Mycosporine-like Amino Acids Concentrations Research Articles

The relative contributions of diet and associated microbiota to the accumulation of UV‐absorbing mycosporine‐like amino acids in the freshwater copepod Boeckella antiqua

Summary1. Mycosporine‐like amino acids (MAAs) are ubiquitous compounds in aquatic organisms that are usually considered sunscreens that protect them from harmful ultraviolet radiation. Given that virtually all animals lack the metabolic pathways to synthesise MAAs de novo, they must acquire them either from their diet or from microorganisms living in close association. In freshwater copepods, accumulation of MAAs is stimulated by exposure to ultraviolet and/or visible radiation.2. A 2 × 2 factorial experiment was performed to assess the contributions of dietary and microbial sources of MAAs in the freshwater copepod Boeckella antiqua. The treatments consisted of two different diets: an MAA‐free diet, including only Chlamydomonas reinhardtii, and an MAA‐rich diet, including both C. reinhardtii and Peridinium inconspicuum, crossed with two antibiotic treatments, with and without chloramphenicol. Treatment with chloramphenicol was intended to inhibit the development of bacteria associated with the copepods.3. MAA concentration in B. antiqua was affected by the experimental conditions: (i) exposure to artificial PAR + UVR stimulated the accumulation of several MAAs (up to 62% increase in total MAA concentration with respect to the initial concentration); (ii) the presence of chloramphenicol in the culture media reduced the MAA concentration in copepods fed an MAA‐free diet; (iii) in the absence of chloramphenicol, copepods fed the MAA‐rich diet had significantly higher total MAA concentration than those fed the MAA‐deficient diet; but (iv) dietary supplementation with an MAA‐rich algae in the presence of chloramphenicol failed to significantly increase total MAA concentration.4. Analysis of profiles from denaturing gradient gel electrophoresis (DGGE) showed that the prokaryotic community associated with the copepods was affected by chloramphenicol. Dendograms constructed from digitalised DGGE images consistently grouped the antibiotics treatments separately from the initial samples and the treatments without antibiotics. Two band positions were exclusive to treatments without antibiotics.5. We conclude that when offered an MAA‐rich diet, B. antiqua may accumulate a proportion of MAAs from diet. However, we suspect that in the absence of an MAA‐rich dietary source (as in its natural habitat), virtually all MAAs present in B. antiqua are produced by copepod‐associated prokaryotes.

Non-enzymatic antioxidant photoprotection against potential UVBR-induced damage in an Antarctic diatom (Thalassiosira sp.)

In January 1999, unialgal cultures of the diatom Thalassiosira sp., solate from natural phyto- plankton assemblages from Potter Cove, Antarctica, were exposed to solar ultraviolet radiation (UVR, 280- 400 nm) in order to study the long-term acclimation of this species. Ultraviolet radiation B (UVBR, 280-315 nm) inhibited the growth rate during the first and second days of exposure. No UVBR inhibition was observed on the third day. The initial content of α-tocopherol (13 pmol (10 4 cell) -1 ) showed a marked decrease during the exponential growth phase (4 pmol (10 4 cell -1 ) by day 3). The initial content of β-carotene (3 pmol (10 4 cell) -1 ) did not show significant differences over time in cells exposed to UVBR. Two mycosporine-like amino acids (MAAs) were identified: porphyra-334 and shinorine. Cellular concentrations of MAAs increased significantly on days 2 and 3, and exposure of the algae to UVBR significantly enhanced this value. The relative importance of MAAs concentration was significant (P < 0.05) in relation to the α-tocopherol content. A positive correlation was shown between cellular MAAs concentration and growth rate. Our results suggest that photoprotection against UV-induced damage is characterized by short-term consumption of α-tocopherol and longer-term synthesis of MAAs. The UVBR damage/repair ratio during long-term exposure involves the combined action of several endogenous factors within the cell, with MAAs synthesis being the most effective factor related to photoprotection.

EFFECTS OF ULTRAVIOLET-A RADIATION AND NUTRIENT AVAILABILITY ON THE CELLULAR COMPOSITION OF PHOTOPROTECTIVE COMPOUNDS IN GLENODINIUM FOLIACEUM (DINOPHYCEAE)(1).

The photoprotective response in the dinoflagellate Glenodinium foliaceum F. Stein exposed to ultraviolet-A (UVA) radiation (320-400 nm; 1.7 W · m(2) ) and the effect of nitrate and phosphate availability on that response have been studied. Parameters measured over a 14 d growth period in control (PAR) and experimental (PAR + UVA) cultures included cellular mycosporine-like amino acids (MAAs), chls, carotenoids, and culture growth rates. Although there were no significant effects of UVA on growth rate, there was significant induction of MAA compounds (28 ± 2 pg · cell(-1) ) and a reduction in chl a (9.6 ± 0.1 pg · cell(-1) ) and fucoxanthin (4.4 ± 0.1 pg · cell(-1) ) compared to the control cultures (3 ± 1 pg · cell(-1) , 13.3 ± 3.2 pg · cell(-1) , and 7.4 ± 0.3 pg · cell(-1) , respectively). In a second investigation, MAA concentrations in UVA-exposed cultures were lower when nitrate was limited (P < 0.05) but were higher when phosphate was limiting. Nitrate limitation led to significant decreases (P < 0.05) in cellular concentration of chls (chl c1 , chl c2 , and chl a), but other pigments were not affected. Phosphate availability had no effect on final pigment concentrations. Results suggest that nutrient availability significantly affects cellular accumulation of photoprotective compounds in G. foliaceum exposed to UVA.

Body distribution and source of mycosporine-like amino acids in the cyclopoid copepod Cyclops abyssorum tatricus

Mycosporine-like amino acids (MAAs) are an important family of photoprotective compounds accumulated by zooplankton, but little is known about their localization in the body or their source in natural lake populations. Here, we assessed first, the distribution of MAAs in the body of the copepod Cyclops abyssorum tatricus and second, measured stable isotope signatures (dC, dN) in different life stages of the copepod, as well as in seston and benthic primary producers to reveal the potential source of these compounds in a UV transparent alpine lake. The stable isotope analysis showed that this copepod was herbivorous throughout its ontogeny, from nauplius to adult. Mixing models based on the stable isotope signatures suggest that this copepod feeds predominantly on the ,20-mm size fraction of seston. These models also indicated that the probability of feeding in the upper water layers was high in the ice-cover period, but that the copepod switched to deep water layers in the ice-free period following the maximum of phytoplankton biomass. The higher MAA concentration found in C. abyssorum tatricus during the ice-free period was not connected with an enhanced availability in the food source, but seems to be the result of an active accumulation. The highest MAA content in the body of C. abyssorum tatricus females was found in eggs. Our results indicate that MAAs in this copepod are efficiently accumulated through the diet (mainly nanophytoplankton) and transferred to the nauplii as an important strategy to provide photoprotection for the first life stages.

High concentrations of mycosporine‐like amino acids and colored dissolved organic matter in the sea surface microlayer off the Iberian Peninsula

Mycosporine‐like amino acids (MAAs), phytoplankton pigments, and inherent optical properties were analyzed in sea surface microlayer (SSM), near‐surface (0‐2 m), and subsurface (0–110 m) samples from stations off the Iberian Peninsula in June‐July 2005. During a visible surface slick, MAA concentrations reached 290 µg L−1 in the SSM, which correlated with an increase in abundance of the dinoflagellate, Prorocentrum micans (&gt; 4000 cells mL−1) and covaried with low Ekman transport. High levels of colored dissolved organic material (CDOM) were also found in the SSM, and a prominent absorption shoulder between 300 and 340 nm was evident in the CDOM absorption spectra, indicative of MAAs in the dissolved fraction. Low CDOM slope ratios (SCDOM(275‐295) : (350‐400)) during the development of the surface slick suggest microbial production of CDOM, possibly exuded from the sudden proliferation in P. micans. In the absence of a well‐defined SSM, higher SCDOM(275–295) : (350–400) ratios were found, suggesting photobleaching of CDOM at the surface. The particulate absorption spectra of MAAs (aPMAA(λ)) were effective at absorbing ultraviolet (UV) radiation in a narrow spectral band between 310 and 350 nm. Since aCDOM(λ) exhibits an exponential rise in absorption in the UV and was high in the SSM and the subsurface compared with aPMAA (λ), CDOM would be the principal mechanism of UVA and B attenuation in the water column, providing an additional sunscreen to phytoplankton.

Effect of nutrient supply on photosynthesis and pigmentation to short-term stress (UV radiation) in Gracilaria conferta (Rhodophyta)

The effects of increased photosynthetic active radiation (PAR), UV radiation (UVR), and nutrient supply on photosynthetic activity, pigment content, C:N ratio and biomass yield were studied in tank cultivated Gracilaria conferta (Rhodophyta). Electron transport rate (ETR) and biliprotein content were higher under high nutrient supply (HNS), obtained from fishpond effluents, compared to low nutrient supply (LNS), in contrast to mycosporine-like amino acids (MAAs) dynamic. The high MAA content in LNS-algae could be explained by higher UVR penetration in the thallus and by the competition for the use of nutrients with other processes. Effective quantum yield decreased after short-term exposure to high irradiance whereas full recovery in shade was produced only under slightly heat shock. UVA radiation provoked an additional decrease in photosynthesis under high water temperature. UVB radiation reversed UVA’s negative effect mainly with HNS. Results support that nutrient-sufficiency help G. conferta to resist environmental changes as short-term temperature increase.

UV protective compounds transferred from a marine dinoflagellate to its copepod predator

Planktonic organisms living in surface waters can be exposed to harmful levels of ultraviolet radiation (UVR), but few studies have examined accumulation of UV protective compounds in marine zooplankton. Zooplankters are suggested to lack the ability to synthesize these substances and hence must accumulate them from their algal food. Here, we show that both phytoplankton (dinoflagellates) and their zooplankton grazers (copepods) respond strongly to UVR exposure by, respectively, synthesizing and accumulating the natural sunscreens mycosporine-like amino acids (MAAs). In our experiment, the MAAs content increased approximately four times in dinoflagellates exposed to UVR and PAR, as compared to non-UVR controls only receiving PAR (PAR = photosynthetically active radiation). The elevated MAAs level in the dinoflagellates was mirrored in the copepods, which accumulated more MAAs when exposed to UVR as compared to a non-UVR treatment. Overall, copepods accumulated approximately 2–5% of the total MAAs pool. Other UV protective compounds, like carotenoids, were however not accumulated by the copepods. The ability of some species to produce or accumulate photoprotective compounds may lead to increased fitness, and thus these taxa may become more dominant in plankton communities.

Mycosporine-like amino acid content in the sea anemones Aulactinia marplatensis, Oulactis muscosa and Anthothoe chilensis

The occurrence of mycosporine-like amino acids (MAAs) in the sea anemones Aulactinia marplatensis (Zamponi, 1977), Oulactis muscosa (Drayton in Dana, 1846) and Anthothoe chilensis (Lesson, 1830), from the rocky intertidal habitats on the coast of Mar del Plata, Argentina, was assessed by HPLC. The pattern of MAAs in the mussel Brachidontes rodriguezi, main component of the diet for A. marplatensis and O. muscosa, was as well determined. The results were comparatively analyzed together and with previously reported MAA content in species mainly of the genus Anthopleura. The correlation between the MAA concentration and light availability of their habitats is in line with the photoprotective role assigned to the compounds. The high proportion of mycosporine-taurine in the three species and the results for the evaluation of MAAs in the mussels point to a non-dietary origin or a regulated biotransformation metabolism of dietary MAAs and/or their precursors that is common to sea anemones.

Mycosporine‐like Amino Acids in Freshwater Copepods: Potential Sources and Some Factors That Affect Their Bioaccumulation

Mycosporine-like amino acids (MAAs) are ubiquitous photoprotective compounds in aquatic environments. MAAs are synthesized by a wide variety of organisms (i.e. bacteria, fungi and algae) and their production is photoinducible by ultraviolet radiation (UVR) (280-400 nm) and/or photosynthetically active radiation (400-750 nm). Most animals however, are unable to synthesize MAAs and must acquire these compounds through their diet or from symbiotic organisms. In this paper, we investigate the possible sources of MAAs and factors (temperature and initial MAA concentration) that may affect their bioaccumulation in freshwater copepods. We found that MAA accumulation may occur even if the copepods are cultured on a MAA-free diet. In addition, we found that the bacteriostatic antibiotic, chloramphenicol, inhibits the bioaccumulation of MAAs. These two pieces of evidence suggest that the source of MAAs in these copepods may be prokaryotic organisms in close association with the animals. The two factors investigated in this study, temperature and initial MAA concentrations, were found to affect the rates at which MAAs are accumulated. Temperature had positive effects on both uptake and elimination rates. On the other hand, the rate of uptake decreased at the highest assayed initial MAA concentration, probably because the concentration of MAAs was already close to saturation.

Photoprotection mechanisms against ultraviolet radiation in Heterocapsa sp. (Dinophyceae) are influenced by nitrogen availability: Mycosporine‐like amino acids vs. xanthophyll cycle

The photoprotective and acclimation capacity against ultraviolet radiation (UVR) was assessed for the dinoflagellate Heterocapsa sp. We examined the effect of UVR and N availability on photosynthetic activity and on the accumulation of photoprotective substances such as mycosporine‐like amino acids (MAAs) and xanthophyll cycle pigments. Cells were cultivated under two different light treatments, photosynthetically active radiation (PAR) and PAR + UVR, and at two NaNO3 concentrations, mid nitrogen (MN, 0.1 mmol L−1) and high nitrogen (HN, 1 mmol L−1) for 6 d. MAA and photosynthetic pigment contents as well as maximum quantum yield of fluorescence (Fv : Fm) and electron transport rate were analyzed at the initial time and after 3 and 6 d of experimentation. Fv : Fm decreased because of UVR and N limitation. N enrichment reduced the deleterious effect of UVR on photosynthesis. The content of photosynthetic pigments and MAAs was higher at HN than at MN supply and a positive effect of UVR on MAA and pigment accumulation was observed, suggesting that under HN conditions the deleterious UVR effect is counteracted by MAAs. Under N limitation thermal energy dissipation takes place by the xanthophyll cycle, i.e., an increase of the de‐epoxidation degree was observed under these culture conditions. However, UVR seems to favor diadinoxanthin accumulation; thus, no photoprotection through the xanthophyll cycle seems available in cells exposed to PAR + UVR and MN. We conclude that not only MAA accumulation but also N availability is very important to determine the photoprotective capacity against UVR of Heterocapsa sp.

Contribution of mycosporine‐like amino acids and colored dissolved and particulate matter to sea ice optical properties and ultraviolet attenuation

In the Baltic Sea ice, the spectral absorption coefficients for particulate matter (PM) were about two times higher at ultraviolet wavelengths than at photosynthetically available radiation (PAR) wavelengths. PM absorption spectra included significant absorption by mycosporine‐like amino acids (MAAs) between 320 and 345 nm. In the surface ice layer, the concentration of MAAs (1.37 µg L−1) was similar to that of chlorophyll a, resulting in a MAAs‐to‐chlorophyll a ratio as high as 0.65. Ultraviolet radiation (UVR) intensity and the ratio of UVR to PAR had a strong relationship with MAAs concentration (R2 = 0.97, n = 3) in the ice. In the surface ice layer, PM and especially MAAs dominated the absorption (absorption coefficient at 325 nm: 0.73 m−1). In the columnar ice layers, colored dissolved organic matter was the most significant absorber in the UVR (&lt; 380 nm) (absorption coefficient at 325 nm: 1.5 m−1). Our measurements and modeling of UVR and PAR in Baltic Sea ice show that organic matter, both particulate and dissolved, influences the optical properties of sea ice and strongly modifies the UVR exposure of biological communities in and under snow‐free sea ice.

Contribution of mycosporine-like amino acids and colored dissolved and particulate matter to sea ice optical properties and ultraviolet attenuation

In the Baltic Sea ice, the spectral absorption coefficients for particulate matter (PM) were about two times higher at ultraviolet wavelengths than at photosynthetically available radiation (PAR) wavelengths. PM absorption spectra included significant absorption by mycosporine-like amino acids (MAAs) between 320 and 345 nm. In the surface ice layer, the concentration of MAAs (1.37 μg L(-1)) was similar to that of chlorophyll a, resulting in a MAAs-to-chlorophyll a ratio as high as 0.65. Ultraviolet radiation (UVR) intensity and the ratio of UVR to PAR had a strong relationship with MAAs concentration (R(2) = 0.97, n = 3) in the ice. In the surface ice layer, PM and especially MAAs dominated the absorption (absorption coefficient at 325 nm: 0.73 m(-1)). In the columnar ice layers, colored dissolved organic matter was the most significant absorber in the UVR (< 380 nm) (absorption coefficient at 325 nm: 1.5 m(-1)). Our measurements and modeling of UVR and PAR in Baltic Sea ice show that organic matter, both particulate and dissolved, influences the optical properties of sea ice and strongly modifies the UVR exposure of biological communities in and under snow-free sea ice.

Complementary UV protective compounds in zooplankton

Zooplankton accumulate several groups of photoprotective compounds to shield against damaging ultraviolet radiation (UV). One of these groups, the carotenoids, makes the animals more conspicuous to visually hunting predators, whereas others, such as the mycosporine‐like amino acids (MAAs) may not. The blend of photoprotective compounds is therefore important for the UV defense but also for the ability to escape predation through crypsis. Here we assess laboratory and field data from different latitudes to examine how UV, predation threat, and pigment availability (in food) affects the mixture of UV‐protective compounds in copepods. Overall, the blend of MAAs and carotenoids was partly explained by the availability of MAAs in the food, the UV‐threat, and the presence of predators. Copepods upregulated their MAA content when UV threat was increasing (i.e., if MAAs were abundant in food), and in field data this accumulation only occurred at high levels of predation threat. If MAAs were scarce, copepods instead compensated with higher carotenoid accumulation. However, when there was a high predation threat this carotenoid compensatory effect was disadvantageous, and low concentrations of both MAAs and carotenoids at high UV‐threat resulted in lower reproduction. In all, these results showed that carotenoids and MAAs are complementary substances, i.e., one is high when the other is low, and copepods are, hence, able to adjust their blend of different UV‐protective compounds to optimize their defenses to the threats of UV and predation. These defense systems may buffer against direct food‐web interactions and help the zooplankton to survive in environments with high UV threat.

Mycosporine-like amino acids in six scleractinian coral species

Mycosporine-like amino acids (MAAs) were studied in stony coral species (Fungi- idae) along the Eastern coast of the Red Sea. Six species - Fungia scutaria, F. danai, F. corona, F. repanda, Ctenactis echinata and Lithophyllor lobata - were examined for MAAs at water depths of 5, 10, 15 and 20 m. Protein and chlorophyll were also determined and showed higher contents in winter than in summer. Generally, the total content of MAAs in summer was found to be approximately three times greater than in winter. Overall, concentrations of MAAs were greatest at a depth of 5 m. Porphyra-334 was the most abundant MAA in F. scutaria and F. danai, whereas asterina-330 was either not detectable (e.g. L. lobata) or present in low concentrations (e.g. F. danai, F. repanda and C. echinata). Shinorine was not detected in F. danai or L. lobata. Both C. echinata and L. lobata had the lowest concentrations of MAAs, presumably because of their large calcareous skeletons. The variation in MAA concentrations among seasons and water depths is probably due to a number of factors, including the intensity of solar radiation, turbidity and phylogenetic variation.

The effect of re-dissolution solvents and HPLC columns on the analysis of mycosporine-like amino acids in the eulittoral macroalgae Prasiola crispa and Porphyra umbilicalis

Many macroalgal species that are regularly exposed to high solar radiation such as the eulittoral green alga Prasiola crispa and the red alga Porphyra umbilicalis synthesize and accumulate high concentrations of mycosporine-like amino acids (MAAs) as UV-sunscreen compounds. These substances are typically extracted with a widely used standard protocol following quantification by various high performance liquid chromatography (HPLC) techniques. However, further preparation steps prior to HPLC analysis as well as different HPLC column types have not been systematically checked regarding separation quality and reproducibility. Therefore pure methanol, distilled water and HPLC eluent were evaluated as re-dissolution solvent for dried Prasiola and Porphyra extracts, which were subsequently analyzed on three reversed-phase C8 and C18 HPLC columns. The data indicate that distilled water and the HPLC eluent gave almost identical peak patterns and MAA contents on the C8 and C18 columns. In contrast, the application of the widely used methanol led to double peaks or even the loss of specific peaks as well as to a strong decline in total MAA amounts ranging from about 35% of the maximum in P. crispa to 80% of the maximum in P. umbilicalis. Consequently, methanol should be avoided as re-dissolution solvent for the HPLC sample preparation. An improved protocol for the MAA analysis in macroalgae in combination with a reliable C18 column is suggested.

Differential tolerance of UV radiation between Chaoborus species and role of photoprotective compounds

Previous studies with larvae of the American species Chaoborus punctipennis suggest that this genus is highly sensitive to solar ultraviolet radiation (UVR). Here, we first tested the UV sensitivity of late-instar larvae of two European Chaoborus species collected from a fishless shallow lake and second assessed their strategies to minimize UV damage. Exposure of late-instar larvae of C. pallidus to artificial UVR caused 50% mortality at a dose of 63.9 J cm -2 . In contrast, no mortality was observed in C. obscuripes at the same or even at a higher dose (85.1 J cm -2 ). Several UV-absorbing compounds identified as mycosporine-like amino acids (MAAs) were found by high-performance liquid chromatography analysis in C. obscuripes [mean total MAAs: 0.142 μg mg -1 dry weight (DW)], but not in C. pallidus. Larvae of C. flavicans also had the same suite of MAAs (shinorine, mycosporine-glycine, palythine, usujirene and polythene) and similar MAAs concentration (0.186 μg mg -1 DW) as C. obscuripes, whereas C. crystallinus had very low concentrations (0.008 μg mg -1 DW) and only two MAAs. The same suite of MAAs found in larvae of C. obscuripes was also detected in pupae, adult midges and eggs of this species. Whereas C. obscuripes was able to accumulate MAAs through the diet, the origin of these secondary metabolites in the lake remains unclear. Our results indicate that sensitivity to UVR in Chaoborus is species-specific and suggest that resistance in C. obscuripes is associated with the presence of photoprotective compounds. Populations of UV-sensitive Chaoborus species may be at disadvantage in water bodies becoming more transparent to UVR as consequence of climate change.

Accumulation of Mycosporine‐like Amino Acids in Asparagopsis armata Grown in Tanks with Fishpond Effluents of Gilthead Sea Bream, Sparus aurata

Abstract.— The accumulation of ultraviolet (UV)‐screen substances, mycosporine‐like amino acids (MAAs), in the tetrasporophytic phase of the red alga Asparagopsis armata grown in fishpond effluents of gilthead sea bream, Sparus aurata, is analyzed. Both the effects of total ammonium nitrogen (TAN) fluxes and the algal densities on MAA accumulation were investigated. MAAs increased with the TAN flux, but only until values lower than 100 μM/h. Above this flux, the MAA content decreased, whereas algal yield increased. The content of individual MAAs was related to nitrogen (N) status, that is, shinorine percentage slightly decreased and palythine increased with increasing N fluxes. The decrease of MAAs at high flux of N (up to 100 μM/h) is related to the decrease of water residence time and the decrease of the biofiltration efficiency. Under different TAN fluxes and algal densities, MAA content was negatively related to algal yield indicating that MAAs were accumulated only under a high ammonium‐N availability. Thus, an energy allocation between growth (primary metabolism) and MAA accumulation (secondary metabolism) is regulated by the absorption capacity of inorganic N. In conclusion, A. armata, in addition to its high biofiltration capacity of nutrients, is a good source of MAAs as potential UV screen photoprotectors.

Multiple Strategies of Bloom-Forming Microcystis to Minimize Damage by Solar Ultraviolet Radiation in Surface Waters

The occurrence of bloom-forming cyanobacteria is one of the most obvious sign of eutrophication in freshwaters. Although in eutrophic lakes water transparency in the ultraviolet (UV) region is strongly reduced, bloom-forming cyanobacteria are exposed to high solar UV radiation at the surface. Here, we show that, in a natural phytoplankton community from a very eutrophic lake, Microcystis synthesizes UV sunscreen compounds identified as mycosporine-like amino acids (MAAs). The biomass-specific MAA concentration was significantly correlated with the occurrence of Microcystis but not with other algal groups, even though they were dominant in terms of biomass. Based on a photo-optical model, we estimated that the maximum MAA concentration per cell observed (2.5% dry weight) will confer only approximately 40% of internal screening to a single layer of Microcystis cells. Thus, the formation of a colony with several layers of cells is important to afford an efficient UV screening by internal self-shading. Overall, we propose that Microcystis uses a combination of photoprotective strategies (MAAs, carotenoids) to cope with high solar UV radiation at the water surface. These strategies include also the screening of UV radiation by D-galacturonic acid, one of the main chemical components of the slime layer in Microcystis.

Isolate-specific effects of ultraviolet radiation on photosynthesis, growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium Microcoleus chthonoplastes

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12-15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 mumol photon m(-2) s(-1)) = P; PAR + UV-A (8 W m(-2)) = PA; PAR + UV-A + UV-B (0.4 W m(-2)) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F (v)/F (m)) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F (v)/F (m) during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F (v)/F (m )sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.

Analysis of fluorescent and non-fluorescent sea anemones from the Mediterranean Sea during a bleaching event

We examined the physiological responses to bleaching of two shallow water anemones, Anemonia sulcata var. smaragdina and A. rustica, common to the Mediterranean Sea. The tentacle coloration of A. sulcata var. smaragdina is caused by proteins homologous to the green fluorescent protein (GFP) from Aequorea victoria. Minor amounts of the transcripts for the same GFP-like proteins were also detected in tentacles of the non-fluorescent Anemonia rustica. During a bleaching event observed in the Mediterranean Sea, representatives of both species lost ∼ 90% of their zooxanthellae, indicating that the green fluorescent host pigmentation had no influence on the degree of bleaching of the two sea anemone species. Interestingly, the content of fluorescent proteins was also significantly reduced in bleached individuals of A. sulcata. The content of mycosporine-like aminoacids (MAAs) was essentially unaltered in bleached specimens. Bleached anemones were characterized by increased levels of superoxide dismutase activity, whereas the catalase activity in these animals was significantly reduced. The latter results indicate that oxidative stress might have been involved in the observed bleaching of Anemonia spp.