UV effects on stoichiometry and PUFAs of Selenastrum capricornutum and their consequences for the grazer Daphnia magna

1. The effect of ultraviolet (UV) radiation (280–400 nm) on fatty acid composition and elemental ratios of carbon (C), nitrogen (N) and phosphorus (P) of a unialgal culture of the chlorophyte Selenastrum capricornutum was investigated. Algae were cultured in the presence or absence of UV radiation and were subsequently fed to Daphnia magna to assess potential effects of UV on zooplankton fatty acid composition, growth and reproduction. 2. Algal growth rate was substantially reduced by UV radiation, probably because of a severe inhibition of photosynthetic efficiency (measured as optimal quantum yield). 3. Algae exposed to UV radiation had a significantly reduced content of 18 : 1 n‐9, while C18 polyunsaturated fatty acids (PUFAs) were higher under UV radiation. These observations point at an increased demand for and synthesis of PUFAs under UV stress. 4. The C : P and N : P ratios showed a remarkable decrease in UV‐exposed cells primarily owing to an increased uptake of P. The nutritional quality in terms of both fatty acid composition and stoichiometry was therefore higher in the UV treatment relative to the control. 5. Despite the UV‐induced changes in nutritional quality of S. capricornutum, no significant effects on D. magna growth or reproduction were detected. The fatty acid composition of Daphnia fed on UV irradiated algae showed a significantly lower content of 18 : 1 n‐9, but no changes in the essential PUFAs.     

Ambient UV-B radiation reduces PSII performance and net photosynthesis in high Arctic Salix arctica

Ambient ultraviolet-B (UV-B) radiation potentially impacts the photosynthetic performance of high Arctic plants. We conducted an UV-B exclusion experiment in a dwarf shrub heath in NE Greenland (74°N), with open control, filter control, UV-B filtering and UV-AB filtering, all in combination with leaf angle control. Two sites with natural leaf positions had ground angles of 0° (‘level site’) and 45° (‘sloping site’), while at a third site the leaves were fixed in an angle of 45° to homogenize the irradiance dose (‘fixed leaf angle site’). The photosynthetic performance of the leaves was characterized by simultaneous gas exchange and chlorophyll fluorescence measurements and the PSII performance through the growing season was investigated with fluorescence measurements. Leaf harvest towards the end of the growing season was done to determine the specific leaf area and the content of carbon, nitrogen and UV-B absorbing compounds. Compared to a 60% reduced UV-B irradiance, the ambient solar UV-B reduced net photosynthesis in Salix arctica leaves fixed in the 45° position which exposed leaves to maximum natural irradiance. Also a reduced Calvin Cycle capacity was found, i.e. the maximum rate of electron transport (J_max) and the maximum carboxylation rate of Rubisco (V_cmax), and the PSII performance showed a decreased quantum yield and increased energy dissipation. A parallel response pattern and reduced PSII performance at all three sites indicate that these responses take place in all leaves across position in the vegetation. These findings add to the evidence that the ambient solar UV-B currently is a significant stress factor for plants in high Arctic Greenland.     

Exposure to ultraviolet radiation delays photosynthetic recovery in Arctic kelp zoospores

Seasonal reproduction in some Arctic Laminariales coincides with increased UV-B radiation due to stratospheric ozone depletion and relatively high water temperatures during polar spring. To find out the capacity to cope with different spectral irradiance, the kinetics of photosynthetic recovery was investigated in zoospores of four Arctic species of the order Laminariales, the kelps Saccorhiza dermatodea, Alaria esculenta, Laminaria digitata, and Laminaria saccharina. The physiology of light harvesting, changes in photosynthetic efficiency and kinetics of photosynthetic recovery were measured by in vivo fluorescence changes of Photosystem II (PSII). Saturation irradiance of freshly released spores showed minimal I _k values (photon fluence rate where initial slope intersects horizontal asymptote of the curve) values ranging from 13 to 18 μmol photons m⁻² s⁻¹ among species collected at different depths, confirming that spores are low-light adapted. Exposure to different radiation spectra consisting of photosynthetically active radiation (PAR; 400–700 nm), PAR+UV-A radiation (UV-A; 320–400 nm), and PAR+ UV-A+UV-B radiation (UV-B; 280–320 nm) showed that the cumulative effects of increasing PAR fluence and the additional effect of UV-A and UV-B radiations on photoinhibition of photosynthesis are species specific. After long exposures, Laminaria saccharina was more sensitive to the different light treatments than the other three species investigated. Kinetics of recovery in zoospores showed a fast phase in S. dermatodea, which indicates a reduction of the photoprotective process while a slow phase in L. saccharina indicates recovery from severe photodamage. This first attempt to study photoinhibition and kinetics of recovery in zoospores showed that zoospores are the stage in the life history of seaweeds most susceptible to light stress and that ultraviolet radiation (UVR) effectively delays photosynthetic recovery. The viability of spores is important on the recruitment of the gametophytic and sporophytic life stages. The impact of UVR on the zoospores is related to the vertical depth distribution of the large sporophytes in the field.     

Synthesis of fatty acids de novo is required for photosynthetic acclimation of Synechocystis sp. PCC 6803 to high temperature

The role of fatty acid synthesis in the acclimation of the photosynthetic machinery to high temperature was investigated in a mutant of the cyanobacterium Synechocystis sp. PCC 6803 that had a lower than wild-type level of enoyl-(acyl-carrier-protein) reductase FabI, a key component of the type-II fatty acid synthase system. The mutant exhibited marked impairment in the tolerance and acclimation of cells to high temperature: photoautotrophic growth of the mutant was severely inhibited at 40 °C. Moreover, mutant cells were unable to achieve wild-type enhancement of the thermal stability of photosystem II (PSII) when the growth temperature was raised from 25 °C to 38 °C. Enhancement of the thermal stability of PSII was abolished when wild-type cells were treated with triclosan, a specific inhibitor of FabI, and the enhancement of thermal stability was also blocked in darkness and in the presence of chloramphenicol. Analysis of fatty acids in thylakoid membranes revealed that levels of unsaturated fatty acids did not differ between mutant and wild-type cells, indicating that the saturation of fatty acids in membrane lipids might not be responsible for the enhancement of thermal stability at elevated temperatures. Our observations suggest that the synthesis de novo of fatty acids, as well as proteins, is required for the enhancement of the thermal stability of PSII during the acclimation of Synechocystis cells to high temperature.     

Dynamic response of UV-absorbing compounds, quantum yield and the xanthophyll cycle to diel changes in UV-B and photosynthetic radiations in an aquatic liverwort

We studied the diel responses of the liverwort Jungermannia exsertifolia subsp. cordifolia to radiation changes under laboratory conditions. The samples were exposed to three radiation regimes: P (only PAR), PA (PAR + UV-A), and PAB (PAR + UV-A + UV-B). The day was divided in four periods: darkness, a first low-PAR period, the high-PAR plus UV period, and a second low-PAR period. After 15 days of culture, we measured photosynthetic pigments, chlorophyll fluorescence and UV-absorbing compounds in the four periods of the day on two consecutive days. With respect to UV-absorbing compounds, we analyzed their global amount (as the bulk UV absorbance of methanolic extracts) and the concentration of seven hydroxycinnamic acid derivatives, both in the soluble (mainly vacuolar) and insoluble (cell wall-bound) fractions of the plant extracts. PAB samples increased the bulk UV absorbance of the soluble and insoluble fractions, and the concentrations of p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall. Most of these variables showed significant diel changes and responded within a few hours to radiation changes (more strongly to UV-B), increasing at the end of the period of high-PAR plus UV. F_v/F_m, Φ_PSII, NPQ and the components of the xanthophyll cycle showed significant and quick diel changes in response to high PAR, UV-A and UV-B radiation, indicating dynamic photoinhibition and protection of PSII from excess radiation through the xanthophyll cycle. Thus, the liverwort showed a dynamic protection and acclimation capacity to the irradiance level and spectral characteristics of the radiation received.     

Impact of feeding by Arenicola marina (L.) and ageing of faecal material on fatty acid distribution and bacterial community structure in marine sediments: An experimental approach

The fate of ingested eukaryotic photoautotrophic fatty acids during gut transit in the lugworm Arenicola marina (L.) and the influence of A. marina's faeces on the evolution of fatty acid distribution and bacterial community structure in superficial sediments were studied under laboratory conditions. Dead phytoplanktonic cells (food portions) were fed to individual A. marina and subsequently incubated, or allowed to directly incubate in the presence of fresh egesta or non-ingested sediment. Changes in fatty acid composition and genetic structure of bacterial communities during gut transit and/or incubation were monitored using gas-chromatography/mass-spectrometry and a DNA fingerprint approach (RISA), respectively. Results, supported by principal component analyses, suggest that A. marina's feeding activity can directly and indirectly affect the lipid biomarker composition and the bacterial community structure of inhabited sediments. Faecal casts produced from food portions appeared qualitatively enriched in saturated fatty acids relative to (poly)unsaturated ones due, partly, to an increase of some bacterial fatty acids and to the preferential removal of some polyunsaturated fatty acids (PUFAs). The incubation of food portions in the presence of fresh A. marina's egesta (designed to study the indirect impact of feeding by A. marina) induced a significant increase in the concentrations of C₂₀ and C₂₂ polyunsaturated fatty acids (PUFAs), whereas these compounds almost disappeared following direct feeding and subsequent incubation, indicating that some dietary fatty acids may be more accessible to biodegradation following passage through the gut of A. marina. The aforementioned increase in PUFAs was attributed to a bacterial production during incubation, suggesting the presence of PUFA-producing bacteria in the fresh egesta of A. marina. Those bacteria were either enteric bacteria that were released with the egesta or ingested bacteria that have survived gut passage, as suggested by the variations of the bacterial community structure (i.e. RISA profiles) during incubation. The results suggest that aged faeces from A. marina might be, in some circumstances, of relatively high nutritional value to trophic levels which are unable to synthesize essential PUFAs de novo. The presence of PUFA-producing bacteria in guts of marine lugworms deserves further attention.     

Polyunsaturated fatty acids cause apoptosis in C. albicans and C. dubliniensis biofilms

Background

Polyunsaturated fatty acids (PUFAs) have antifungal properties, but the mode by which they induce their action is not always clear. The aim of the study was to investigate apoptosis as a mode of action of antifungal PUFAs (stearidonic acid, eicosapentaenoic acid and docosapentaenoic acid) which are inhibitory towards biofilm formation of C. albicans and C. dubliniensis.

Methods

Candida biofilms were grown in the absence or presence of 1 mM PUFAs (linoleic acid, stearidonic acid, eicosapentaenoic acid, docosapentaenoic acid) for 48 h at 37 °C. The effect of these PUFAs on the membrane fatty acid profile and unsaturation index, oxidative stress, mitochondrial transmembrane potential and apoptosis was evaluated.

Results

When biofilms of C. albicans and C. dubliniensis were exposed to certain PUFAs there was an increase in unsaturation index of the cellular membranes and accumulation of intracellular reactive oxygen species (ROS). This resulted in apoptosis, evidenced by reduced mitochondrial membrane potential and nuclear condensation and fragmentation. The most effective PUFA was stearidonic acid.

Conclusions

The resultant cell death of both C. albicans and C. dubliniensis is due to apoptosis.

General significance

Due to the increase in drug resistance, alternative antifungal drugs are needed. A group of natural antifungal compounds is PUFAs. However, understanding their mechanisms of action is important for further use and development of these compounds as antifungal drugs. This paper provides insight into a possible mode of action of antifungal PUFAs.     

Combined effects of irradiance level and carbon source on fatty acid and lipid class composition in the microalga Pavlova lutheri commonly used in mariculture

Pavlova lutheri, a marine Pavlovophyceae, has been well documented as it is commonly used as a food source in mariculture. In this study, we investigated the combined effects of carbon sources and irradiance levels on the growth, lipid classes and fatty acid profiles of this microalga. The microalga was cultured at 15 °C with a 14 h photoperiod in artificial seawater containing bicarbonate or acetate as carbon source.The growth and lipid composition of P. lutheri were more sensitive to variations in light intensity than in carbon source. However, P. lutheri seems to be able to use acetate to growth cell and lipid metabolism. With the both carbon source, the lowest cellular lipid contents were obtained under low light intensity. The proportions of PUFAs, especially EPA, were significantly higher under low light, and saturating fatty acids and DHA levels were significantly higher under high light. In P. lutheri, galactolipids, a major component of chloroplast lipid membranes, made up approximately 54-66% of total lipids. The highest PUFA levels, such as those of EPA, were predominantly found in the galactolipid fraction when the cells were grown at low light, regardless of the carbon source. The consequent accumulation of n-3 fatty acids in the galactolipids could facilitate thylakoid membrane fluidity, and therefore the velocity of electron flow involved in photosynthesis during light acclimatization. These results could be used to optimize the culture conditions and the nutritional value of this microalga, which is used to feed marine invertebrates and fish larvae in mariculture hatcheries, and to produce n-3 fatty acids for human health care and nutrition.     

Acclimation of Maximal Quantum Yield of Photosynthesis in the Brown Alga Alaria esculenta under High Light and UV Radiation

Abstract: Macroalgae of the upper sublittoral zone of Arctic coastal ecosystems are subjected to darkness or low light for several months during winter and have to withstand large changes in irradiance after the breakup of sea ice in the Arctic spring. Changes in photosynthetic response to high PAR (pho-tosynthetically active radiation) and UV-B radiation (UV-B) in the cold temperate brown alga A/aria esculenta were monitored with a PAM fluorometer to study photoinhibition, recovery and acclimation of maximal quantum yield of photochemistry. Plants collected in the field, as well as specimens raised in the laboratory, were exposed to various radiation conditions including different levels of PAR and UV radiation (UV-A + UV-B). Measurements of variable chlorophyll fluorescence of photosystem II revealed that the photosynthetic apparatus in A. esculenta was able to acclimate to the respective high light and UV treatments within several days. However, two different mechanisms of acclimation seem to be involved. Initially, the rate of recovery of maximal quantum yield increased after only a few exposures to high light or UV. Second, after several exposure cycles, the degree of inhibition was reduced. Data on fluorescence induction kinetics and quenching analysis showed that exposure to the respective UV radiation resulted in an increase of non-photochemical quenching, while effective quantum yield of photochemistry was hardly affected.     

Seasonal variability in feeding behaviour, metabolic rates and carbon and nitrogen balances in the Sydney oyster, Saccostrea glomerata (Gould)

To establish if nutrients limit the growth of bivalves requires information not only on the quality of food available, but also the animals' feeding behaviour and endogenous metabolic demands. We hypothesized that growth of the Sydney rock oyster (Saccostrea glomerata) would vary in response to seasonal changes in food quality rather than quantity. We also predicted that the oysters would show feeding preferences for nitrogen over carbon, and this behaviour would result in carbon/nitrogen (C/N) ratios for ingested and absorbed matter that would be lower than the C/N of both the seston and the oysters' estimated metabolic maintenance requirements. The experiments were done in two phases under natural conditions. In phase 1, feeding behaviour was assessed on a single occasion and the results used to pose hypotheses for testing in phase 2, which included measurements made on three occasions encompassing autumn, winter and spring conditions. Growth rates varied with changes in ambient food quality and not with the concentration of total suspended matter. Feeding behaviour responded to food quality and, in most cases, resulted in nitrogen enrichment. For example, when nitrogen was potentially limiting to growth and/or maintenance, due to high food C/N (July) or high nitrogen demand (March), pre-ingestive selection ensured nitrogen enrichment of ingested matter and C/N ratios of ingested matter which were below the maintenance requirement. However, in November, when endogenous demands indicated an increased requirement for carbon, feeding behaviour resulted in carbon enrichment, an increase in carbon conversion efficiency, and ingested C/N ratios greater than the maintenance requirement. The results support the assertion of variable feeding physiology in oysters, responsive to both exogenous (seasonal differences in carbon and nitrogen availability) and endogenous (cycles of reproduction and growth) factors.     

Changes in cyclic and respiratory electron transport by the movement of phycobilisomes in the cyanobacterium Synechocystis sp. strain PCC 6803

Phycobilisomes (PBS) are the major accessory light-harvesting complexes in cyanobacteria and their mobility affects the light energy distribution between the two photosystems. We investigated the effect of PBS mobility on state transitions, photosynthetic and respiratory electron transport, and various fluorescence parameters in Synechocystis sp. strain PCC 6803, using glycinebetaine to immobilize and couple PBS to photosystem II (PSII) or photosystem I (PSI) by applying under far-red or green light, respectively. The immobilization of PBS at PSII inhibited the increase in cyclic electron flow, photochemical and non-photochemical quenching, and decrease in respiration that occurred during the movement of PBS from PSII to PSI. In contrast, the immobilization of PBS at PSI inhibited the increase in respiration and photochemical quenching and decrease in cyclic electron flow and non-photochemical quenching that occurred when PBS moved from PSI to PSII. Linear electron transport did not change during PBS movement but increased or decreased significantly during longer illumination with far-red or green light, respectively. This implies that PBS movement is completed in a short time but it takes longer for the overall photosynthetic reactions to be tuned to a new state.     

The role of ammonium in photoprotection against high irradiance in the red alga Grateloupia lanceola

Combined and/or interactive effects of inorganic nitrogen (as ammonium) and irradiance on the accumulation of nitrogenous compounds, like UV-absorbing mycosporine-like amino acids (MAAs), chlorophyll a and phycobiliproteins, were examined in the red alga Grateloupia lanceola (J. Agardh) J. Agardh in a high irradiance laboratory exposure and a subsequent recovery period under low light. Also, photosynthetic activity as in vivo chlorophyll fluorescence of photosystem II, i.e. optimum quantum yield (F_v/F_m), electron transport rate (ETR) and quantum efficiency, were examined. Photosynthetic activity, phycobiliproteins and internal nitrogen content declined during the 3-day PAR (photosynthetically active radiation; 600 μmol s⁻¹ m⁻²) and PAR + UVR (ultraviolet radiation; UVB 280–315 nm 0.8 W m⁻², UVA 315–400 nm 16 W m⁻²) exposure. Ammonium supplied in the culture medium (0, 100 and 300 μM NH₄Cl) modified the responses of the alga to high irradiance exposures in a concentration dependent manner, mainly with respect to recovery, as the highest recovery during a 10-day low light period was produced under elevated concentration of ammonium (300 μM). The recovery of photosynthetic activity and phycobiliproteins was enhanced in the algae previously incubated under PAR + UVR as compared to exposure to only PAR, suggesting a beneficial effect of UVR on recovery or photoprotective processes under enriched nitrogen conditions. However, the content of MAAs did not follow the same pattern and thus it could not be concluded as the cause of observed enhanced recovery.     

Composition of the summer photosynthetic pico and nanoplankton communities in the Beaufort Sea assessed by T-RFLP and sequences of the 18S rRNA gene from flow cytometry sorted samples

The composition of photosynthetic pico and nanoeukaryotes was investigated in the North East Pacific and the Arctic Ocean with special emphasis on the Beaufort Sea during the MALINA cruise in summer 2009. Photosynthetic populations were sorted using flow cytometry based on their size and pigment fluorescence. Diversity of the sorted photosynthetic eukaryotes was determined using terminal-restriction fragment length polymorphism analysis and cloning/sequencing of the 18S ribosomal RNA gene. Picoplankton was dominated by Mamiellophyceae, a class of small green algae previously included in the prasinophytes: in the North East Pacific, the contribution of an Arctic Micromonas ecotype increased steadily northward becoming the only taxon occurring at most stations throughout the Beaufort Sea. In contrast, nanoplankton was more diverse: North Pacific stations were dominated by Pseudo-nitzschia sp. whereas those in the Beaufort Sea were dominated by two distinct Chaetoceros species as well as by Chrysophyceae, Pelagophyceae and Chrysochromulina spp.. This study confirms the importance of Arctic Micromonas within picoplankton throughout the Beaufort Sea and demonstrates that the photosynthetic picoeukaryote community in the Arctic is much less diverse than at lower latitudes. Moreover, in contrast to what occurs in warmer waters, most of the key pico- and nanoplankton species found in the Beaufort Sea could be successfully established in culture.     

Chromatic photoacclimation, photosynthetic electron transport and oxygen evolution in the Chlorophyll d-containing oxyphotobacterium Acaryochloris marina

Changes in photosynthetic pigment ratios showed that the Chlorophyll d-dominated oxyphotobacterium Acaryochloris marina was able to photoacclimate to different light regimes. Chl d per cell were higher in cultures grown under low irradiance and red or green light compared to those found when grown under high white light, but phycocyanin/Chl d and carotenoid/Chl d indices under the corresponding conditions were lower. Chl a, considered an accessory pigment in this organism, decreased respective to Chl d in low irradiance and low intensity non-white light sources. Blue diode PAM (Pulse Amplitude Modulation) fluorometry was able to be used to measure photosynthesis in Acaryochloris. Light response curves for Acaryochloris were created using both PAM and O₂ electrode. A linear relationship was found between electron transport rate (ETR), measured using a PAM fluorometer, and oxygen evolution (net and gross photosynthesis). Gross photosynthesis and ETR were directly proportional to one another. The optimum light for white light (quartz halogen) was about 206 ± 51 μmol m^− 2 s^− 1 (PAR) (Photosynthetically Active Radiation), whereas for red light (red diodes) the optimum light was lower (109 ± 27 μmol m^− 2 s^− 1 (PAR)). The maximum mean gross photosynthetic rate of Acaryochloris was 73 ± 7 μmol mg Chl d^− 1 h^− 1. The gross photosynthesis/respiration ratio (P_g/R) of Acaryochloris under optimum conditions was about 4.02 ± 1.69. The implications of our findings will be discussed in relation to how photosynthesis is regulated in Acaryochloris.     

Fractionation of stable isotopes in the Arctic marine copepod Calanus glacialis: Effects on the isotopic composition of marine particulate organic matter

Fractionation of δ¹³C and δ¹⁵N between food, consumer, and faecal pellets was studied in the Arctic marine copepod Calanus glacialis Jaschnov, fed with isotopically distinct algal monocultures. Temporal variations in δ¹³C and δ¹⁵N of copepods that were fed ice algae and phytoplankton followed those of a control group consisting of starved animals. There were no significant trends in the δ¹³C and δ¹⁵N values of copepods that were starved for 42 days, suggesting that the isotopic composition of non-lipid body tissues is unaffected by the metabolic processes during prolonged periods of starvation. The stable isotopic composition of starved copepods therefore seems to reflect food consumed during the previous period of feeding and growth. Faecal pellets produced by feeding copepods were depleted in ¹³C and ¹⁵N by 6.3-11.2‰ and 0.7-9.1‰, respectively, relative to the food ingested. These results indicate that faecal pellet production is an important pathway for the trophic fractionation of δ¹³C, whereas other fractionation pathways, such as excretion of ammonia, may be relatively more important for δ¹⁵N. The strong depletion of ¹³C in faecal pellets compared to the food suggests that grazing by herbivorous copepods on primary production adds to the variability of δ¹³C in marine particulate organic matter.     

Interaction Effects of Ambient UV Radiation and Nutrient Limitation on the Toxic Cyanobacterium <Emphasis Type="Italic">Nodularia spumigena</Emphasis>

Nodularia spumigena is one of the dominating species during the extensive cyanobacterial blooms in the Baltic Sea. The blooms coincide with strong light, stable stratification, low ratios of dissolved inorganic nitrogen, and dissolved inorganic phosphorus. The ability of nitrogen fixation, a high tolerance to phosphorus starvation, and different photo-protective strategies (production of mycosporine-like amino acids, MAAs) may give N. spumigena a competitive advantage over other phytoplankton during the blooms. To elucidate the interactive effects of ambient UV radiation and nutrient limitation on the performance of N. spumigena, an outdoor experiment was designed. Two radiation treatments photosynthetic active radiation (PAR) and PAR +UV-A + UV-B (PAB) and three nutrient treatments were established: nutrient replete (NP), nitrogen limited (−N), and phosphorus limited (−P). Variables measured were specific growth rate, heterocyst frequency, cell volume, cell concentrations of MAAs, photosynthetic pigments, particulate carbon (POC), particulate nitrogen (PON), and particulate phosphorus (POP). Ratios of particulate organic matter were calculated: POC/PON, POC/POP, and PON/POP. There was no interactive effect between radiation and nutrient limitation on the specific growth rate of N. spumigena, but there was an overall effect of phosphorus limitation on the variables measured. Interaction effects were observed for some variables; cell size (larger cells in −P PAB compared to other treatments) and the carotenoid canthaxanthin (highest concentration in −N PAR). In addition, significantly less POC and PON (mol cell⁻¹) were found in −P PAR compared to −P PAB, and the opposite radiation effect was observed in −N. Our study shows that despite interactive effects on some of the variables studied, N. spumigena tolerate high ambient UVR also under nutrient limiting conditions and maintain positive growth rate even under severe phosphorus limitation.     

Photosynthetic response of Nodularia spumigena to UV and photosynthetically active radiation depends on nutrient (N and P) availability

Biomass of N. spumigena is distributed within the dynamic photic zone that changes in both light quantity and quality. This study was designed to determine whether nutrient status can mitigate the negative impacts of experimental radiation treatments on the photosynthetic performance of N. spumigena. Cyanobacterial suspensions were exposed to radiation consisting of photosynthetically active radiation (PAR=400-700 nm), PAR+UV-A (=PA, 320-700 nm), and PAR+UV-A+UV-B (=PAB, 280-700 nm) under different nutrient media either replete with external dissolved nitrate (N) and orthophosphate (P; designated as +N/+P), replete with P only (-N/+P), or replete with N only (+N/-P). Under low PAR (75 micromol photons m(-2) s(-1)), nutrient status had no significant effect on the photosynthetic performance of N. spumigena in terms of rETRmax, alpha, and E(k). Nodularia spumigena was able to acclimate to high PAR (300 micromol photons m(-2) s(-1)), with a corresponding increase in rETRmax and E(k). The photosynthetic performance of N. spumigena cultured with supplemental nitrogen was more susceptible to experimental PAR irradiance. Under UVR, P-enrichment in the absence of additional external N (-N/+P) induced lower photoinhibition of photosynthesis compared with +N/-P cultures. However, the induction of NPQ may have provided PSII protection under P-deplete and PAR+UVR conditions. Because N. spumigena are able to fix nitrogen, access to available P can render them less susceptible to photoinhibition, effectively promoting blooms. Under a P-deficient condition, N. spumigena were more susceptible to radiation but were capable of photosynthetic recovery immediately after removal of radiation stress. In the presence of an internal P pool in the Baltic Sea, which may be seasonally available to the diazotrophic cyanobacteria, summer blooms of the resilient N. spumigena will persist.     

Feeding behavior and differential absorption of biochemical components by the infaunal bivalve Mulinia edulis and the epibenthic Mytilus chilensis in response to changes in food regimes

We measured changes in the feeding rate and food absorption efficiency of two suspension feeding bivalves, cross-trasplanted between habitats with special emphasis on their capacity for differential absorption of biochemical components from their food supply. Mulinia edulis were moved from the intertidal zone to the subtidal zone, and Mytilus chilensis from the subtidal to the intertidal zone for a period of 7 days, and then compared with animal that had not been transplanted. Experimentally prepared diets similar to those available in the two different environments were offered to the bivalves, and their rates of feeding and differential uptake of biochemical components were determined and statistically compared. The two species did not achieve complete acclimation of their feeding behaviour during the transplant period since the highest ingestion rates for biochemical components occurred under dietary conditions that reflected their habitats of origin. Absorption efficiency showed greater acclimation than the other physiological parameters measured, indicating the capacity of these species to modulate their enzymatic-digestive activity depending on food composition. We conclude that both Mytilus and Mulinia have a certain degree of physiological plasticity in their feeding behaviour and assimilatory balance of biochemical components, being greater in Mytilus. When both species encounter ambient food conditions characteristic of their normal habitats, they show maximum values of food absorption, while under conditions where their typical diets are exchanged (Mytilus in intertidal and Mulinia in subtidal), the energy absorbed declines in each, but in ways very different between the two species. Thus, Mytilus exposed to high concentrations of low quality seston reduced the energy absorbed by 31.7% compared to its normal habitat, while Mulinia exposed to low concentrations of high-quality food reduced their energy absorption by 64%.     

Effect of dietary polyunsaturated fatty acids on reproductive output and larval growth of bivalves

The pre-spawning condition of adult bivalves is influenced by quantity and quality of available food. For bivalves, the essential polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA) 20:5(n-3) and docosahexaenoic acid (DHA) 22:6(n-3) are presumed to determine the nutritional value of algae. Macoma balthica kept on a broodstock diet supplemented with PUFAs spawned a larger number of eggs (average 22220) per female and larger sized eggs (106.7 μm) compared to adults kept on a diet without PUFA supplementation (962.5 eggs with average size 99.8 μm).Larvae of M. balthica from the same parental pool however did not profit from a diet where a part was replaced with PUFA spheres. Instead, larvae reared on Isochrysis sp. showed lower mortality and higher growth rates than larvae fed on the same algae supplemented with lipid spheres. Crassostrea gigas larvae showed no clear response to a PUFA supplemented diet.