Unit Chlorophyll Research Articles

Native hemiparasite and light effects on photoprotection and photodamage in a native host.

Plants infected with hemiparasites often have lowered rates of photosynthesis, which could make them more susceptible to photodamage. However, it is also possible that infected plants increase their photoprotective capacity by changing their pigment content and/or engagement of the xanthophyll cycle. There are no published studies investigating infection effects on host pigment dynamics and how this relates to host susceptibility to photodamage whether in high (HL) or low light (LL). A glasshouse experiment was conducted where Leptospermum myrsinoides Schltdl. either uninfected or infected with Cassytha pubescens R.Br. was grown in HL or LL and pigment content of both host and parasite were assessed. Infection with C. pubescens significantly decreased all foliar pigment concentrations (except chlorophyll b) in L. myrsinoides in both HL and LL. Xanthophyll cycle (violaxanthin, antheraxanthin, zeaxanthin; VAZ) and chlorophyll (Chl) pigments decreased in parallel in response to infection, hence, VAZ/Chl of the host was unaffected by C. pubescens in either HL or LL. Pre-dawn and midday de-epoxidation state [(A+Z)/(V+A+Z)] of L. myrsinoides was also unaffected by infection in both HL and LL. Thus, L. myrsinoides infected with C. pubescens maintained similar photoprotective capacity per unit chlorophyll and engagement of the xanthophyll cycle as uninfected plants. Even though midday quantum yield (ΦPSII) of HL plants was affected by infection, pre-dawn maximum quantum yields (Fv/Fm) of hosts were the same as uninfected plants whether in HL or LL. This ability of L. myrsinoides to maintain photoprotective capacity/engagement when infected by C. pubescens thereby preventing photodamage could explain this host's tolerance to hemiparasite infection.

Gap effects on leaf traits of tropical rainforest trees differing in juvenile light requirement

The relationships of 16 leaf traits and their plasticity with the dependence of tree species on gaps for regeneration (gap association index; GAI) were examined in a Neotropical rainforest. Young saplings of 24 species with varying GAI were grown under a closed canopy, in a medium-sized and in a large gap, thus capturing the full range of plasticity with respect to canopy openness. Structural, biomechanical, chemical and photosynthetic traits were measured. At the chloroplast level, the chlorophyll a/b ratio and plasticity in this variable were not related to the GAI. However, plasticity in total carotenoids per unit chlorophyll was larger in shade-tolerant species. At the leaf level, leaf mass per unit area (LMA) decreased with the GAI under the closed canopy and in the medium gap, but did not significantly decrease with the GAI in the large gap. This was a reflection of the larger plasticity in LMA and leaf thickness of gap-dependent species. The well-known opposite trends in LMA for adaptation and acclimation to high irradiance in evergreen tropical trees were thus not invariably found. Although leaf strength was dependent on LMA and thickness, plasticity in this trait was not related to the GAI. Photosynthetic capacity expressed on each basis increased with the GAI, but the large plasticity in these traits was not clearly related to the GAI. Although gap-dependent species tended to have a greater plasticity overall, as evident from a principle component analysis, leaf traits of gap-dependent species are thus not invariably more phenotypically plastic.

Sex-related differences in lipid peroxidation and photoprotection in Pistacia lentiscus.

Sex-related differences in the response of dioecious plants to abiotic stress have been poorly studied to date. This work explored to what extent sex may affect plant stress responses in Pistacia lentiscus L. (Anacardiaceae), a tree well adapted to Mediterranean climatic conditions. It was hypothesized that a greater reproductive effort in females may increase oxidative stress in leaves, particularly when plants are exposed to abiotic stress. Measurements of oxidative stress markers throughout the year revealed increased lipid peroxidation in females, but only during the winter. Enhanced lipid peroxidation in females was associated with reduced photoprotection, as indicated by reduced tocopherol levels and nonphotochemical quenching (NPQ) of chlorophyll fluorescence. Enhanced lipid peroxidation in females was also observed at predawn, which was associated with increased lipoxygenase activity and reduced cytokinin levels. An analysis of the differences between reproductive (R) and nonreproductive (NR) shoots showed an enhanced photoprotective capacity in R shoots compared to NR shoots in females. This capacity was characterized by an increased NPQ and a better antioxidant protection (increased carotenoid and tocopherol levels per unit of chlorophyll) in R compared to NR shoots. It is concluded that (i) females exhibit higher lipid peroxidation in leaves than males, but only during the winter (when sex-related differences in reproductive effort are the highest), (ii) this is associated with a lower photoprotective capacity at midday, as well as enhanced lipoxygenase activity and reduced cytokinin levels at predawn, and (iii) photoprotection capacity is higher in R relative to NR shoots in females.

Study of dynamics of phytoplankton and photosynthetic parameters using opportunity ships in the western English Channel

The dynamics of physicochemical, phytoplankton communities and photosynthetic parameters were monitored monthly along a transect in the western English Channel from a ship of opportunity. The transect was divided into three areas, based on the hydrological properties of the water mass: the French area (area 1), a station in the centre of the Channel (area 2), and the English area (area 3). In area 1, a bloom occurred in summer/autumn. Due to the gradual changes of physicochemical parameters, area 1 can be described as an ecocline where species adapted to a wide range of physicochemical properties were observed. The area was characterised by the co-dominance of diatoms and dinoflagellates. In area 2, a spot with a high concentration of dissolved inorganic phosphate (DIP) was sampled before and after a bloom of Karenia mikimotoï, suggesting that DIP availability may explain the occurrence of this dinoflagellate species in this area. Area 3 was characterised by two phytoplankton peaks, one in spring and one in late summer/early autumn. Production was controlled by light in area 1, by nutrients in area 2, and by light in spring and by nutrients in summer/autumn in area 3. A link between the chlorophyll a (Chla) biomass and primary production was not systematically observed, suggesting that Chla is not a good proxy for production. On the other hand, a strong link was observed between the relative maximum electron transport rate (rETRmax) and the maximum electron transport rate per chlorophyll unit (Chl_ETRmax). rETRmax consequently appears to be a good indicator of the dynamics of productivity in this area, making it possible to monitor it using PAM (pulse amplitude modulated) from ships of opportunity.

Photosynthetically relevant foliar traits correlating better on a mass vs an area basis: of ecophysiological relevance or just a case of mathematical imperatives and statistical quicksand?

Plant biologists have long had a choice of the dimensions in which to express their studied traits and/or processes. For example, a plant physiologist working at the leaf level might typically measure and report photosynthetic rates as a flux per unit leaf area (e.g. μmol CO₂ m⁻² s⁻¹) whereas a biochemist might typically express the same process per unit chlorophyll. An agronomist or forester would usually be more interested in dry matter accumulation rate per unit ground area (Mg ha⁻¹ yr⁻¹) or sometimes even as a relative growth rate (g DW g⁻¹ DW d⁻¹). Although expressing leaf-level photosynthesis on an area basis seems intuitive and was for decades the standard practice, starting with Field & Mooney (1986) and then Reich & Walters (1994), there has been an increasing tendency to express the photosynthetic characteristics of leaves on a dry-weight basis (typically nmol CO₂ g⁻¹ s⁻¹). This trend has been due, at least in part, to stronger correlations for mass-based photosynthetic rates with foliar properties thought to be important in their modulation (Reich et al., 1998). Weaker associations between foliar properties when expressed on an area-basis have also provided one rationale for the inclusion of mass-based measures of photosynthesis, nitrogen and phosphorus into a so-called 'leaf economics spectrum' (Wright et al., 2004) and with mass-based measures of photosynthetic carbon exchange subsequently underlying further analyses (Shipley et al., 2006). Some modelling studies investigating the relative importance of nitrogen vs phosphorus as modulators of leaf photosynthetic capacity have likewise been parameterized on a mass rather than an area-basis because of the apparently superior model fit of the former (Domingues et al., 2010).

Effect of inland salt-alkaline stress on C<sub>4</sub> enzymes, pigments, antioxidant enzymes, and photosynthesis in leaf, bark, and branch chlorenchyma of poplars

The effects of soil salt-alkaline (SA) stress on leaf physiological processes are well studied in the laboratory, but less is known about their effect on leaf, bark and branch chlorenchyma and no reports exist on their effect on C4 enzymes in field conditions. Our results demonstrated that activities of C4 enzymes, such as phospholenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME), pyruvate orthophosphate dikinase (PPDK), and NADP-dependent malate dehydrogenase (NADP-MDH), could also be regulated by soil salinity/alkalinity in poplar (Populus alba × P. berolinensis) trees, similarly as the already documented changes in activities of antioxidative enzymes, such as superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR), pigment composition, photosynthesis, and respiration. However, compared with 50-90% changes in a leaf and young branch chlorenchyma, much smaller changes in malondialdehyde (MDA), antioxidative enzymes, and C4 enzymatic activities were observed in bark chlorenchyma, showing that the effect of soil salinity/alkalinity on enzymatic activities was organ-dependent. This suggests that C4 enzymatic ratios between nonleaf chlorenchyma and leaf (the commonly used parameter to discern the operation of the C4 photosynthetic pathway in nonleaf chlorenchyma), were dependent on SA stress. Moreover, much smaller enhancement of these ratios was seen in an improved soil contrary to SA soil, when the fresh mass (FM) was used as the unit compared with a calculation on a chlorophyll (Chl) unit. An identification of the C4 photosynthesis pathway via C4 enzyme difference between chlorenchyma and leaf should take this environmental regulation and unit-based difference into account.

Retrospective satellite ocean color analysis of purposeful and natural ocean iron fertilization

Significant effort has been invested in understanding the role of iron in marine ecosystems over the past few decades. What began as shipboard amendment experiments quickly grew into a succession of in situ, mesoscale ocean iron fertilization (OIF) experiments carried out in all three high nutrient low chlorophyll (HNLC) regions of the world ocean. Dedicated process studies have also looked at regions of the ocean that are seasonally exposed to iron-replete conditions as natural OIF experiments. However, one problem common to many OIF experiments is determination of biological response beyond the duration of the experiment (typically<1 month). Satellite-derived products have been used to address this shortcoming with some success, but thus far, have been limited snapshots of a single parameter, chlorophyll. Here, we investigate phytoplankton responses to OIF in both purposeful and naturally iron enriched systems using estimates of chlorophyll (Chl), phytoplankton carbon biomass (Cphyto), their ratio (Chl:Cphyto) and two fluorescence indices, fluorescence per unit chlorophyll (FLH:Chl) and the chlorophyll fluorescence efficiency (ϕf). These quantities allow partitioning of the biological response to OIF into that due to changes in biomass and that due to phytoplankton physiology. We find that relative increases in Chl (∼10–20x) following OIF far exceed increases in Cphyto (<4–5x), suggesting that a significant fraction of the observed Chl increase is associated with physiological adjustment to increased growth rates, photoacclimation, and floristic shifts in the phytoplankton community. Further, a consistent pattern of decreased satellite fluorescence efficiency (FLH:Chl or ϕf) following OIF is observed that is in agreement with current understanding of phytoplankton physiological responses to relief from iron stress. The current study extends our ability to retrieve phytoplankton physiology from space-based sensors, strengthens the link between satellite fluorescence and iron availability, and shows that satellite ocean color analyses provide a unique tool for monitoring OIF experiments.

Compositional differences of chromophoric dissolved organic matter derived from phytoplankton and macrophytes

Chromophoric dissolved organic matter (CDOM) is an important component in the aquatic environment and plays a key role in light attenuation and in carbon biogeochemical cycles. We examined CDOM production in each of two laboratory experiments in which phytoplankton and macrophyte degradation were monitored using absorption and excitation–emission matrix fluorescence spectroscopy (EEMs). During the incubation period, CDOM was produced from phytoplankton and macrophytes, and partly decomposed by microorganisms. The absorption spectra of the phytoplankton derived and the macrophyte derived CDOM were distinct and characterized by peaks and shoulders in the UV bands. Production of CDOM absorption at 350nm, a(350), was 0.0125m2/g per unit of chlorophyll a from phytoplankton CDOM from 0–3d. Meanwhile a(350) production was 2.708×10−4m2/g per unit of wet biomass from macrophytes CDOM from 1–7d. Despite the high production of CDOM by phytoplankton and macrophytes, extrapolation of these values to the field indicated that about 15% of total CDOM was produced from phytoplankton during algal blooms in Meiliang Bay in summer and about 8% of total CDOM was produced from macrophytes in the macrophyte dominated littorals. The mean value of the spectral slope (S) describing the exponential decrease of the absorption spectrum, which was strongly correlated to an optical index of molecular size, for the phytoplankton derived CDOM was 10.26±2.05μm−1, which was significantly lower than the mean S of 14.47±2.88μm−1 for the macrophyte derived CDOM (t-test, p<0.001). The mean value of the spectral slope ratio (SR) for the phytoplankton derived CDOM was 1.79±0.52, which was significantly higher than that of 0.35±0.58 for the macrophyte derived CDOM (t-test, p<0.001). Three fluorescent components were validated in parallel factor analysis (PARAFAC) models calculated separately for phytoplankton derived and macrophyte derived CDOM, each CDOM source resulting in distinct excitation and emission maxima for each component. The significant differences in CDOM absorption spectra, S, SR and PARAFAC fluorescence component characteristics, all showed that phytoplankton derived CDOM was compositionally distinct from macrophyte derived CDOM. Overall both sources were important to the CDOM pool in the shallow temperate lake.

Reply to comment by J. Marra on “Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA”

Reply to comment by J. Marra on “Measuring primary production rates in the ocean: Enigmatic results between incubation and non‐incubation methods at Station ALOHA”

Spectral model of depth‐integrated water column photosynthesis and its inhibition by ultraviolet radiation

Depth‐integrated models of primary production (DIMs) are used to estimate water column photosynthesis as a function of chlorophyll concentration, irradiance at the surface, the penetration of photosynthetically available radiation (PAR), and parameters of the relationship between photosynthesis and PAR. These models are inherently unable to account for variability in the ratio of photosynthetically utilizable radiation (PUR) to PAR with depth and water type, and they cannot account for the inhibition of photosynthesis by ultraviolet radiation, UVR. These important spectral effects — all sensitive to climate change — are readily described with numerical models that require many computations and are unsuitable for some important applications, including the estimation of aquatic productivity from remote sensing. We present a simple DIM that accounts for the spectral effects of irradiance on photosynthesis, including inhibition by UVR. Water column photosynthesis, normalized to surface chlorophyll and scaled to the maximum rate per unit chlorophyll, is described as a function of four dimensionless derived variables:E*PUR, PUR at the surface scaled to the saturation irradiance for photosynthesis; T*PUR, water transparency, normalized to a depth scale and weighted spectrally for photosynthetic absorption; E*PIR, surface irradiance weighted spectrally for inhibition of photosynthesis; and T*PIR, scaled transparency weighted for photosynthesis‐inhibiting radiation. Simple functions of these variables closely approximate (within 6%) the results of a full‐spectral numerical model of instantaneous and daily integrated water column photosynthesis with and without UVR for a broad range of water types, solar angles, stratospheric ozone concentrations and biological properties of phytoplankton. The spectral DIM is suitable for examining patterns in global ocean productivity and can be used to assess the biological effects of variations in solar radiation (e.g., ozone depletion) and water clarity in climate‐change scenarios for lakes and oceans.

Effect of vertical mixing on short-term mycosporine-like amino acid synthesis in the Antarctic diatom, &lt;i&gt;Thalasiossira&lt;/i&gt; sp.

One of the adaptations whereby phytoplankton can alleviate damage induced by ultraviolet radiation (280-400 nm) is the synthesis of mycosporine-like amino acids (MAAs). The synthesis of MAAs was studied after exposure of the Antarctic diatom Thalassiosira sp. isolated from Potter Cove (South Shetland Is., Antarctica) to 2 treatments with a solar simulator: surface (Sfix) and vertical mixing (Mix) irradiance conditions. Light exposure was simulated in daily cycles with maximum irradiance at noon. Only 2 MAAs, Porphyra-334 (82-85%) and Shinorine (15-18%), were identified. The concentration of the two compounds increased during experimental light exposure (50-55%) and declined in the dark (10-15%). During the light period the synthesis rate of MAAs per unit of chlorophyll a was higher in the Sfix treatment (µ=0.17 h -1 ) than in the Mix treatment (µ=0.05 h -1 ). In spite of the higher MAA levels, low cell numbers were observed in the Sfix treatment, suggesting that the algae synthesized photoprotective compounds at the expense of growth. Our results document overlapping effects of both daily light cycles and vertical mixing affecting the synthesis of MAAs. This, and the high thermal dissipation of the ultraviolet B radiation energy (280-320 nm) absorbed by these substances, suggest a rapid photoadaptive response of Thalasiossira sp. upon exposure to elevated irradiance in a stratified water column, as well as the complementary role of vertical mixing in photo-protection.

Xanthophyll cycle pigment and antioxidant profiles of winter-red (anthocyanic) and winter-green (acyanic) angiosperm evergreen species

Leaves of many angiosperm evergreen species change colour from green to red during winter, corresponding with the synthesis of anthocyanin pigments. The ecophysiological function of winter colour change (if any), and why it occurs in some species and not others, are not yet understood. It was hypothesized that anthocyanins play a compensatory photoprotective role in species with limited capacity for energy dissipation. Seasonal xanthophyll pigment content, chlorophyll fluorescence, leaf nitrogen, and low molecular weight antioxidants (LMWA) of five winter-red and five winter-green angiosperm evergreen species were compared. Our results showed no difference in seasonal xanthophyll pigment content (V+A+Z g(-1) leaf dry mass) or LMWA between winter-red and winter-green species, indicating red-leafed species are not deficient in their capacity for non-photochemical energy dissipation via these mechanisms. Winter-red and winter-green species also did not differ in percentage leaf nitrogen, corroborating previous studies showing no difference in seasonal photosynthesis under saturating irradiance. Consistent with a photoprotective function of anthocyanin, winter-red species had significantly lower xanthophyll content per unit chlorophyll and less sustained photoinhibition than winter-green species (i.e. higher pre-dawn F(v)/F(m) and a lower proportion of de-epoxidized xanthophylls retained overnight). Red-leafed species also maintained a higher maximum quantum yield efficiency of PSII at midday (F'(v)/F'(m)) during winter, and showed characteristics of shade acclimation (positive correlation between anthocyanin and chlorophyll content, and negative correlation with chlorophyll a/b). These results suggest that the capacity for photon energy dissipation (photochemical and non-photochemical) is not limited in red-leafed species, and that anthocyanins more likely function as an alternative photoprotective strategy to increased VAZ/Chl during winter.

Ontogenetic changes in leaf traits of tropical rainforest trees differing in juvenile light requirement

Relationships between leaf traits and the gap dependence for regeneration, and ontogenetic changes therein, were investigated in juvenile and adult tropical rainforest tree species. The juveniles of the 17 species included in the study were grown in high light, similar to the exposed crowns of the adult trees. The traits were structural, biomechanical, chemical and photosynthetic. With increasing species gap dependence, leaf mass per area (LMA) decreased only slightly in juveniles and remained constant in adults, whereas punch strength together with tissue density decreased, and photosynthetic capacity and chlorophyll increased. Contrary to what has been mostly found in evergreen tropical rainforest, the trade-off between investment in longevity and in productivity was evident at an essentially constant LMA. Of the traits pertaining to the chloroplast level, photosynthetic capacity per unit chlorophyll increased with gap dependence, but the chlorophyll a/b ratio showed no relationship. Adults had a twofold higher LMA, but leaf strength was on average only about 50% larger. Leaf tissue density, and chlorophyll and leaf N per area were also higher, whereas chlorophyll and leaf N per unit dry mass were lower. Ranking of the species, relationships between traits and with the gap dependence of the species were similar for juveniles and adults. However, the magnitudes of most ontogenetic changes were not clearly related to a species’ gap dependence. The adaptive value of the leaf traits for juveniles and adults is discussed.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-011-2175-x) contains supplementary material, which is available to authorized users.

Bio-optical properties of east coast Malaysia waters in relation to remote sensing of chlorophyll

Surface water samples collected during the monsoon and inter-monsoon seasons of 2009 off the east coast of Peninsular Malaysia have been analysed for concentrations of total chlorophyll, suspended particulates and coloured dissolved organic matter (CDOM). Spectral absorption coefficients of dissolved and particulate materials have also been measured. Significant seasonal variabilities in concentrations and optical properties were reported with high concentrations of all parameters during the northeast monsoon (NEM) season and low during the southwest monsoon (SWM) and inter-monsoon seasons. Contrary to previous reports on the oligotrophic nature of the waters during the inter-monsoon season, relatively high concentrations of chlorophyll (>3 mg m−3) were observed at offshore stations in the study area in the spring and fall inter-monsoon months. The chlorophyll-specific absorption spectrum changes with the seasons with the greatest absorption per unit chlorophyll during the SWM and the least during the inter-monsoon seasons, probably in response to seasonal changes in phytoplankton community and cell size structure. The water is classified as optical case 2. At the blue end of the spectrum (440 nm), light absorption by non-phytoplankton materials (CDOM and detritus) accounts for nearly 70% of the total non-water absorption regardless of the season. At the wavelength (676 nm) of the secondary chlorophyll absorption peak in the red part of the spectrum, light absorption by chlorophyll contributes 80–90% to total non-water absorption at most stations and this may provide the basis for remote sensing of phytoplankton chlorophyll in these waters.

Context-Specific Bioturbation Mediates Changes to Ecosystem Functioning

Species are often grouped according to their biological or functional traits to better understand their contribution to ecosystem functioning. However, it is becoming clear that a single species can perform different roles in different habitats. Austrohelice crassa, a burrow-building mud crab shifts its primary bioturbational role to that of a vertical mixer in non-cohesive sediments as frequent burrow collapse greatly enhances sediment reworking. We conducted in situ crab density manipulations in two sediment environments (a non-cohesive sand and a cohesive muddy-sand) to examine if the context-specific functional roles were linked to changes in solute fluxes across the sediment–water interface. Across both habitats, we show that A. crassa regulated nutrient cycling, creating strong density driven effects on solute exchanges. Increasing crab density increased sediment O2 demand and the flux of NH4 + from the sediment, indicating much of the response was physiologically driven. Clear interactions between A. crassa and microphytobenthos were also detected in both habitats. Despite lowering microphyte standing stock through deposit feeding, A. crassa increased benthic primary production per unit of chlorophyll a. Our experiment also revealed important context-specific differences, most notably for NH4 + fluxes, which were higher where burrows and their associated microbial communities were most stable (muddy-sand). This study highlights the need to integrate interactions between organism behavior and habitat type into functional group studies to broaden conceptual frameworks and avoid oversimplification of highly complex organism–sediment interactions.

Effect of light conditions of wheat growing on sensitivity of photosynthetic machinery to salt stress

Sensitivity of wheat (Triticum aestivum L.) seedlings to salt stress was investigated as dependent on light conditions of plant growing. In two-week-old seedlings grown on salt-free medium, aboveground organs were detached from the roots and subjected to a brief stress at different concentrations of NaCl. The extent of salt stress effect expressed as a decrease in the rate of the photosynthetic release of oxygen and the relative content of water and chlorophyll in the leaves greatly depended on light conditions of growing. The plants grown at low light intensity were notable for a greater sensitivity to NaCl in the medium. Plant responses to salt stress were different at low and high salt concentrations. At low NaCl concentrations (0.05–0.10 M) in the solution, in plants grown at low light intensity, the rate of photosynthesis calculated per unit of chlorophyll increased. This effect was not observed in plants grown at the higher light intensity. At high NaCl concentrations (0.2–0.4 M) in the medium, the rate of photosynthesis rapidly decreased in all the types of treatment, with the effect being most pronounced in plants grown at low light intensity. The obtained results suggest a narrow range of NaCl concentrations with an optimum at 0.1 M positively affecting the wheat seedlings physiological state upon salt stress development depending on light conditions of plant growing.

Phosphonate metabolism by Trichodesmium IMS101 and the production of greenhouse gases

Herein we present the results of a series of laboratory experiments conducted to delineate the relationship between dissolved organic phosphorus (DOP) utilization and organic matter production by the model diazotroph Trichodesmium IMS101. The rate and ratio of Trichodesmium carbon (C) and nitrogen (N) production driven by phosphorus (P) derived from select phosphonates and P monoesters were not found to be significantly different than growth rates based on dissolved inorganic phosphorus (DIP). Conversely, despite the observed similarities in C and N fixation rates between parallel DIP and DOP incubations, cultures grown solely on methylphosphonate (MPn) had reduced maximal P content per unit chlorophyll a or C relative to cultures grown on P monoesters or DIP. So while Trichodesmium IMS101 appears to be able to maintain similar rates of growth on DIP and select DOP substrates, the P uptake efficiency and, hence, the P content per cell is greater for growth on DIP or tested P monoesters relative to growth on the phosphonate MPn. The primary conclusion arising from these results is that neither the observation of P monoester hydrolysis nor highly P‐deficient organic matter or expression of phosphonate hydrolyzing genes can be interpreted to indicate P limitation of C or N fixation of Trichodesmium spp. under light‐saturated, DIP‐deficient conditions.

Microalgal biomass production: challenges and realities

The maximum quantum yield (Φ (max)), calculated from the maximum chlorophyll a specific photosynthetic rate divided by the quantum absorption per unit chlorophyll a, is 8 photons or 0.125 mol C per mol Quanta light energy. For the average solar radiation that reaches the earth's surface this relates to a photosynthetic yield of 1.79 g(dw) m(-2) day(-1) per percentage photosynthetic efficiency and it could be doubled for sunny, dry and hot areas. Many factors determine volumetric yields of mass algal cultures and it is not simply a question of extrapolating controlled laboratory rates to large scale outdoor production systems. This is an obvious mistake many algal biotechnology start-up companies make. Closed photobioreactors should be able to outperform open raceway pond cultures because of the synergistic enhancement of a reduced boundary layer and short light/dark fluctuations at high turbulences. However, this has not been shown on any large scale and to date the industrial norm for very large production systems is open raceway production ponds. Microalgal biomass production offers real opportunities for addressing issues such as CO(2) sequestration, biofuel production and wastewater treatment, and it should be the preferred research emphasis.

Inactivation of Genes Encoding Plastoglobulin-Like Proteins in Synechocystis sp. PCC 6803 Leads to a Light-Sensitive Phenotype

Plastoglobulins (PGL) are the predominant proteins of lipid globules in the plastids of flowering plants. Genes encoding proteins similar to plant PGL are also present in algae and cyanobacteria but in no other organisms, suggesting an important role for these proteins in oxygenic photosynthesis. To gain an understanding of the core and fundamental function of PGL, the two genes that encode PGL-like polypeptides in the cyanobacterium Synechocystis sp. PCC 6803 (pgl1 and pgl2) were inactivated individually and in combination. The resulting mutants were able to grow under photoautotrophic conditions, dividing at rates that were comparable to that of the wild-type (WT) under low-light (LL) conditions (10 microeinsteins x m(-2) x s(-1)) but lower than that of the WT under moderately high-irradiance (HL) conditions (150 microeinsteins x m(-2) x s(-1)). Under HL, each Deltapgl mutant had less chlorophyll, a lower photosystem I (PSI)/PSII ratio, more carotenoid per unit of chlorophyll, and very much more myxoxanthophyll (a carotenoid symptomatic of high light stress) per unit of chlorophyll than the WT. Large, heterogeneous inclusion bodies were observed in cells of mutants inactivated in pgl2 or both pgl2 and pgl1 under both LL and HL conditions. The mutant inactivated in both pgl genes was especially sensitive to the light environment, with alterations in pigmentation, heterogeneous inclusion bodies, and a lower PSI/PSII ratio than the WT even for cultures grown under LL conditions. The WT cultures grown under HL contained 2- to 3-fold more PGL1 and PGL2 per cell than cultures grown under LL conditions. These and other observations led us to conclude that the PGL-like polypeptides of Synechocystis play similar but not identical roles in some process relevant to the repair of photooxidative damage.

Effects of photoacclimation on the light niche of corals: a process-based approach

The ecology of photosynthetic organisms is influenced by the need to adjust the photosynthetic apparatus to variable light environments (photoacclimation). In this study, we quantified different components of the photoacclimation process for a reef-building coral (Turbinaria mesenterina, Lamarck, 1816): including, variation in absorption cross-section, size of photosynthetic units, turnover time, chlorophyll content, and colony respiration. We used these calibrations to characterize this species’ light niche, and to determine the sensitivity of the niche boundaries to different processes of photoacclimation. Results showed that the breadth of the light niche was most sensitive to the size of the photosynthetic unit, absorption cross-section, and rates of respiration. Habitats with the highest light availability did not lead to maximal energy acquisition. This was because, although corals acclimated to high light have high rates of photosynthesis per unit chlorophyll, their chlorophyll content was strongly reduced. This suggests that potential energetic benefits that could be achieved through increased light harvesting (i.e., increased chlorophyll content) in high-light habitats are outweighed by costs associated with photoprotection. Such costs appear to place an upper bound on the habitat distributions of coral species. Our approach reveals how the photophysiological processes involved in photoacclimation interact to determine the light niche.