threads and some uniseriate filaments were observed to have penetrated the fibers of the dispersed ropes. The algal filaments were easily isolated and prepared for cultivation, in comparison to the methods of enzymatically isolated algal protoplasts. Under low light (60–100 μmol photons · m^?2 · s^?1), the algal filaments grew to form a filamentous mass. When cultivated under stronger light (300–600 μmol photons · m^?2 · s^?1), they grew to initially form tubular thalli and then, when cultivated under light intensities >700 μmol photons · m^?2 · s^?1, formed foliaceous thalli. Consequently, the filaments were homogenized into small sections and then sewed on the nylon rope for algal mass cultivation. Under high‐intensity natural light, they grew to form leafy thalli.     

Effects of salinity,light intensity and sediment on growth,pigments, agar production and reproduction in Gracilaria tenuistipitata from Songkhla Lagoon in Thailand

The effects of salinity, light intensity and sediment on Gracilaria tenuistipitata C.F. Chang & B.M. Xia on growth, pigments, agar production, and net photosynthesis rate were examined in the laboratory under varying conditions of salinity (0, 25 and 33 psu), light intensity (150, 400, 700 and 1000 µmol photons m^?2 s^?1) and sediment (0, 0.67 and 2.28 mg L^?1). These conditions simulated field conditions, to gain some understanding of the best conditions for cultivation of G. tenuistipitata. The highest growth rate was at 25 psu, 700 µmol photons m^?2 s^?1 with no sediments, that provided a 6.7% increase in weight gain. The highest agar production (24.8 ± 3.0 %DW) was at 25 psu, 150–400 µmol photons m^?2 s^?1 and no sediment. The highest pigment contents were phycoerythrin (0.8 ± 0.5 mg g^?1FW) and phycocyanin (0.34 ± 0.05 mg g^?1 FW) produced in low light conditions, at 150 µmol photons m^?2 s^?1. The highest photosynthesis rate was 161.3 ± 32.7 mg O₂ g^?1 DW h^?1 in 25 psu, 400 µmol photons m^?2 s^?1 without sediment in the short period of cultivation, (3 days) and 60.3 ± 6.7 mg O₂ g^?1 DW h^?1 in 25 psu, 700 µmol photons m^?2 s^?1 without sediment in the long period of cultivation (20 days). The results indicated that salinity was the most crucial factor affecting G. tenuistipitata growth and production. This would help to promote the cultivation of Gracilaria cultivation back into the lagoon using these now determined baseline conditions. Extrapolation of the results from the laboratory study to field conditions indicated that it was possible to obtain two crops of Gracilaria a year in the lagoon, with good yields of agar, from mid‐January to the end of April (dry season), and from mid‐July to the end of September (first rainy season) when provided sediment was restricted.     

Effects of flashing light-emitting diodes on algal biomass productivity

To reduce power consumption and enhance algal biomass productivity in a thin flat-plate bioreactor (called a sliver tank bioreactor), flashing (pulsing) light was used. Biomass productivity and power consumption were monitored in controlled experiments using various photon flux levels, including a constant (non-flashing) flux of 75 μmol photons m^?2 s^?1 and three flashing experiments with photon fluxes of 375, 275, and 175 μmol photons m^?2 s^?1. Flashing experiments were performed at 10 kHz and a duty cycle of 20 %. A sliver tank bioreactor with a chamber width of 6.4 mm was used for its short optical path. Data from the experiments where light was flashed with a photon flux of 375 μmol photons m^?2 s^?1 indicated 9.6 % less power and 2.86 times the biomass productivity compared to the constant photon flux experiments. Similar results were obtained for the other flashing light regimes, which had lower biomass yields but also less input power per unit biomass produced, indicating that a large fraction of the continuously applied photons are shed or wasted, even at levels approximately 1/30th the intensity of full sun.     

Dominance of a 675?nm chlorophyll(ide) form upon selective 632.8 or 654?nm laser illumination after partial protochlorophyllide phototransformation

The phototransformation pathways of protochlorophyllide forms were studied in 8?C14-day-old leaves of dark-germinated wheat (Triticum aestivum L.) using white, 632.8?nm He?CNe laser and 654?nm laser diode light. The photon flux density (PFD) values (0.75?C360???mol photons?m^?2?s^?1), the illumination periods (20?ms?C10?s) and the temperature of the leaves (between ?60?°C and room temperature) were varied. The 77?K fluorescence spectra of partially phototransformed leaves showed gradual accumulation or even the dominance of the 675?nm emitting chlorophyllide or chlorophyll form at room temperature with 632.8?nm of PFD less than 200???mol photons?m^?2?s^?1 or with 654?nm of low PFD (7.5???mol photons?m^?2?s^?1) up to 1?s. Longer wavelength (685 or 690?nm) emitting chlorophyllide forms appeared at illuminations under ?25?°C with both laser lights or at room temperature when the PFD values were higher or the illumination period was longer than above. We concluded that the formation of the 675?nm emitting chlorophyllide form does not indicate the direct photoactivity of the 633?nm emitting protochlorophyllide form; it can derive from 644 and 657?nm forms via instantaneous disaggregation of the newly-produced chlorophyllide complexes. The disaggregation is strongly influenced by the molecular environment and the localization of the complex.     

Evaluation of Arthrospira platensis extracellular polymeric substances production in photoautotrophic, heterotrophic and mixotrophic conditions

The kinetic study of Arthrospira platensis extracellular polymeric substances (EPS) production under different trophic modes??photoautotrophy (100???mol photons m^?2?s^?1), heterotrophy (1.5?g/L glucose), and mixotrophy (100???mol photons m^?2?s^?1 and 1.5?g/L glucose)??was investigated. Under photoautotrophic and heterotrophic conditions, the maximum EPS production 219.61?±?4.73 and 30.30?±?1.97?mg/L, respectively, occurred during the stationary phase. Under a mixotrophic condition, the maximum EPS production (290.50?±?2.21?mg/L) was observed during the early stationary phase. The highest specific EPS productivity (433.62?mg/g per day) was obtained under a photoautotrophic culture. The lowest specific EPS productivity (38.33?mg/g per day) was observed for the heterotrophic culture. The effects of glucose concentration, light intensity, and their interaction in mixotrophic culture on A. platensis EPS production were evaluated by means of 32 factorial design and response surface methodology. This design was carried out with a glucose concentration of 0.5, 1.5, and 2.5?g/L and at light levels of 50, 100, and 150???mol photons m^?2?s^?1. Statistical analysis of the model demonstrated that EPS concentration and EPS yield were mainly influenced by glucose concentration and that conditions optimizing EPS concentration were dissimilar from those optimizing EPS yield. The highest maximum predicted EPS concentration (369.3?mg/L) was found at 150???mol photons m^?2?s^?1 light intensity and 2.4?g/L glucose concentration, while the highest maximum predicted EPS yield (364.3?mg/g) was recorded at 115???mol photons m^?2?s^?1 light intensity and 1.8?g/L glucose concentration.     

Rapid, reversible alterations in spinach thylakoid appression upon changes in light intensity

Changes in light quantity and quality cause structural changes within the thylakoid membrane; long‐term responses have been described for so‐called ‘sun’ and ‘shade’ leaves. Many leaves, however, experience changes in irradiance on a time scale of minutes due to self‐shading and sun flecks. In this study, mature, attached spinach leaves were grown at 300 µmol photons m^?2 s^?1 then rapidly switched to a different light treatment. The treatment irradiances were 10, 800 or 1500 µmol m^?2 s^?1 for 10 min, or 10 or 20 min of self‐shading (about 10 µmol m^?2 s^?1). Image analysis of transmission electron micrographs revealed that a 10 min switch to a lower light intensity increased grana size and number per chloroplast profile by 10–20%. Returning the leaves to 300 µmol m^?2 s^?1 for 10 min reversed the phenomenon. Chlorophyll fluorescence measurements of detached, intact leaves at 77 K were suggestive of a transition from state 2 to state 1 upon shading. Diurnal ultrastructural measurements of granal size and number did not reveal a significant net change in ultrastructure over the time scale of hours. It is concluded that spinach chloroplasts can alter the degree of thylakoid appression in response to irradiance changes on a time scale of minutes. These ultrastructural responses are caused by biochemical and biophysical adjustments within the thylakoid membrane that serve to maximize photosynthesis and minimize photo‐inhibition under rapidly fluctuating light environments.     

Regulation of shoot growth, root development and manganese allocation in wheat (Triticum aestivum) genotypes by light intensity

The objective of this study was to assess effects of different light intensities on shoot growth, root development and allocation of root-borne solutes via the transpiration stream to various shoot parts of young wheat plants (Triticum aestivum L.). Hydroponic culture allowed direct access to the roots and shoots throughout the experiment. Under low light intensity (100?μmol photons m^?2?s^?1), shoot growth was restricted, less (but larger) leaves were produced at the main shoot and only a few tillers became visible as compared to plants under high light intensity (380?μmol photons m^?2?s^?1). The root system was indirectly also affected by the illumination of the aerial parts. A larger number of shorter roots were produced under high light leading to a denser root system, while only a small number of longer roots were present under low light. The distribution of ⁵⁴Mn (xylem-mobile, but essentially phloem-immobile in wheat) from the roots to the shoot lead to the conclusion that light regime strongly influences the distribution of root-borne solutes within the shoots. Labels introduced into the roots may allow a deeper insight into the transfer of solutes from the root system to the various shoot parts under different light regimes.     

Behavior of red king crab larvae: Phototaxis,geotaxis and rheotaxis

Zoeae of Paralithodes camtschatica were positively phototactic to white light intensities above 1 × 10¹³ q cm^?2 s^?1. Negative phototaxis occurred at low (1 × 10¹² q cm^?2 s^?1), but not high intensities (2.2 × 10¹⁶q cm^?2 s^?1). Phototactic response was directly related to light intensity. Zoeae also responded to red, green and blue light. Zoeae were negatively geotactic, but geotaxis was dominated by phototaxis. Horizontal swimming speed of stage 1 zoeae <4 d old was 2.4 ± 0.1 (SE) cms^?1 and decreased to 1.7 ± 0.1 cm s^?1 in older zoeae (P <0.01). Horizontal swimming speed of stage 2 zoeae was not significantly different from ≥4 d old stage 1 zoeae. Vertical swimming speed, 1.6 ± 0.1 cm s^?1, and sinking rate, 0.7 ± 0.1 cm s^?1, did not change with ontogeny. King crab zoeae were positively rheotactic and maintained position in horizontal currents less than 1.4 cm s^?1. Starvation reduced swimming and sinking rates and phototactic response.     

Effects of simulated trawling on sablefish and walleye pollock: the role of light intensity, net velocity and towing duration

Laboratory apparatus which simulated capture of fish in the cod-end of a towed trawl was used to induce post-capture stress as measured by alterations in behavioural, physiological and mortality indices in juvenile walleye pollock Theragra chalcogramma and juvenile and adult sablefish Anoplopoma fimbria. Differences in resistance to net entrainment varied between species with the severity of stress and the potential for recovery depending on light intensity, net velocity and towing duration. At a light intensity which simulated daylight at depth in clear ocean water (0.5 μmol photons m^?2 s^?1), walleye pollock juveniles were able to maintain swimming in nets towed at 0.65 m s^?1 for 3h with no discernible effects on behaviour or mortality. However, when net velocity was increased to >0.75m s^?1 or light intensity was decreased to <0.002 μmol photons m^?2 s^?1, fish became entrained in the meshes of the net and exhibited significant alterations in feeding behaviour, predator evasion and increases in plasma cortisol concentrations. Marked increases in stress-induced mortality also occurred, in some cases after a delay of 6 days and eventually reaching 100%. In comparison with walleye pollock, sablefish juveniles became entrained in the meshes of the net at higher velocities (>0.92m s^?1) or lower light intensities (<0.0004 μmol photons m^?2 s^?1) and were much more resistant to post-capture stress. Towing of net-entrained fish for 15 min caused no detectable changes in feeding and cortisol and for 2 h, no changes in feeding although mortality increased from 0% for 15-min tows to 19% for 2-h tows. Towing for 4 h caused significant alterations in feeding and cortisol with feeding recovering to control levels by 6 days and cortisol by 3 days; mortality was 25%. When adult sablefish were towed for 4 h followed by 15-min exposure to air, feeding was inhibited 6 days after towing, but recovered within 30 days with no mortality observed after 30 days. The results demonstrate the value of using laboratory-based behavioural and biochemical indices to identify factors that may potentially affect post-capture survival among different species of fish.     

Induced change in <Emphasis Type="Italic">Arthrospira</Emphasis> sp. (<Emphasis Type="Italic">Spirulina</Emphasis>) intracellular and extracellular metabolites using multifactor stress combination approach

The interactive effects of light intensity, NaCl, nitrogen, and phosphorus on intracellular biomass content and extracellular polymeric substance production were assessed for Arthrospira sp. (Spirulina) in a two-phase culture process using principal component analysis and central composite face design. Under high light intensity (120 μmol photons m^?2?s^?1) and low NaCl (1 gL^?1), NaNO₃, and K₂HPO₄ (0.5 g L^?1), the carbohydrate content was maximized to 26.61%. Interaction of both K₂HPO₄ (1.6 gL^?1) and NaCl (1.19 gL^?1) with low NaNO₃ (0.5 gL^?1) achieved the maximum content of lipids (15.62%), while high NaCl (40 gL^?1), K₂HPO₄, and NaNO₃ (4.5 gL^?1) enhanced mainly total carotenoids (0.85%). Conversely, under low light intensity of 10 μmol photons m^?2?s^?1 combined with 11.76 gL^?1 of NaCl, 0.5 gL^?1 of NaNO₃, and 2.68 gL^?1 of K₂HPO₄, the phycobiliprotein content reached its highest level (16.09%). The maximum extracellular polymeric substance (EPS) production (0.902 gg^?1?DW) was triggered under moderate light of 57.25 μmol photons m^?2?s^?1 and interaction of high NaCl (40 gL^?1) and K₂HPO₄ (4.5 gL^?1) with low NaNO₃ (0.5 gL^?1). The maximization ratios of intracellular biomass content in terms of carbohydrate, lipid, total carotenoid, phycobiliprotein, and EPS production were 3.55-, 1.73-, 9.55-, 2.92-, and 1.46-fold, respectively, greater than those obtained at optimal growth conditions. This study demonstrated that the multiple stress factors applied to the adopted two-phase culture process could be a promising strategy to produce biomass enriched in various high-value compound.     

Luminescence from the yeast Candida utilis and comparisons across three genera

Weak luminescence was detected from oxygenated liquid cultures of the yeast Candida utilis during two stages of its growth cycle. The first period of emission occurred during the exponential phase of growth and comprised an ultraviolet band (270-390 nm; ca 19 photons s^{? 1} cm^?2 of culture surface) and a visible band (450-620 nm; ca 68 photons s^{? 1} cm^{? 2}). The second period of emission occurred late in the stationary phase of growth and was comprised almost entirely of a visible region band (450-620 nm; 6.8 × 10² photons s^{? 1} cm^{? 2}). No luminescence was observed when the yeast was grown anaerobically. These observations are compared with those previously obtained for two other yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe. The ratios of the intensities of blue/red emissions in the stationary phase luminescences correlated with the ratio of the saturated/unsaturated lipid content for the three yeasts. This result provided further support for the claim that the stationary phase luminescence arises from the reactions associated with lipid peroxidation. A number of previously suggested sources of the exponential phase luminescence are discussed and rejected. Oxidative side reactions accompanying protein synthesis remain a possible source of that emission.     

Photosynthetic characteristics of charophytes from tropical lotic ecosystems

Responses of photosynthetic rates, determined by oxygen evolution using the light and dark bottles technique, to different temperatures, irradiances, pH, and diurnal rhythm were analyzed under laboratory conditions in four charophyte species (Chara braunii Gmelin, C. guairensis R. Bicudo, Nitella subglomerata A. Braun and Nitella sp.) from lotic habitats in southeastern Brazil. Parameters derived from the photosynthesis versus irradiance curves indicated affinity to low irradiances for all algae tested. Some degree of photoinhibition, [β= ‐(0.30–0.13) mg O2 g^?1 dry weight Ir¹ (μmol photons m^?2 s^?1)^?1], low light compensation points (I_c= 4–20 μmol photons m^?2 s^?1) were found for all species analyzed, as well as low values of light saturation parameter (I_k) and saturation (I_s) 29–130 and 92–169 μmol photons m^?2 s^?1, respectively. Photoacclimation was observed in two populations of N. subglomerata collected from sites with different irradiances, consisting of variations in photosynthetic parameters (higher values of a, and lower of I_k and maximum photosynthetic rate, P_max, in the population under lower irradiance). The highest photosynthetic rates for Chara species were observed at 10–15°C, while for Nitella the highest photosynthetic rate was observed at 20–25°C, despite the lack of significant differences among most levels tested. Rates of dark respiration significantly increase with temperature, with the highest values at 25°C. The results from pH experiments showed highest photosynthetic rates under pH 4.0 for all algae, suggesting higher affinity for inorganic carbon in the form of carbon dioxide, except in one population of N. subglomerata, with similar rates under the three levels, suggesting indistinct use of bicarbonate and carbon dioxide. Diurnal changes in photosynthetic rates revealed a general pattern for most algae tested, which was characterized by two peaks: the first (higher) during the morning (07.00–11.00) and the second (lower) in the afternoon (14.00–17.00). This suggests an endogenous rhythm determining the daily variations in photosynthetic rates.     

Early development of germlings of <Emphasis Type="Italic">Sargassum thunbergii</Emphasis> (Fucales,Phaeophyta) under laboratory conditions

Morphology and culture studies on germlings of Sargassum thunbergii (Mertens et Roth) Kuntze were carried out under controlled laboratory conditions. Growth characteristics of these germlings grown under different temperatures (from 10 to 25°C), irradiances (from 9 to 88 μmol photons m⁻² s⁻¹), and under blue and white light conditions are described. The development of embryonic germlings follows the classic “8 nuclei 1 egg” type described for Sargassaceae. Fertilized eggs spent 5–6 h developing into multicellular germlings with abundant rhizoids after fertilization. Under conditions of 20°C, 44 μmol photons m⁻² s⁻¹ and photoperiod of 12 h, young germlings with one or two leaflets reached 2–3 mm in length after 8 weeks. Temperature variations (10, 15, 20, 25°C) under 88 μmol photons m⁻² s⁻¹ significantly influenced the growth rate within the first week, although this effect became less obvious after 8 weeks, especially at 15 and 20°C. Variation in germling growth was highly significant under different irradiances (9, 18, 44, 88 μmol photons m⁻² s⁻¹) at 25°C. Low temperature (10°C) reduced germling growth. Growth of germlings cultured under blue light was lower than in white light. Optimal growth of these germlings occurred at 25°C and 44 μmol photons m⁻² s⁻¹.     

Controls on isoprene emission from trees in a subtropical dry forest

Isoprene emission from vegetation is the single most important source of photochemically active reduced compounds to the atmosphere. We present the first controlled-environment measurements of isoprene emission from leaves of tropical forest trees. Our studies were conducted in the Guanica State Forest in Puerto Rico. We report the effects of temperature and light variations on biogenic isoprene emissions during 1995. Maximum emission rates varied among species from 0 to 268 nmol m^?2 s^?1. Values at the upper end of this range of maximum emission rates are 2–3 times higher than values reported from any temperate taxa. Isoprene emission showed strong sensitivity to light and temperature variations. In contrast to temperate plants, whose emissions tend to saturate at a light intensity of ～1000 μmol m^?2 s^?1, emissions from the tropical species increased with light intensity up to 2500 μmol m^?2 s^?1. The temperature optima for emissions from these plants were similar to those previously reported for temperate plants: ～40 °C. The high maximum emission rates and lack of light saturation indicate that estimates of isoprene emission from tropical forests need to be revised upwards.     

Temperature × light interaction and tolerance of high water temperature in the planktonic freshwater flagellates Cryptomonas (Cryptophyceae) and Dinobryon (Chrysophyceae)

Using microcosm experiments, we investigated the interactive effects of temperature and light on specific growth rates of three species each of the phytoplanktonic genera Cryptomonas and Dinobryon. Several species of these genera play important roles in the food web of lakes and seem to be sensitive to high water temperature. We measured growth rates at three to four photon flux densities ranging from 10 to 240 μmol photon · m^?2 · s^?1 and at 4–5 temperatures ranging from 10°C to 28°C. The temperature × light interaction was generally strong, species specific, and also genus specific. Five of the six species studied tolerated 25°C when light availability was high; however, low light reduced tolerance of high temperatures. Growth rates of all six species were unaffected by temperature in the 10°C–15°C range at light levels ≤50 μmol photon · m^?2 · s^?1. At high light, growth rates of Cryptomonas spp. increased with temperature until the temperature optimum was reached and then declined. The Dinobryon species were less sensitive than Cryptomonas spp. to photon flux densities of 40 μmol photon · m^?2 · s^?1 and 200 μmol photon · m^?2 · s^?1 over the entire temperature range but did not grow under a combination of very low light (10 μmol photon · m^?2 · s^?1) and high temperature (≥20°C). Among the three Cryptomonas species, cell volume declined with temperature and the maximum temperature tolerated was negatively related to cell size. Since Cryptomonas is important food for microzooplankton, these trends may affect the pelagic carbon flow if lake warming continues.