LIGHT AND TEMPERATURE AS FACTORS REGULATING SEASONAL GROWTH AND DISTRIBUTION OF ULOTHRIX ZONATA (ULVOPHYCEAE)1

Ulothrix zonata (Weber and Mohr) Kütz. is an unbranched filamentous green alga found in rocky littoral areas of many northern lakes. Field observations of its seasonal and spatial distribution indicated that it should have a low temperature and a high irradiance optimum for net photosynthesis, and at temperatures above 10°C it should show an increasingly unfavorable energy balance. Measurements of net photosynthesis and respiration were made at 56 combinations of light and temperature. Optimum conditions were 5°C and 1100 μE·m^?2·s^?1 at which net photosynthesis was 16.8 mg O₂·g^?1·h^?1. As temperature increased above 5° C optimum irradiance decreased to 125 μE·m^?2·s^?1 at 30°C. Respiration rates increased with both temperature and prior irradiance. Light-enhanced respiration rates were significantly greater than dark respiration rates following irradiance exposures of 125 μE·m^?2·s^?1 or greater. Polynomials were fitted to the data to generate response surfaces. Polynomial equations represent statistical models which can accurately predict photosynthesis and respiration for inclusion in ecosystem models.     

COMPARATIVE STUDY ON THE PHOTOSYNTHETIC PROPERTIES OF PRASIOLA (CHLOROPHYCEAE) AND NOSTOC (CYANOPHYCEAE) FROM ANTARCTIC AND NON‐ANTARCTIC SITES1

The terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault occurs worldwide, including in Japan and on the Antarctic continent. The terrestrial green alga Prasiola crispa (Lightf.) Kütz. is also distributed in Antarctica. These two species need to acclimate to the severe Antarctic climate including low ambient temperature and desiccation under strong light conditions. To clarify this acclimation process, the physiological characteristics of the photosynthetic systems of these two Antarctic terrestrial organisms were assessed. The relative rate of photosynthetic electron flow in N. commune collected in Japan and in Antarctica reached maxima at 900 and 1,100 μmol photons · m^?2 · s^?1, respectively. The difference seemed to reflect the presence of high amounts of UV‐absorbing substances within the Antarctic cyanobacterium. On the other hand, the optimal temperatures for photosynthesis at the two locations were 30°C–35°C and 20°C–25°C, respectively. This finding suggested a decreased photosynthetic thermotolerance in the Antarctic strain. P. crispa exhibited desiccation tolerance and dehydration‐induced quenching of PSII fluorescence. Re‐reduction of the photooxidized PSI reaction center, P700, was also inhibited at fully dry states. Photosynthetic electron flow in P. crispa reached a maximum at 20°C–25°C and at a light intensity of 700 μmol photons ? m^?2 ? s^?1. Interestingly, the osmolarity of P. crispa cells suggested that photosynthesis is performed using water absorbed in a liquid form rather than water absorbed from the air. Overall, these data suggest that these two species have acclimated to optimally photosynthesize under conditions of the highest light intensity and the highest temperature for their habitat in Antarctica.     

MICROCLIMATIC EFFECTS ON WATER RELATIONS,LEAF TEMPERATURES,AND THE DISTRIBUTION OF HERACLEUM LANATUM AT HIGH ELEVATIONS

The small-scale distribution of an understory herb, Heracleum lanatum, was evaluated in terms of leaf temperature and water relations limitations due to a large leaf size (630 cm²). Diurnal variations in transpiration (4 to 60 mg m⁻² s⁻¹) were influenced by fluctuations in solar irradiance, wind speed, leaf temperature and stomatal conductance. Computer simulations indicated that leaf temperatures in a forest clearing would be > 12 C above air temperature, with maximum transpiration rates of 140 mg m⁻² s⁻¹, and daily water loss to be over 200% greater than values at natural understory locations. Simulations of nocturnal temperature relations indicated ~100 W m ⁻² less incident longwave irradiance in the forest clearing as compared to the understory (560 vs. 660 W m⁻² at 400 hr). This difference led to predicted leaf temperatures being as low as 6 C below air temperature in the forest clearing while measured leaf temperatures in the forest understory were within 1.5 C of air temperature throughout the night. Furthermore, minimum air temperatures were at or below 6 C on 36% of the nights during the summer growth period indicating that in open areas leaves of H. lanatum would frequently be below 0 C and subject to possible freeze damage. Heracleum lanatum may be more abundant in the shaded understory of the subalpine forest because exposure in open environments would result in high leaf temperatures and increased transpirational water loss during the day, as well as low leaf temperatures with the possibility of freeze damage at night.     

Photosynthesis and respiration in bladelets of Ecklonia cava Kjellman (Laminariales, Phaeophyta) in two localities with different temperature conditions

Characteristics of photosynthesis and respiration of bladelets were compared between Ecklonia cava Kjellman sporophytes growing in a warmer temperate locality (Tei, Kochi Pref., southern Japan) and in a cooler temperate locality (Nabeta, Shizuoka Pref., central Japan). Photosynthesis and respiration were measured with a differential gas-volumeter (Productmeter). In photosynthesis-light curves at 20°C, the rate of net photosynthesis was almost the same at light intensities lower than 25 μmol m⁻² s⁻¹ and the light-saturation occurred at 200–400 μmol m⁻²s⁻¹ in plants of both localities. The light-saturated net photosynthetic rates were higher in winter and spring than in summer and autumn in both plants. The optimum temperature for net photosynthesis at 400 μmol m⁻²s⁻¹ was 27°C throughout the year in the Tei plant and 25–27°C in the Nabeta plant. The decrease of net photosynthetic rates in the supraoptimal temperature range up to 29°C was sharper in winter and spring than in summer and autumn in both plants, being smaller in the Tei plant than in the Nabeta plant in all seasons. The dark respiration rate always increased with water temperature rise in both plants. No clear differences were found in the dark respiration rate between Tei and Nabeta plants except that when measured against dry weight, the Tei plant showed a slightly lower rate as compared with the Nabeta plant.     

Low temperature and high light dependent dynamic photoprotective strategies in Arabidopsis thaliana

Arabidopsis thaliana has been recognized as a chilling tolerant species based on analysis of resistance to low temperature stress, however, the mechanisms involved in this tolerance are not yet clarified. The low temperature-induced effects are exacerbated when plants are exposed to low temperatures in the presence of high light irradiance but the experimental data on the impact of light intensity during cold stress and its influence during recovery from stress are rather limited. The main objective of this study was to re-examine the photosynthetic responses of A. thaliana plants to short term (6 days) low temperature stress (12/10°C) under optimal (150 μmol m⁻² s⁻¹) and high light (500 μmol m⁻² s⁻¹) intensity and the subsequent recovery from the stress. Simultaneous measurements of the in vivo and in vitro functional performance of both photosystem II (PSII) and photosystem I (PSI), as well as, net photosynthesis, low temperature (77 K) chlorophyll fluorescence and immunoblot analysis of the relative abundance of PSII and PSI reaction center proteins were used to evaluate the role of light in the development of possible protective mechanisms during low temperature stress and the consequent recovery from exposure to low temperature and different light intensities. The results presented clearly suggest that Arabidopsis plants can employ a number of highly dynamic photoprotective strategies depending on the light intensity. These strategies include one based on LHCII quenching and two other quenching mechanisms localized within the PSII and PSI reaction centers, which are all expressed to different extent depending on the severity of the photoinhibitory treatments under low temperature stress conditions.     

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.     

Effects of light history on primary productivity in a phytoplankton community dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii

1. The effects of instantaneous irradiance and short‐term light history on primary production were determined for samples from a subtropical water reservoir dominated by the toxic cyanobacterium Cylindrospermopsis raciborskii. ¹⁴C‐bicarbonate uptake incubations were conducted on water samples from the reservoir, for irradiance (photosynthetically active radiation) ranging from 0 to 1654 μmol quanta m⁻² s⁻¹. Prior to the ¹⁴C incubations, cells were pre‐treated at irradiance levels ranging from 0 to 1006 μmol quanta m⁻² s⁻¹. 2. The average irradiance experienced by cells during the 2–2.5 h pre‐treatment incubations affected the productivity–irradiance (P–I) parameters: exposure to high light in pre‐treatment conditions caused a substantial decrease in maximum rate of primary production P_max and the photoinhibition parameter β when compared to cells pre‐treated in the dark. 3. While the data collected in this study were not sufficient to develop a full dynamic model of C. raciborskii productivity, P_max and β were modelled as a function of pre‐treatment irradiance, and these models were applied to predict the rate of primary production as a function of both instantaneous and historical irradiance. The results indicated that while cells with a history of exposure to high irradiance will be the most productive in high irradiance, production rates will be highest overall for dark‐acclimated cells in moderate irradiance. 4. Our results may explain why optically‐deep mixing favours C. raciborskii. If the mixing depth z_m exceeds the euphotic depth z_eu, cells will be dark‐acclimated, which will increase their rate of production when they are circulated through the euphotic zone. These results also predict that production rates will be higher during morning hours than for the same irradiance in the afternoon, which is consistent with other phytoplankton studies. 5. Since the rate of production of C. raciborskii‐dominated systems cannot be described by a single P–I curve, accurate estimates of production rates will require measurements over the daily light cycle.     

Desiccation independence of terrestrialNostoc commune ecotypes (cyanobacteria)

The subspeciesNostoc commune var.flagelliforme andN. commune var.commune are found in China (Ningxia Province, Inner Mongolia) as two morphologically different ecotypes of the desiccation-independent cyanobacteriumN. commune. The first ecotype, but not the second, colonizes arid areas. Various biochemical parameters and water dependence of photosynthesis and nitrogen fixation were compared for both ecotypes. Different patterns of water stress proteins were found in the two ecotypes. Repeated desiccation resulted in an enhanced desiccation independence for photosynthesis and, in the case of the ecotypecommune, for nitrogen fixation. The different response of nitrogenase of both ecotypes towards repeated cycles of rewetting and desiccation under conditions simulating the natural environment is discussed in terms of the energy balance of the colonies that are adapted to different environmental conditions.     

EFFECTS OF LIGHT AND TEMPERATURE ON NET PHOTOSYNTHESIS AND DARK RESPIRATION OF GAMETOPHYTES AND EMBRYONIC SPOROPHYTES OF MACROCYSTIS PYRIFERA1

The rates of net photosynthesis as a function of irradiance and temperature were determined for gametophytes and embryonic sporophytes of the kelp, Macrocystis pyrifera (L.) C. Ag. Gametophytes exhibited higher net photosynthetic rates based on oxygen and pH measurements than their derived embryonic sporophytes, but reached light saturation at comparable irradiance levels. The net photosynthesis of gametophytes reached a maximum of 66.4 mg O₂ g dry wt^?1 h^?1 (86.5 mg CO₂ g dry wt^?1 h^?1), a value approximately seven times the rate reported previously for the adult sporophyte blades. Gametophytes were light saturated at 70 μE m^?2 s^?1 and exhibited a significant decline in photosynthetic performance at irradiances 140 μE m^?1 s^?1. Embryonic sporophytes revealed a maximum photosynthetic capacity of 20.6 mg O₂ g dry wt^?1 h^?1 (25.3 mg CO₂ g dry wt^?1 h^?1), a rate about twice that reported for adult sporophyte blades. Embryonic sporophytes also became light saturated at 70 μE m^?2 s^?1, but unlike their parental gametophytes, failed to exhibit lesser photosynthetic rates at the highest irradiance levels studied; light compensation occurred at 2.8 μE m^?2 s^?1. Light-saturated net photosynthetic rates of gametophytes and embryonic sporophytes varied significantly with temperature. Gametophytes exhibited maximal photosynthesis at 15° to 20° C, whereas embryonic sporophytes maintained comparable rates between 10° and 20° C. Both gametophytes and embryonic sporophytes declined in photosynthetic capacity at 30° C. Dark respiration of gametophytes was uniform from 10° to 25° C, but increased six-fold at 30° C; the rates for embryonic sporophytes were comparable over the entire range of temperatures examined. The broader light and temperature tolerances of the embryonic sporophytes suggest that this stage in the life history of M. pyrifera is well suited for the subtidal benthic environment and for the conditions in the upper levels of the water column.     

Soil–atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture, and Pinus radiata and Eucalyptus globulus plantations

Soils provide the largest terrestrial carbon store, the largest atmospheric CO₂ source, the largest terrestrial N₂O source and the largest terrestrial CH₄ sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We measured soil–atmosphere exchange of CO₂, N₂O and CH₄ in four adjacent land‐use systems (native eucalypt woodland, clover‐grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia. Mean N₂O emission in the pasture was 26.6 μg N m⁻² h⁻¹, significantly greater than in the natural and managed forests (< 2.0 μg N m⁻² h⁻¹). N₂O emission from pasture soil increased after rainfall events (up to 100 μg N m⁻² h⁻¹) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through ¹⁵N isotope dilution in all land‐use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg⁻¹ h⁻¹, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH₄ sink, up to −20 μg C m⁻² h⁻¹ in the native woodland. Pasture soil was an occasional CH₄ source, but weak CH₄ sink overall (−3 μg C m⁻² h⁻¹). There were no strong correlations (R < 0.4) between CH₄ flux and soil moisture or temperature. Soil CO₂ emissions (35–55 mg C m⁻² h⁻¹) correlated with soil water content (R < 0.5) in all but the E. globulus plantation. Soil N₂O emissions from improved pastures can be considerable and comparable with intensively managed, irrigated and fertilised dairy pastures. In all land uses, soil N₂O emissions exceeded soil CH₄ uptake on a carbon dioxide equivalent basis. Overall, afforestation of improved pastures (i) decreases soil N₂O emissions and (ii) increases soil CH₄ uptake.     

KINETICS OF GROWTH AND DEATH IN ANABAENA FLOS-AQUAE (CYANOBACTERIA) UNDER LIGHT LIMITATION AND SUPERSATURATION

The kinetics of population growth and death were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown at light intensities ranging from limitation to photoinhibition (5 W·m⁻² to 160 W·m⁻²) in a nutrient-replete turbidostat. Steady-state growth rate (μ, or dilution rate, D) increased with light intensity from 0.44·day⁻¹ at a light intensity of 5 W·m⁻² to 0.99·day⁻¹ at 20 W·m⁻² and started to decrease above about 22 W·m⁻², reaching 0.56·day⁻¹ at 160 W·m⁻². The Haldane function of enzyme inhibition fit the growth data poorly, largely because of the unusually narrow range of saturation intensity. However, it produced a good fit (P < 0.001) for growth under photoinhibition. Anabaena flos-aquae died at different specific death rates (γ) below and above the saturation intensity. When calculated as the slope of a v_x⁻¹ and D⁻¹ plot, where v_x and D are cell viability (or live cell fraction) and dilution rate, respectively; γ was 0.047·day⁻¹ in the range of light limitation and 0.103·day⁻¹ under photoinhibition. Live vegetative cells and heterocysts, either in numbers or as a percentage of the total cells, showed a peak at the saturation intensity and decreased at lower and higher intensities. The ratio of live heterocysts to live vegetative cells increased with intensity when light was limiting but decreased when light was supersaturating. In cells growing at the same growth rate, the ratio was significantly lower under light inhibition than under subsaturation and the cell N:C ratio was also lower under inhibition. The steady-state rate of dissolved organic carbon (DOC) production increased with light intensity. However, its production as a percentage of the total C fixation was lowest at the optimum intensity and increased as the irradiance decreased or increased. The rate and percentage was significantly higher under photoinhibition than limitation in cells growing at the same growth rate. About 22% of the total fixed carbon was released as DOC at the highest light intensity. No correlation was found between the number of dead cells and DOC.     

Effects of environmental parameters on net photosynthesis of a free-living brown seaweed,Cystoseira barbata formarepens: determination of optimal photosynthetic culture conditions

The net photosynthesis of the Mediterranean brown seaweedCystoseira barbata f.repens is measured according to irradiance, temperature and salinity. There is not only, a good utilization of low light intensities (light-shade adaptation), but also a specific ability to use a broad range of irradiance, which corresponds in the photosynthesis-irradiance curves to a high initial slope and an extended light saturation level from 300 to 1500 mol photon m^–2 s^–1; only very high irradiances induce photoinhibition. Maximum net photosynthesis occurred at temperatures ranging from 20 °C to 30 °C. The alga tolerates not only a low level of salinity, but also a slight increase in salinity; however, at more than 47.5 g 1^–1 NaCl, oxygen exchange is significantly reduced.Light, temperature and salinity requirements are discussed, taking into account ecological considerations. Yields and quality of alginic acid are presented according to the irradiance and yearly evolutionin situ in order to aid future cultivation of this species.     

Photosynthetic activity including the thermal‐ and chilling‐light sensitivities of a temperate Japanese brown alga Sargassum macrocarpum

The effects of irradiance, temperature, thermal‐ and chilling‐light sensitivities on the photosynthesis of a temperate alga, Sargassum macrocarpum (Fucales) were determined by a pulse amplitude modulation (PAM)‐chlorophyll fluorometer and dissolved oxygen sensors. Oxygenic photosynthesis–irradiance curves at 8, 20, and 28°C revealed that the maximum net photosynthetic rates (NP _max) and saturation irradiance were highest at 28°C, and lowest at 8°C. Gross photosynthesis and dark respiration determined over a range of temperatures (8–36°C) at 300 μmol photons m^?2 s^?1 revealed that the maximum gross photosynthetic rate (GP_max) occurred at 27.8°C, which is consistent with the highest seawater temperature in the southern distributional limit of this species in Japan. Additionally, the maximum quantum yields of photosystem II (F _v/F _m) during the 72‐h temperature exposures were stable at 8–28°C, but suddenly dropped to zero at higher temperatures, indicative of PSII deactivation. Continuous exposure (12 h) to irradiance of 200 (low) and 1000 (high) μmol photons m^?2 s^?1 at 8, 20, and 28°C revealed greater declines in their effective quantum yields (Φ _PSII) under high irradiance. While Φ _PSII under low irradiance were very similar with the initial F _v/F _m under 20 and 28°C, values rapidly decreased with exposure duration at 8°C. At this temperature, F _v/F _m did not recover to initial values even after 12 h of dark acclimation. Final F _v/F _m of alga at 28°C under high irradiance treatment also did not recover, suggesting its sensitivity to photoinhibition at both low and high temperatures. These photosynthetic characteristics reflect both the adaptation of the species to the general environmental conditions, and its ability to acclimate to seasonal changes in seawater temperature within their geographical range of distribution.     

Assessment of photosynthetic performance in the two life history stages of Alaria crassifolia (Laminariales,Phaeophyceae)

Understanding of the physiological responses of kelp to environmental parameters is crucial, especially in the context of environmental change that may have contributed to the decline of kelp forests all over the world. The current study presents the photosynthetic characteristics of the macroscopic sporophyte and microscopic gametophyte stages of the brown alga Alaria crassifolia from Hokkaido, Japan, as determined by examining their photosynthetic responses over a range of temperature and irradiance using dissolved oxygen and chlorophyll fluorescence measurements. Net photosynthetic rates of the sporophyte were consistently higher than those of gametophyte across temperature gradients and irradiance levels. Photosynthesis–irradiance curves at 8°C, 16°C, and 20°C revealed similar initial slopes (α = 0.4–0.9) on the two life history stages, but higher compensation (E _c = 4–7 μmol photons m^?2 s^?1) and saturation irradiances (E _k = 53–103 μmol photons m^?2 s^?1) for the sporophyte than for the gametophyte (E _c = 0–7 μmol photons m^?2 s^?1; E _k = 7–10 μmol photons m^?2 s^?1). Both stages exhibited chronic photoinhibition, as shown by the failure of recovery in their maximum quantum yields (F _v/F _m) following high irradiance stress, with greater possibility of photodamage at low temperature. Gametophytes were less sensitive to low temperatures than sporophytes, given their relatively stable F _v/F _m response. Nevertheless, temperature optima for photosynthesis of both stages coincide with each other at 20–23°C, which correspond to the growth and maturation periods of A. crassifolia in Japan. This species is also likely to suffer from thermal inhibition as both GP rates and F _v/F _m decreased above 24°C.     

THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM1

The red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night-breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (<0.05 μmol·m⁻²·s⁻¹ vs. 3 μmol·m⁻²·s⁻¹). The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 65 μmol·m⁻²·s⁻¹ vs. threshold at <0.5 μmol·m⁻²·s⁻¹ after a main photoperiod at ca. 35 μmol·m⁻²·s⁻¹). The spectral dependence of the response was investigated in day-extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol·m⁻²·s⁻¹) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum-dose of 2.3 mmol·m⁻². However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far-red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum-doses of up to 20 mmol·m⁻².     

Plasticity of crassulacean acid metabolism at subtropical latitudes: a pineapple case study

Plants with the crassulacean acid metabolism (CAM) express high‐metabolic plasticity, to adjust to environmental stresses. This article hypothesizes that irradiance and nocturnal temperatures are the major limitations for CAM at higher latitudes such as the Azores (37°45'N). Circadian CAM expression in Ananas comosus L. Merr. (pineapple) was assessed by the diurnal pattern of leaf carbon fixation into l ‐malate at the solstices and equinoxes, and confirmed by determining maximal phosphoenolpyruvate carboxylase (PEPC) activity in plant material. Metabolic adjustments to environmental conditions were confirmed by gas exchange measurements, and integrated with environmental data to determine CAM's limiting factors: light and temperature. CAM plasticity was observed at the equinoxes, under similar photoperiods, but different environmental conditions. In spring, CAM expression was similar between vegetative and flowering plants, while in autumn, flowering (before anthesis) and fructifying (with fully developed fruit before ripening) plants accumulated more l ‐malate. Below 100 µmol m^?2 s^?1, CAM phase I was extended, reducing CAM phase III during the day. Carbon fixation inhibition may occur by two major pathways: nocturnal temperature (<15°C) inhibiting PEPC activity and l ‐malate accumulation; and low irradiance influencing the interplay between CAM phase I and III, affecting carboxylation and decarboxylation. Both have important consequences for plant development in autumn and winter. Observations were confirmed by flowering time prediction using environmental data, emphasizing that CAM expression had a strong seasonal regulation due to a complex network response to light and temperature, allowing pineapple to survive in environments not suitable for high productivity.     

First description of the environmental niche of the epibenthic dinoflagellate species Coolia palmyrensis,C. malayensis,and C. tropicalis (Dinophyceae) from Eastern Australia

Environmental variables such as temperature, salinity, and irradiance are significant drivers of microalgal growth and distribution. Therefore, understanding how these variables influence fitness of potentially toxic microalgal species is particularly important. In this study, strains of the potentially harmful epibenthic dinoflagellate species Coolia palmyrensis, C. malayensis, and C. tropicalis were isolated from coastal shallow water habitats on the east coast of Australia and identified using the D1‐D3 region of the large subunit (LSU) ribosomal DNA (rDNA). To determine the environmental niche of each taxon, growth was measured across a gradient of temperature (15–30°C), salinity (20–38), and irradiance (10–200 μmol photons · m^?2 · s^?1). Specific growth rates of Coolia tropicalis were highest under warm temperatures (27°C), low salinities (ca. 23), and intermediate irradiance levels (150 μmol photons · m^?2 · s^?1), while C. malayensis showed the highest growth at moderate temperatures (24°C) and irradiance levels (150 μmol photons · m^?2 · s^?1) and growth rates were consistent across the range of salinity levels tested (20–38). Coolia palmyrensis had the highest growth rate of all species tested and favored moderate temperatures (24°C), oceanic salinity (35), and high irradiance (>200 μmol photons · m^?2 · s^?1). This is the first study to characterize the environmental niche of species from the benthic harmful algal bloom genus Coolia and provides important information to help define species distributions and inform risk management.     

SEASONAL VARIATIONS OF PHOTOSYNTHETIC PIGMENTS, TOTAL C, N, AND P CONTENT, AND PHOTOSYNTHESIS IN PHYLLARIOPSIS PURPURASCENS: (PHAEOPHYTA) FROM THE STRAIT OF GIBRALTAR

Photosynthetic pigments, C, N, and P tissue composition, and photosynthetic rate were measured from April to October in the brown alga Phyllariopsis purpurascens (C. Agardh) Henry et South (Laminariales, Phaeophyta) growing at a 30-m depth in the Strait of Gibraltar. Ir-radiance reaching the population ranged from 13.5 to 27.5 mol.m^-2.mo^-1. The available light for this species, expressed as a percentage of the irradiance above the water, was 1.8%. Dissolved inorganic nitrogen forms, NO3-and NH₄+, were constant from April to October, whereas phosphate was depleted in August. Chlorophyll a decreased from 520.0 ± 165.0 to 199.6 ± 159.9 μg.g^-1 dry weight; in contrast, chlorophyll c and carotenoids did not change until September but increased threefold in October. C:N and N:P ratios changed in the same way and in the same range. They were constant until July but increased from 15–17 up to 42 (C:N) and from 14 to 40 (N:P) in October, suggesting a severe P limitation of growth of this species. The dark respiration rate and the light compensation point were constant from April to October (0.5 ± 0.1 μmol O₂. m^-2.s^-1 and 6.5 ± 0.2 μmol.m^-2. s^-1, respectively), whereas the maximum rate of apparent photosynthesis, light onset saturation parameter, and half saturation constant for light were maximum in April to May (3.7 μmol O₂. m^-2.s^-1and 40 and 41.5 μmol.m^-2. s^-1, respectively) and October (3.6 μmol O₂. m^-2.s^-1 and 50 and 53.7 μmol.m^-2. s^-1, respectively). They were minimum in August (1.2 μmol O₂.m^-2.s^-1 and 11.3 and 12 μmol.m^-2.s^-1, respectively). These minimum figures yielded a negative carbon budget in August and 0 in September, whereas it was positive the rest of the year. Photosynthetic efficiency, estimated by the ratio between maximum apparent photosynthesis and light half saturation constant, showed a strong agreement with productivity measured by means of an independent method. These results indicate that lamina expansion in this species is controlled by photosynthetic efficiency.     

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.     

Accumulation of trehalose in response to desiccation and salt stress in the terrestrial cyanobacterium Nostoc commune

Changes in photosynthetic activity and trehalose levels in field‐isolated, natural colonies of the terrestrial cyanobacterium Nostoc commune responding to desiccation and salt stress were investigated. As the water content decreased in N. commune colonies during desiccation, photosynthetic O₂‐evolving activity decreased and no activity was detected in desiccated colonies. A high level of O₂ evolution was restored in the colonies as they absorbed atmospheric moisture, indicating that only a small amount of water is required for reactivation of photosynthesis. No detectable trehalose was found in fully hydrated N. commune colonies; however, trehalose accumulation occurred in response to water loss during desiccation and high levels of trehalose were detected in the air‐dried colonies. Moreover, a 0.2 M NaCl treatment also induced trehalose accumulation to a level equivalent to that by desiccation. Photosynthetic O₂ evolution was inhibited by 0.2 M NaCl, indicating that N. commune can tolerate only low levels of salt. These results suggest that cessation of photosynthesis and trehalose accumulation occur in response to both matric water stress (desiccation) and osmotic water stress (high salt concentration), and that while trehalose may be a less effective osmoprotective compound than others, it is important for the extreme tolerance to desiccation observed in terrestrial cyanobacterium.