PHOTOPHYSIOLOGY OF A TURF ALGAL COMMUNITY: INTEGRATED RESPONSES TO AMBIENT LIGHT AND STANDING BIOMASS1

This study investigated the variation in the relationship between photosynthesis and ambient light (P‐E curves) for turf algal communities on a temperate reef off the coast of South Australia, analyzing the integrated effects of ambient light and standing biomass. The photophysiology of turfs was studied in situ on a seasonal basis, examining algal communities growing on artificial substrate (plates) at depths of 4 m and 10 m. P‐E curves and estimates for the photokinetic parameters (P_m, R_d, α, E_k, and E_c) were obtained through oxygen evolution methods, using an automated underwater respirometer. Photoacclimation responses to changes in ambient light were strongly affected by the biomass of the community. P_m showed an inverse relationship to standing biomass, irrespective of depth and season, which was considered to be a response to self‐shading and boundary layer effects. Biomass effects imposed a high variance on estimates for all photosynthetic parameters, overshadowing differences observed for season and depth. Biomass also affected photoinhibition on turf communities, where significant afternoon depression of photosynthesis was observed in sparse turf patches when compared to denser patches. High areal productivity rates were maintained across all seasons with a significant decrease only being observed during winter.     

PHOTOSYNTHESIS-IRRADIANCE PATTERNS IN BENTHIC MICROALGAE: VARIATIONS AS A FUNCTION OF ASSEMBLAGE THICKNESS AND COMMUNITY STRUCTURE

Photosynthesis-irradiance (P-I) characteristics of periphyton (microphytobenthos) have been considered primarily for entire assemblages. How P-I responses vary with mat thickness and with community composition has not been considered in detail. We used a combined approach of modeling, microscale determinations of photosynthetic rate and light attenuation, and whole-assemblage O₂ flux measurements to explore P-I relationships. The modeling approach suggested that the onset of photosynthetic saturation and photoinhibition will occur at higher irradiance and that whole-mat photoinhibition (decreased photosynthesis at very high irradiance), biomass-specific maximum photosynthetic rate, and initial slope of the P-I function (α) should decrease as assemblage thickness increases or light attenuation increases. Spherical light microsensor profiles for a variety of stream algae indicated a strongly compressed photic zone with attenuation coefficients of 70–1791 m^?1 for scalar photosynthetic photon fluence density. The O₂ microelectrode measurements showed little if any photoinhibition at 2 and 4 mm depths in one filamentous green algal (Ulothrix) assemblage, with a relatively low attenuation coefficient, and no photoinhibition in a second Ulothrix community. An assemblage dominated by a unicellular cyanobacterium exhibited little photoinhibition at 2 and 4 mm, and a dense cyanobacterial (Phormidium)/xanthophyte (Vaucheria) community exhibited no photoinhibition at all. The microelectrode data revealed increases in α over several millimeters of depth (photoacclimation). These data supported the model predictions with regard to the effects of mat optical thickness on whole-assemblage values for α and photoinhibition. Whole-community O₂ flux data from 15 intact assemblages revealed positive relationships between chlorophyll a density and maximum photosynthetic rate or α expressed per unit area; the relationships with chlorophyll a were negative when photosynthetic rates were expressed per unit chlorophyll a. None of the whole assemblages exhibited photoinhibition. Thus, the data from the whole communities were consistent with model predictions.     

Limits to physiological plasticity of the coral Pocillopora verrucosa from the central Red Sea

Many coral species display changing distribution patterns across coral reef depths. While changes in the underwater light field and the ability to associate with different photosynthetic symbionts of the genus Symbiodinium explain some of the variation, the limits to physiological plasticity are unknown for most corals. In the central Red Sea, colonies of the branching coral Pocillopora verrucosa are most abundant in shallow high light environments and become less abundant in water depths below 10 m. To further understand what determines this narrow distribution, we conducted a cross-depths transplant experiment looking at physiological plasticity and acclimation in regard to depth. Colonies from 5, 10, and 20 m were collected, transplanted to all depths, and re-investigated after 30 and 210 d. All coral colonies transplanted downward from shallow to deep water displayed an increase in photosynthetic light-harvesting pigments, which resulted in higher photosynthetic efficiency. Shallow-water specimens transplanted to deeper water showed a significant decrease in total protein content after 30 and 210 d under low light conditions compared to specimens transplanted to shallow and medium depths. Stable isotope data suggest that heterotrophic input of carbon was not increased under low light, and consequently, decreasing protein levels were symptomatic of decreasing photosynthetic rates that could not be compensated for through higher light-harvesting efficiency. Our results provide insights into the physiological plasticity of P. verrucosa in changing light regimes and explain the observed depth distribution pattern. Despite its high abundance in shallow reef waters, P. verrucosa possesses limited heterotrophic acclimation potential, i.e., the ability to support its mainly photoautotrophic diet through heterotrophic feeding. We conclude that P. verrucosa might be a species vulnerable to sudden changes in underwater light fields resulting from processes such as increased turbidity caused by coastal development along the Saudi Arabian Red Sea coast.     

Natural succession of macroalgal-dominated epibenthic assemblages at different water depths and after transplantation from deep to shallow water on Spitsbergen

In the current study, we investigated the primary succession of seaweeds over different time periods at different water depths. Furthermore, we followed the succession of field-grown benthic communities of different successional age, developing on ceramic tiles, prior to and after transplantation from 8 to 0.5 m water depth. The transplantation simulated changes associated with the break up of sea-ice cover, e.g. light regime or wave exposure. For this purpose, we transplanted 12 and 21-month old communities, grown at 8 m water depth, together with a set of sterile tiles, onto rafts, floating in 0.5 m water depth. Our results describe for the first time the succession of macroalgal communities in the Arctic and give important insights into the effect of disturbance of differently aged communities. The primary succession at 0.5 m water depth was mainly driven by Bacillariophyta and filamentous green algae like Urospora sp. and Ulothrix implexa. Twelve-month old communities at 8 m water depth are dominated by members of the Ectocarpales (Phaeophyceae), like Pylaiella littoralis, P. varia, and Ectocarpus siliculosus and the green alga U. implexa, whereas the 21-month old community showed a higher cover of the green algal class Ulvophyceae and sessile invertebrates. After transplantation to near surface conditions, species composition of the communities changed, but this effect was differently strong between communities of different age.     

Influence of nitrogen supply and growth irradiance on photoinhibition and recovery in Heuchera americana (Saxifragaceae)

Prior work demonstrated that Heuchera americana, an evergreen herb inhabiting the deciduous forest understory in the southeastern United States, has a 3-4-fold greater photosynthetic capacity under the low-temperature, strong-light, open canopies of winter compared to the high-temperature, weak-light, closed canopies of summer. Moreover, despite the reductions in soil nitrogen, the chilling temperatures, and the increased quantum flux associated with winter, chronic photoinhibition was not observed in this species at this time of the year. We were interested in the photosynthetic acclimation and photoinhibition characteristics of this species when grown under contrasting light and nitrogen regimes. Newly expanded shade-acclimated leaves of forest-grown plants exposed to strong light varying in intensity and duration at 25°C showed a reduction in F_v/F_m (the ratio of variable to maximum room temperature chlorophyll fluorescence measured after dark adaptation), which was correlated with a decline in ø_a (the intrinsic quantum yield of CO₂-saturated O₂ evolution on an absorbed light basis). Plants grown in the glasshouse under contrasting light (high and low light; HL and LL, respectively) and nitrogen supply (high and low nitrogen; HN and LN, respectively) regimes showed that photosynthetic acclimation to HL was impaired in LN regimes. The HL-LN plants also had the lowest values of F_v/F_m and of ø on both incident and absorbed light bases and had 50% less chlorophyll (per unit area) compared to plants from other growth regimes. Controlled exposure to bright light at low temperatures (2-3°C) for 3 h resulted in a sharp decrease in F_v/F_m (and rise in F_o, the minimum fluorescence yield) in all plants. Shade-grown plants from both N regimes were highly susceptible to chronic photoinhibition, as indicated by a greater reduction in F_v/F_m and incomplete recovery after 18 h in weak light at 25°C. The HL-HN plants were the least susceptible to chronic photoinhibition, having the smallest decrease in F_v/F_m with near full recovery within 6 h. The decline in Fv/Fm in HL-LN plants was comparable to that of shade-acclimated plants, but recovered fully within 6 h. Low-N plants from both light regimes displayed greater increases in F_o which did not return to pretreatment levels after 18 h of recovery. These studies indicate that HL-LN plants were sensitive to chronic photoinhibition and, at the same time, had a high capacity for dynamic photoinhibition. Experimental garden studies showed that H. americana grown in an open field in summer were photoinhibited and did not fully recover overnight or during extended periods of weak light. These results are discussed in relation to the photosynthetic acclimation of H. americana under natural conditions.     

The importance of shore height and host identity for amphipod assemblages

The spatial distribution of organisms associated with marine intertidal macroalgae may be a direct result of their tolerance to air exposure or an indirect consequence of the distribution of their host. We compared amphipod assemblages from five intertidal macroalgae to investigate their relationship with algal identity. To test the effect of height regardless of algal characteristics, we transplanted coralline algal turfs to three different levels within the intertidal zone and compared amphipod assemblages after 1 and 14 days. Interstitial volume was positively correlated to the abundance of amphipods, suggesting that this attribute may correspond better to the potential space for their occupation when compared to algal biomass, thallus volume or the ratio between thallus and interstitial volume. Algal level determined the structure of the amphipod assemblages. Upper-level (Acanthophora spicifera and Caulerpa racemosa) and intermediate-level (coralline) algae host similar amphipod assemblages dominated by Apohyale media, but different from lower-level algae (Padina gymnospora and Sargassum cymosum), which were dominated by Hyale niger. Ten of the 15 amphipod species reported from natural communities were found in the transplanted plots. Distinct pools of amphipod species colonized coralline transplants at upper and lower levels after 1 day. However, regardless of the position on the shore, transplanted coralline turfs supported similar assemblages after 14 days, indicating that algal identity is also important for species assemblages. Our results suggest that both height on the shore and host identity combine to determine the vertical structure of amphipod assemblages in the rocky intertidal.     

Nitrogen supply as a factor influencing photoinhibition and photosynthetic acclimation after transfer of shade-grown Solanum dulcamara to bright light

We have compared the ability of shadegrown clones of Solamum dulcamara L. from shade and sun habitats to acclimate to bright light, as a function of nitrogen nutrition before and after transfer to bright light. Leaves of S. dulcamara grown in the shade with 0.6 mM NO ₃ ^- have similar photosynthetic properties as leaves of plants grown with 12.0 mM NO ₃ ^- . When transferred to bright light for 1–2 d the leaves of these plants show substantial photoinhibition which is characterized by about 50% decrease in apparent quantum yield and a reduction in the rate of photosynthesis in air at light saturation. Photoinhibition of leaf photosynthesis is associated with reduction in the variable component of low-temperature fluorescence emission, and with loss of in-vitro electron transport, especially of photosystem II-dependent processes.We find no evidence for ecotypic differentiation in the potential for photosynthetic acclimation among shade and sun clones of S. dulcamara, or of differentiation with respect to nitrogen requirements for acclimation. Recovery from photoinhibition and subsequent acclimation of photosynthesis to bright light only occurs in leaves of plants provided with 12.0 mM NO ₃ ^- . In these, apparent quantum yield is fully restored after 14 d, and photosynthetic acclimation is shown by an increase in light-saturated photosynthesis in air, of light-and CO₂-saturated photosynthesis, and of the initial slope of the CO₂-response curve. The latter changes are highly correlated with changes in ribulose-bisphosphate-carboxylase activity in vitro. Plants supplied with 0.6 mM NO ₃ ^- show incomplete recovery of apparent quantum yield after 14 d, but CO₂-dependent leaf photosynthetic parameters return to control levels.Symbols and abbreviations F_o initial level of fluorescence at 77 K - F_m maximum level of fluorescence at 77 K - F_v variable components of fluorescence at 77 K (F_v=F_m-F_o) - PSI, PSII photosystem I and II, respectively - RuBP ribulose-1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39)     

Photosynthetic acclimation and photoinhibition on exposure to high light in shade-developed leaves of Fagus crenata seedlings

The physiological response of leaves developed in low light (L) on Fagus crenata seedlings exposed to different levels of high light (H: high light, M: medium light) was studied. Measurements were conducted on potted seedlings in the F. crenata forest understory. The seedlings with leaves developed in L were transferred to H (L–H) and M (L–M) in summer. On exposure to high light, the photochemical efficiency of dark-adapted PSII (F_v/F_m) immediately decreased and was followed by a subsequent recovery in both L–H and L–M leaves. The mean value of F_v/F_m in L–H leaves was lower than that in L–M leaves through experiments, indicating that the degree of photoinhibition in L–H leaves was greater than that in L–M leaves. About 1 month after transfer, 37% and 5% of leaves had fallen in L–H and L–M seedlings, respectively. This result also indicated the greater photoinhibition in L–H leaves. Moreover, the photosynthetic capacity (P_Nmax) of L–H leaves decreased. In contrast, the P_Nmax of L–M leaves increased, although the P_Nmax was lower than that of M control leaves. An increase in the xanthophyll cycle pool (VAZ), indicating an increase of the photoprotective function, was found in both L–H and L–M leaves. Especially, the VAZ pool in L–M leaves was higher than that in M leaves by the end of experiments. L–M leaves may avoid photoinhibition effectively by the decrease in excess light with the increase of the P_Nmax or VAZ pool, compared to L–H leaves. Thus, the physiological acclimation on exposure to high light depended on the degree of high light. To achieve successful photosynthetic acclimation with slight photoinhibition, the variation of light intensity before and after exposure to high light would be an important factor because of the difference in excess light.     

Photosynthetic responses to short-term and long-term light variation in <Emphasis Type="Italic">Pinus sylvestris</Emphasis> and <Emphasis Type="Italic">Salix dasyclados</Emphasis>

Pinus sylvestris and Salix dasyclados, which differ in leaf longevity, were compared with respect to four aspects of photosynthetic light use and response: high light acclimation, photoinhibition resistance and recovery, lightfleck exposure and use and chloroplast acclimation across leaves. The first two aspects were examined using seedlings under controlled conditions and the other two were tested using trees in the field. When exposed to high light, shade leaves of Pinus acclimated completely, achieving the same photosynthetic capacities as sun leaves, whereas shade leaves of Salix did not reach sun leaf capacities although the absolute magnitude of their acclimation was larger. Shade leaves of Pinus were also more resistant to photoinhibition than those of Salix. Much of the direct light supplied within the canopy was in the form of rapid fluctuations, lightflecks, for Pinus and Salix alike. They exploited short lightflecks with similar efficiency. The greater proportion of diffuse light in the canopy for Pinus than Salix seems to lead to a lesser degree of differential intra-leaf acclimation of chloroplasts, in turn leading to lower efficiency of photosynthesis under unilateral light as reflected by a lower convexity, rate of bending, of the light–response curve. The differences in light use and responses are discussed in relation to possible differences in characteristics of the long and short-lived leaf.     

Primary production of epipsammic algal communities in Lake Balaton (Hungary)

Benthic algal communities can play an important role in matter and energy flux of shallow lakes. Their contribution to total primary production of lakes has been largely unexplored. The aim of this study was to estimate the primary production of the epipsammic algal communities at different water depths in Lake Balaton (Hungary) with photosynthetic measurements performed in laboratory. The photosynthesis of the benthic algae of different origin was studied at nine different irradiance levels, in three replicates. The maximum photosynthetic rate (P _max) was always higher in samples from the shallow parts than those from the deeper regions of the lake. Along the west–east longitudinal axis of the lake P _max decreased in the southern part and increased in the middle of the lake as a consequence of differences in the chlorophyll-a concentrations. Knowing P _max, I _k, global radiation and extinction coefficient, the primary production (mg C m⁻² day⁻¹) of the epipsammic algal community was calculated at different water depths. In the shallow regions at 0.5 and 1 m water depth 75–95% and 60–85% of the production was attributable to the epipsammon. The percentage contribution of epipsammon was at 2 m water depth 20–65%. In the deeper pelagic region (>3 m) more than 85% of the primary production originated from the phytoplankton.     

Fluorescence imaging of light acclimation of brazilian atlantic forest tree species

In the pursuit of knowledge on the biological behavior of Brazilian Atlantic Forest tree species, this study evaluated the susceptibility of the light-demanding species, Schinus terebinthifolia Raddi., Pseudobombax grandiflorum (Cav.) A. Robyns and Joannesia princeps Vell., and of the shade-tolerant species, Hymenaea courbaril L. var. stilbocarpa and Lecythis pisonis Camb, to photoinhibition and acclimation capacity. These species were first cultivated under two irradiance conditions, I₂₀ (20% direct sunlight radiation) and I₁₀₀ (all-sky or direct sunlight) and then transferred from I₂₀ to I₁₀₀. The effects of the sudden increase in light radiation intensity on photosynthetic activity were then evaluated through chlorophyll (Chl) fluorescence imaging, HPLC xanthophylls analysis, and cell membrane lipid peroxidation measurements. Light-demanding species were found to present a higher photochemical efficiency and higher acclimation capacity under high light irradiance than shade-tolerant species. The higher photoinhibition tolerance observed in light-demanding species was associated to their higher capacity for photochemical dissipation and dissipation of excess excitation energy via the xanthophyll cycle, leading to a lower ROS generation. The obtained results suggested that a knowledge of acclimation capacity, by means of Chl fluorescence imaging yields, is a useful indicator of species successional grouping.     

Short‐ and long‐term acclimation patterns of the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae) along a depth gradient

The giant kelp, Macrocystis pyrifera, is exposed to highly variable irradiance and temperature regimes across its geographic and vertical depth gradients. The objective of this study was to extend our understanding of algal acclimation strategies on different temporal scales to those varying abiotic conditions at various water depths. Different acclimation strategies to various water depths (0.2 and 4 m) between different sampling times (Jan/Feb and Aug/Sept 2012; long‐term acclimation) and more rapid adjustments to different depths (0.2, 2 and 4 m; short‐term acclimation) during 14 d of transplantation were found. Adjustments of variable Chl a fluorescence, pigment composition (Chl c, fucoxanthin), and the de‐epoxidation state of the xanthophyll cycle pigments were responsible for the development of different physiological states with respect to various solar radiation and temperature climates. Interestingly, the results indicated that phlorotannins are important during long‐term acclimation while antioxidants have a crucial role during short‐term acclimation. Furthermore, the results suggested that modifications in total lipids and fatty acid compositions apparently also might play a role in depth acclimation. In Aug/Sept (austral winter), M. pyrifera responded to the transplantation from 4 m to 0.2 m depth with a rise in the degree of saturation and a switch from shorter‐ to longer‐chain fatty acids. These changes seem to be essential for the readjustment of thylakoid membranes and might, thus, facilitate efficient photosynthesis under changing irradiances and temperatures. Further experiments are needed to disentangle the relative contribution of solar radiation, temperature and also other abiotic parameters in the observed physiological changes.     

In situ study of relative electron transport rates in the marine macroalga <Emphasis Type="Italic">Fucus vesiculosus</Emphasis> in the Baltic Sea at different depths and times of year

The brown alga Fucus vesiculous is one of the few marine species in the Baltic Sea. Fucus vesiculosus shows high morphological and physiological variability as a response to its environmental conditions. The salinity in the Baltic Sea is 4–5 psu, compared to 35 psu in the Atlantic. Photosynthesis of algae is usually measured after collection and transportation to constant culture conditions. However, in this study, relative photosynthetic electron transport rates, calculated from chlorophyll a fluorescence parameters were compared in algae collected from 1 and 4 m depths by SCUBA divers. Measurements of light response curves from the same individuals of F. vesiculosus at different depths and times of the year have, to our knowledge, not been made previously. Measurements were performed on four different occasions during the spring of 2005 (25 February, 3 and 29 April, and 26 May) in the Baltic Sea, using rapid light curves generated with a Diving PAM. In addition, samples were collected for photoinhibition studies in the laboratory. The light response curves obtained in situ at 1 and 4 m depths for F. vesiculosus showed lower values of light saturation with depth. When algae from 1 and 4 m depths were exposed to high irradiances of photosynthetically active radiation (1,400 μmol photons m⁻² s⁻¹), algae from 1 m depth showed a higher degree of photoinhibition in comparison to algae from 4 m depth.     

Cold acclimation and photoinhibition of photosynthesis in Scots pine

Cold acclimation of Scots pine did not affect the susceptibility of photosynthesis to photoinhibition. Cold acclimation did however cause a suppression of the rate of CO₂ uptake, and at given light and temperature conditions a larger fraction of the photosystem II reaction centres were closed in cold-acclimated than in nonacclimated pine. Therefore, when assayed at the level of photosystem II reaction centres, i.e. in relation to the degree of photosystem closure, cold acclimation caused a significant increase in resistance to photoinhibition; at given levels of photosystem II closure the resistance to photoinhibition was higher after cold acclimation. This was particularly evident in measurements at 20° C. The amounts and activities of the majority of analyzed active oxygen scavengers were higher after cold acclimation. We suggest that this increase in protective enzymes and compounds, particularly Superoxide dismutase, ascorbate peroxidase, glutathione reductase and ascorbate of the chloroplasts, enables Scots pine to avoid excessive photoinhibition of photosynthesis despite partial suppression of photosynthesis upon cold acclimation. An increased capacity for light-induced de-epoxidation of violaxanthin to zeaxanthin upon cold acclimation may also be of significance.Abbreviations APX ascorbate peroxidase - DHA dehydroascorbate - DHAR dehydroascorbate reductase - Fm maximal fluorescence when all reaction centres are closed - Fv/Fm maximum photochemical yield of PSII - GR glutathione reductase - GSH reduced glutathione - Je rate of photosynthetic electron transport - MDAR monodehydroascorbate reductase - q_N nonphotochemical quenching of fluorescence - q_P photochemical quenching of fluorescence - SOD superoxide dismutase This work was supported by the Swedish Natural Science Research Council and the National Natural Science Foundation of China.     

Photosynthetic characteristics and chloroplast ultrastructure of C3 and C4 tree species grown in high- and low-light environments

Plants of the C₄ tree species, Euphorbia forbesii, Sherff and the C₃ tree species, Claoxylon sandwicense Muell-Arg., were grown in a full sun and a shade environment designed to simulate the understory of their native Hawiian forest habitat. When grown under shade conditions, both species exhibited a photosynthetic light response typical of shade plants with low light compensation points and low dark respiration rates. E. forbesii, however, exhibited greater acclimation of light saturated photosynthetic rates and no evidence of photoinhibition in high light. In contrast, quantum yields for CO₂ uptake and chlorophyll contents were reduced in the high-light as compared to the low-light grown C. sandwicense plants. Both species exhibited similar changes in the intercellular CO₂ response curves and chloroplast whole-chain electron transport capacities, suggesting that the underlying mechanisms of light acclimation are similar. Chloroplasts of E. forbesii exhibited large changes in ultrastructure, with much greater thylakoid membrane development in low than high light. In contrast, C. sandwicense exhibited different starch contents, but otherwise similar membrane development in high and low light. The results show that E. forbesii possesses a very flexible photosynthetic apparatus which may account for its ability to survive in the understory of shaded forests.Abbreviations g_s = stomatal conductance - HL = high light - LL = low light - P_i = intercellular CO₂ partial pressure - PFD = photon flux density     

Effect of vertical cycling on photosynthesis during the annual algal succession in the northern Baltic

During the phytoplankton succession in 1984 and 1985, the effect of fluctuating light on algal photosynthesis (incorporation of ¹⁴C, acidified water sample) was studied in the northern Baltic. Bottles were mounted on moving racks that mimicked vertical transport caused by Langmuir circulations in the trophogenic layer. Assuming that the photoinhibition observed near the surface in fixed-depth incubations (from 1 to 8% of integral photosynthesis) was avoided in cycled samples, vertical cycling conducted around noon resulted in on average 10% lower photosynthesis than fixed-depth incubations (n = 17). This difference lies within the 5% confidence limits of the measurement, and hence it was concluded that the lack of short-term fluctuations in light associated with the vertical circulation of natural phytoplankton communities does not seriously bias conventional in situ ¹⁴CO₂ fixation measurements performed at fixed depths in the study area.     

Nitrogen fixation rates in algal turf communities of a degraded versus less degraded coral reef

Algal turf communities are ubiquitous on coral reefs in the Caribbean and are often dominated by N₂-fixing cyanobacteria. However, it is largely unknown (1) how much N₂ is actually fixed by turf communities and (2) which factors affect their N₂ fixation rates. Therefore, we compared N₂ fixation activity by turf communities at different depths and during day and night-time on a degraded versus a less degraded coral reef site on the island of Curaçao. N₂ fixation rates measured with the acetylene reduction assay were slightly higher in shallow (5–10-m depth) than in deep turf communities (30-m depth), and N₂ fixation rates during the daytime significantly exceeded those during the night. N₂ fixation rates by the turf communities did not differ between the degraded and less degraded reef. Both our study and a literature survey of earlier studies indicated that turf communities tend to have lower N₂ fixation rates than cyanobacterial mats. However, at least in our study area, turf communities were more abundant than cyanobacterial mats. Our results therefore suggest that turf communities play an important role in the nitrogen cycle of coral reefs. N₂ fixation by turfs may contribute to an undesirable positive feedback that promotes the proliferation of algal turf communities while accelerating coral reef degradation.     

Acclimation to low ultraviolet-B radiation increases photosystem I abundance and cyclic electron transfer with enhanced photosynthesis and growth in the cyanobacterium Nostoc sphaeroides

Ultraviolet-B radiation is known to harm most photosynthetic organisms with the exception of several studies of photosynthetic eukaryotes in which UV-B showed positive effects. In this study, we investigated the effect of acclimation to low UV-B radiation on growth and photosynthesis of the cyanobacterium Nostoc sphaeroides. Exposure to 0.08 W m⁻² UV-B plus low visible light for 14 d significantly increased the growth rate and biomass production by 16% and 30%, respectively, compared with those under visible light alone. The UV-B acclimated cells showed an approximately 50% increase in photosynthetic efficiency (α) and photosynthetic capacity (P_max), a higher PSI/PSII fluorescence ratio, an increase in PSI content and consequently enhanced cyclic electron flow, relative to those of non-acclimated cells. Both the primary quinone-type acceptor and plastoquinone pool re-oxidation were up-regulated in the UV-B acclimated cells. In parallel, the UV-B acclimated colonies maintained a higher rate of D1 protein synthesis following exposure to elevated intensity of UV-B or visible light, thus functionally mitigating photoinhibition. The present data provide novel insight into photosynthetic acclimation to low UV-B radiation and suggest that UV-B may act as a positive ecological factor for the productivity of some photosynthetic prokaryotes, especially during twilight periods or in shaded environments.     

Photosynthesis,photoinhibition and low temperature acclimation in cold tolerant plants

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep Q_A oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO₂ fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.     

Ecological studies of Chloroflexis,a gliding photosynthetic bacterium

Summary Chloroflexis, a gliding, filamentous, photosynthetic bacterium, is present in the stratified algal-bacterial mats which occur in the 50°–70°C temperature range of alkaline hot spring effluents. The organism is in association with the alga in the upper, algal layer, and also forms thick, orange mats beneath the algal layer. Natural populations of Chloroflexis from these mats demonstrated light-stimulated uptake of some ¹⁴C-labelled organic compounds. Photosynthetic ¹⁴CO₂ fixation by natural samples of Chloroflexis was investigated with respect to temperature, light intensity and mat depth. Bacterial photosynthesis was determined in samples in which algae were present by use of the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). Bacterial photosynthesis was maximal at depths down to about 3 mm and then decreased rapidly to very low levels at greater depths. The greatest amounts of bacteriochlorophyll pigments were also concentrated in the top 3–4 mm of the mat. The optimum light intensity for bacterial photosynthesis (about 400 ft-c) was considerably lower than the normal summer light intensity at the surface of the mat (5000-8000 ft-c).The temperature optima for photosynthesis by the bacterial component of natural mat samples from several sites of different temperatures in a hot spring thermal gradient were determined. Temperature optima approximated the environmental temperatures, indicative of the occurrence of strains of Chloroflexis adapted to different temperatures. Although bacterial standing crop was greatest in the temperature range 50°–55°C, maximum photosynthetic efficiency was observed at about 45°C. Sulfide was stimulatory to photosynthetic ¹⁴CO₂ fixation by naturally occurring populations of Chloroflexis under field conditions. These data are consistent with the hypothesis that Chloroflexis may utilize sulfide as an electron donor for photosynthetic CO₂ reduction. However, it is also likely that Chloroflexis grows photoheterotrophically in these mats, obtaining organic compounds from algal excretory products.