MAA Synthesis and Accumulation in Polar Macroalgae are Controlled by Abiotic Factors

Mycosporine‐like amino acids (MAAs) are regarded as powerful sunscreens protecting the algae against harmful UV radiation. The MAA protection efficiency was tested in algal samples by measuring the optimum quantum yield of photosynthesis using photosystem II fluorescence. It could be demonstrated that the recovery of photosynthesis after exposure to enhanced UV radiation is faster in individuals with high MAA content. MAAs can be synthesized in several polar macroalgae in response to different radiation conditions. Although MAA induction patterns are very species‐specific, some similarities can be found. Field studies indicate that plants from different growth habitats providing distinct radiation climate can be grouped into three physiological categories depending on their MAA content. The first group (I) includes mainly deep‐water species, typically lacking MAAs. The second group (II), algal species found in a broad range of water depths (eu‐ and sublittoral), which are able to flexibly synthesize and accumulate MAAs. The third group (III) includes supra‐ and eulittoral taxa, which always contain high MAA concentrations. In laboratory studies, we showed that taxa of group II and III responded in three different ways based on MAA accumulation when exposed to different radiation conditions (PAR, PAR + UVA, PAR + UVA + UVB). Either they: (a) exhibit highest total MAA concentration under the full artificial spectrum; (b) increase their MAA concentration after exposure to PAR and PAR + UVA or (c) MAA concentration declines after exposure to the full spectrum. Our studies have indicated that when coupled with UVR, exposure to temperature fluctuations ranging from 0 to 10 °C also affect MAA biosynthesis.     

Dynamic exchange between stabilized conformations predicted for hyaluronan tetrasaccharides: comparison of molecular dynamics simulations with available NMR data

Studies of the hyaluronan (HA) tetrasaccharides are important for understanding hydrogen-bonding in the HA polymer, as they are probably the smallest oligomers in which characteristics of the constituent monosaccharides and the polymer are simultaneously exhibited. Here we present extensive molecular dynamics simulations of the two tetrasaccharides of HA in dilute aqueous solution. These simulations have confirmed the existence of intramolecular hydrogen-bonds between the neighboring sugar residues of HA in solution, as proposed by Scott (1989). However, our simulations predict that these intramolecular hydrogen-bonds are not static as previously proposed, but are in constant dynamic exchange on the sub-nanosecond time-scale. This process results in discrete internal motion of the HA tetrasaccharides where they rapidly move between low energy conformations. Specific interactions between water and intramolecular hydrogen-bonds involving the hydroxymethyl group were found to result in differing conformations and dynamics for the two alternative tetrasaccharides of HA. This new observation suggests that this residue may play a key role in the entropy and stability of HA in solution, allowing it to stay soluble up to high concentration. The vicinal coupling constants3 J NHCH of the acetamido groups have been calculated from our aqueous simulations of HA. We found that high values of 3J NHCH approximately 8 Hz, as experimentally measured for HA, are consistent with mixtures of both trans and cis conformations, and thus3 J NHCH cannot be used to imply a purely trans conformation of the acetamido. The rapid exchange of intramolecular hydrogen-bonds indicates that although the structure is at any moment stabilized by these hydrogen-bonds, no one hydrogen-bond exists for an extended period of time. This could explain why NMR often fails to provide evidence for intramolecular hydrogen-bonds in HA and other aqueous carbohydrate structures.      

Acclimation of brown algal photosynthesis to ultraviolet radiation in Arctic coastal waters (Spitsbergen, Norway)

In field studies conducted at the Kongsfjord (Spitsbergen) changes of the irradiance in the atmosphere and the sublittoral zone were monitored from the beginning of June until the end of August 1997, to register the minimum and maximum fluxes of ultraviolet and photosynthetically active radiation and to characterise the underwater light climate. Measurements of photosynthesis in three abundant brown algal species (Alaria esculenta, Laminaria saccharina, Saccorhiza dermatodea) were conducted to test whether their photosynthetic performance reflects changing light climate in accordance with depth. Plants sampled at various depths were exposed to controlled fluence rates of photosynthetically active radiation (400–700 nm), UV-A (320–400 nm) and UV-B (280–320 nm). Changes in photosynthetic performance during the treatments were monitored by measuring variable chlorophyll fluorescence of photosystem II. In each species, the degree of inhibition of photosynthesis was related to the original collection depth, i.e. shallow-water isolates were more resistant than plants from deeper waters. The results show that macroalgae acclimate effectively to increasing irradiance levels for both photosynthetically active and ultraviolet radiation. However, the kinetics of acclimation are different within the different species. It is shown that one important strategy to cope with higher irradiance levels in shallow waters is the capability for a faster recovery from high light stress compared to isolates from deeper waters. Received: 13 March 1998 / Accepted: 16 May 1998     

Photosynthesis of Marine Macroalgae from Antarctica: Light and Temperature Requirements

The photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae Himantothallus grandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsbergii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta, Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at 0°C, the green algae were cultivated at 10°C. I_K values were between 18 and 53 μmol m^?2 s^?1 characteristic or low light adapted algae. Only the two Ulothrix species showed higher I_K values between 70 and 74 μmol m^?2 s^?1. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 μmol m^?2 s^?1. Values of α were high: between 0.4 and 1.1 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the brown and red algae and between 2.1 and 4.9 μmol O₂ g^?1 FW h^?1 (μmol m^?2 s^?1)^?1 in the green algal species. At 0°C P_max values of the brown and red algae ranged from 6.8 to 19.1 μmol O₂ g^?1 FW h^?1 and were similarly high or higher than those of comparable Arctic-cold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis, 10°C in H. grandifolius, 15°C in G. skottsbergii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q₁₀ values for photosynthesis (1.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here.     

Growth kinetics related to physiological parameters in young Saccorhiza dermatodea and Alaria esculenta sporophytes exposed to UV radiation

Young sporophytes of Saccorhiza dermatodea and Alaria esculenta cultured from Spitsbergen isolates were exposed in the laboratory to either only photosynthetically active radiation (PAR) or to a spectrum including UV-radiation (PAR+UVA+UVB) by use of cutoff glass filters. The plants were grown at 8±2°C and 16:8 h light–dark cycles with 6 h additional UV exposure in the middle of the light period. Growth was measured every 10 min using growth chambers with online video measuring technique for 18–21 days. Tissue morphology and absorption spectra were measured in untreated young sporophytes while tissue chlorophyll-a content and DNA damage were measured from treated thalli at the end of the experiment. Under UVR, growth rates of S. dermatodea were significantly reduced while A. esculenta have a potential to acclimate. Tissue chlorophyll-a contents in both species were not significantly different between treatments suggesting that these algae may acclimate to moderate UVR fluence. Higher DNA damage in S. dermatodea effectively diverted photosynthetic products for repair constraining growth. Tissue optics (opacity and translucence) was correlated to the tissue absorbance in the UVR region characteristics of phlorotannin, an important UV-absorbing compound in brown macroalgae. Growth rates of sporophytes of both species exposed to PAR without UV was similar during day and night. The results showed that both species can recruit and inhabit a similar coastal zone when appropriate strategies are expressed to minimize damage in response to the stress factor.     

Adaptation and acclimation of growth and photosynthesis of five Antarctic red algae to low temperatures

Temperature requirements for growth, photosynthesis and dark respiration were determined for five Antarctic red algal species. After acclimation, the stenothermal species Gigartina skottsbergii and Ballia callitricha grew at 0 or up to 5 °C, respectively; the eurythermal species Kallymenia antarctica, Gymnogongrus antarcticus and Phyllophora ahnfeltioides grew up to 10 °C. The temperature optima of photosynthesis were between 10 and 15 °C in the stenothermal species and between 15 and 25 °C in the eurythermal species, irrespective of the growth temperature. This shows that the temperature optima for photosynthesis are located well below the optima from species of other biogeographical regions, even from the Arctic. Respiratory rates rose with increasing temperatures. In contrast to photosynthesis, no temperature optimum was evident between 0 and 25 °C. Partial acclimation of photosynthetic capacity to growth temperature was found in two species. B. callitricha and Gymnogongrus antarcticus acclimate to 0 °C, and 5 and 0 °C, respectively. But acclimation did in no case lead to an overall shift in the temperature optimum of photosynthesis. B. callitricha and Gymnogongrus antarcticus showed acclimation of respiration to 5 °C, and P. ahnfeltioides to 5 and 10 °C, resulting in a temperature independence of respiration when measured at growth temperature. With respect to the acclimation potential of the species, no distinction can be made between the stenothermal versus the eurythermal group. (Net)photosynthetic capacity:respiration (P:R) ratios showed in all species highest values at 0 °C and decreased continuously to values lower than 1.0 at 25 °C. In turn, the low P:R ratios at higher temperatures are assumed to determine the upper temperature growth limit of the studied species. Estimated daily carbon balance reached values between 4.1 and 30.7 mg C g⁻¹ FW day⁻¹ at 0 °C, 16:8 h light/dark cycle, 12–40 μmol m⁻² s⁻¹. Received: 4 November 1999 / Accepted: 7 March 2000     

Photosynthetic characteristics and mycosporine-like amino acids under UV radiation: a competitive advantage of Mastocarpus stellatus over Chondrus crispus at the Helgoland shoreline?

Chondrus crispus and Mastocarpus stellatus both inhabit the intertidal and upper sublittoral zone of Helgoland, but with C. crispus generally taking a lower position. Measurements of chlorophyll fluorescence, activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), and content and composition of UV absorbing mycosporine-like amino acids (MAAs) were conducted in the laboratory, to test whether susceptibility to UV radiation may play a role in the vertical distribution of these two species. Effective and maximal quantum yield of photochemistry as well as maximal electron transport rate (ETR_max) in C. crispus were more strongly affected by UV-B radiation than in M. stellatus. In both species, no negative effects of the respective radiation conditions were found on total activity of RubisCO. Total MAA content in M. stellatus was up to 6-fold higher than in C. crispus and the composition of MAAs in the two species was different. The results indicate that, among others, UV-B sensitivity may be a factor restricting C. crispus to the lower intertidal and upper sublittoral zone, whereas M. stellatus is better adapted to UV radiation and is therefore more competitive in the upper intertidal zone. Received: 15 November 1999 / Received in revised form: 28 February 2000 / Accepted: 10 March 2000     

Solar ultraviolet radiation affects the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase and the composition of photosynthetic and xanthophyll cycle pigments in the intertidal green alga Ulva lactuca L

The effect of solar UV radiation on the physiology of the intertidal green macroalga Ulva lactuca L. was investigated. A natural Ulva community at the shore of Helgoland was covered with screening foils, excluding UV-B or UV-B + UV-A from the solar spectrum. In the sampled material, changes in the activity and concentration of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), and the concentration of photosynthetic and xanthophyll cycle pigments were determined. Exclusion of UV radiation from the natural solar spectrum resulted in an elevated overall activity of Rubisco, related to an increase in its cellular concentration. Among the photosynthetic pigments, lutein concentration was substantially elevated under UV exclusion. In addition, marked UV effects on the xanthophyll cycle were found: exclusion of solar UV radiation (and particularly UV-B) resulted in an increased ratio of zeaxanthin concentration to the total xanthophyll content, indicating adverse effects of UV-B on the efficiency of photoprotection under high irradiances of photosynthetically active radiation. The results confirm a marked impact of present UV-B levels on macroalgal physiology under field conditions.     

Interactive effects of ultraviolet radiation and salinity on the ecophysiology of two Arctic red algae from shallow waters

In a comparative ecophysiological study, the abundant red macroalgae Devaleraea ramentacea (L.) Guiry and Palmaria palmata (L.) O. Kuntze from shallow waters of the Arctic Kongsfjord (Spitsbergen) were exposed to hyposaline and hypersaline media, in combination with and without artificial UV radiation, to evaluate the interactive effects of both environmental parameters on optimum quantum yield of photosynthesis, as well as on the physiological capability to synthesise and accumulate photoprotective mycosporine-like amino acids (MAAs). While D. ramentacea exhibited euryhaline features and acclimated well to the UV radiation applied, P. palmata can be characterised as a stenohaline plant because of its high mortality even under mild hyposaline conditions (15 PSU). In addition, the latter species showed a limited ability to acclimate to changing PAR/UV radiation, pointing to a relatively low physiological plasticity. Both species synthesised and accumulated MAAs after UV treatment. However, only in D. ramentacea was a correlation between increasing MAA concentration and decreasing photosynthetic sensitivity under UV observed. All ecophysiological data from the laboratory correlate well with field observations, where both red-algal species co-exist in the same shallow-water habitat of the Kongsfjord. However, while P. palmata becomes more often greenish, sometimes slightly bleached over the summer months, D. ramentacea appears much more healthy under the prevailing environmental conditions.