Frithjof C. Küpper

Heavy metal-induced inhibition of photosynthesis: targets of in vivo heavy metal chlorophyll formation

The targets of heavy metal (here Cu2+ and Zn2+) attack on the photosynthetic apparatus of algae belonging to different phyla were investigated. Experiments with the green alga Scenedesmus quadricauda confirmed previous findings that according to the irradiance level two different phenomena occur, which were further characterized by specific changes in several photosynthetic parameters. The reaction occurring under low irradiance (shade reaction) is characterized by heavy metal substitution of Mg2+ in chl molecules bound predominantly in the light harvesting complex II of Chlorophyta (LHC II). Under high irradiance (sun reaction) the LHC II chls are inaccessible to substitution and the damage occurs in the PSII reaction center instead. Algae with antenna proteins other than the LHC II did not show the two types of heavy metal attack at different irradiances. In red algae (Antithamnion plumula), low Cu2+ concentrations induced the sun reaction even at very low irradiance. In brown algae (Ectocarpus siliculosus) the shade reaction occurred even in saturating irradiance. These results also indicate that despite some similarity in their features, the primary step of the sun reaction and photoinhibition is different.

In situ detection of heavy metal substituted chlorophylls in water plants

The in vivo substitution of magnesium, the central atom of chlorophyll, by heavy metals (mercury, copper, cadmium, nickel, zinc, lead) leads to a breakdown in photosynthesis and is an important damage mechanism in heavy metal-stressed plants. In this study, a number of methods are presented for the efficient in situ detection of this substitution (i.e. in whole plants or in chloroplasts). While macroscopic observations point to the formation of heavy metal chlorophylls at higher concentrations, fluorescence microscopy enables the detection of this reaction at very low substitution rates. Therefore, the course of the reaction can be followed by continuously measuring the fluorescence of whole plants. Furthermore absorbance spectroscopy of whole cells or isolated chloroplasts also enables the in situ detection of heavy metal chlorophylls. These methods provide practicable approaches in detecting the formation of these compounds in situ, avoiding artefacts that might occur using extraction methods based on polar solvents. In addition to the new methods for in situ detection, an extreme heterogeneity in the reaction of cells in the same tissue upon heavy metal stress was observed: while some cells are already disintegrating, others still show normal fluorescence and photosynthetic activity. Measurements of fluorescence kinetics gave a further hint that in high light intensity a substitution of Mg by heavy metals might take place specifically in PS II reaction centres.

Novel biogenic iodine-containing trihalomethanes and other short-lived halocarbons in the coastal East Atlantic

Reactive halogen photochemistry and its impact on tropospheric oxidant levels have recently attracted intense research interest following the observation of the iodine oxide radical at midlatitudes. During September 1998, short-lived organoiodines including CH3I, C2H5I, CH2ICl, CH2IBr, CH2I2, and the hitherto undetected CHIBr2, as well as the organobromines CHBr3, CH2Br2, CHBr2Cl, CH3Br, and C2H5Br, were measured in air and seawater at and around Mace Head, on the west coast of Ireland. The release rates of organic bromines and iodines from seaweeds were determined from incubations of 10 species of brown, red, and green macroalgae collected in the intertidal or subtidal zones of the rocky shore. For all the brown algae studied, iodine was released mainly as CH2I2. However, for several seaweeds, the novel iodine-containing trihalomethanes CHIBr2 and CHI2Cl represented a significant fraction of the released organic iodine. The macroalgae incubation experiments as well as monitoring of the in situ concentrations in a rock pool indicated that natural halocarbon production by seaweeds was stimulated by incident light. The halocarbon fluxes derived from the seaweed incubations, coupled with published detailed biomass surveys, enabled coastal organohalogen seawater concentrations to be estimated. The CHBr3, CH2Br2, and CHBr2Cl concentrations calculated by this method compared well with coastal surface seawater measurements, implying that macroalgae were the major sources of the polybromomethanes. Measured CH3Br, CH3I, and CH2ICl levels were higher than calculated, which may be due to the existence of additional sources. CH3Br production by macroalgae accounted for less than 10% of measured levels in coastal waters. Short-lived iodocarbons such as CH2I2 and CHIBr2 were depleted in surface seawater compared to calculated levels, implying their photolytic loss within the upper water column.

Oligoalginate Recognition and Oxidative Burst Play a Key Role in Natural and Induced Resistance of Sporophytes of Laminariales

Forty-five species of brown algae (Phaeophyceae) were surveyed for their capacity to respond by an oxidative burst to challenges with alginate oligosaccharides. Intertidal frondose brown algae (Fucales) constitutively released high quantities of peroxide. The capacity to recognize oligoguluronates and to react with an oxidative burst was confined to alginate-rich taxa with complex thallus morphology, epitomized by the sporophytes of Laminariales. When kelp sporophytes were impaired in their capacity to perform an oxidative burst by the NAD(P)Hoxidase inhibitor diphenylene iodonium, they were readily degraded by their bacterial epiflora. Thus, in these algae, the oxidative response is an essential element of natural resistance. We also report on the establishment of a well-defined experimental system for investigations on kelp immunity, with Laminaria digitata as the host and its phaeophycean endophyte, Laminariocolax tomentosoides, as the pathogen. We found that an alginate-triggered oxidative burst significantly induces resistance in Laminaria digitata against infection. From these findings we infer that oligoalginate signals are important cues in the interaction between laminarialean kelps and potential pathogens.

Oligosaccharide recognition signals and defence reactions in marine plant-microbe interactions

Recent findings on the involvement of oligosaccharide signals in pathogen recognition and defence reactions in marine algae shine a new light on the ecology of their interactions with associated microorganisms. Since the marine environment encompasses lineages that have diverged a long time ago from the terrestrial phyla, these results suggest that cell-cell recognition pathways typical of terrestrial plants appeared very early in the evolution of eukaryotes. Production of oligosaccharides from marine algae using microbial recombinant polysaccharidases is also of industrial interest as plants can be protected from infections by preincubation in the presence of appropriate signals that mimic the attacks by pathogens.

Free Fatty Acids and Methyl Jasmonate Trigger Defense Reactions in Laminaria digitata

Arachidonic acid, linolenic acid and methyl jasmonate (MeJA) were found to be strong triggers of an oxidative burst in the kelp Laminaria digitata. These findings constitute the first report of an oxidative burst in an algal system induced by free fatty acids. The source of reactive oxygen species can be at least partially inhibited by diphenylene iodonium (DPI). Treatment with arachidonic acid increases the levels of a number of free fatty acids [including myristic (C14:0), linoleic (C18:2), linolenic (C18:3) and eicosapentaeneoic (C20:5) acids] and hydroxylated derivatives [such as 15-hydroxyeicosatetraenoic acid (15-HETE), 13-hydroxyoctadecatrienoic acid (13-HOTE) and 15-hydroxyeicosapentaenoic acid (15-HEPE)]. Similar to a previous report of the function of an alginate oligosaccharide-triggered oxidative burst in the establishment of resistance in L. digitata against infection by its brown algal endophyte Laminariocolax tomentosoides, C20:4- and MeJA-induced oxidative bursts seem to be involved in establishing the same protection in L. digitata. Altogether, this study supports the notion that lipid oxidation signaling plays a key role in defense induction in marine brown algae.

Infection experiments reveal broad host ranges of Eurychasma dicksonii (Oomycota) and Chytridium polysiphoniae (Chytridiomycota), two eukaryotic parasites in marine brown algae (Phaeophyceae)

Unialgal cultures of the brown alga Pylaiella littoralis (L.) Kjellman infected by either Eurychasma dicksonii (Wright) Magnus (Oomycota) or Chytridium polysiphoniae (Cohn) H. E. Petersen (Chytridiomycota) were used to elaborate the host ranges of these pathogens. Infection experiments with 48 host species covering 13 orders of the Phaeophyceae showed that 45 species were susceptible to attack by Eurychasma and 23 to Chytridium. The two pathogens showed host‐specific differences in generation times: while in Pylaiella the shortest cycles were 16 days for Eurychasma and 6 days for Chytridium, one and five days more, respectively, were required in Acinetospora. Heavy parasite attack on the microscopic stages of host species with heteromorphic life histories, like kelps (Laminariales), is documented and discussed as a potential regulatory factor for the population dynamics of macroalgae.

X-ray absorption spectroscopic studies on model compounds for biological iodine and bromine

X-ray absorption spectra of a number of organic iodine and bromine compounds of biological relevance, as well as of a series of iodine compounds with different oxidation states, have been measured. The iodine K-edge spectra (XANES) are found to be relatively featureless but the position of the edge is found to be sensitive to formal valence (among other factors), and the edge shape to the number of bound O atoms. EXAFS spectra of organohalogen compounds (both iodine and bromine) can be used to discriminate between aliphatic and aromatic compounds. There are differences both in the distances from the halogens to the first shell of C atoms, which are shorter for aromatic compounds, and in the patterns of shells in the Fourier transforms. This result is expected to be relevant to studies at these edges in biological systems.

The Potential Impact of Climate Change on Endophyte Infections in Kelp Sporophytes

There is a strong scientific consensus that coastal marine ecosystems are threatened by global climate change. These ecosystems are particularly vulnerable as many disturbances act at the terrestrial–marine interface and are predicted to increase, such as increased land run-off after floods or higher wave energies owing to increased storm frequency (Helmuth et al., 2006; IPCC, 2007). An alarming decrease in the density and biomass of canopy-forming kelps has been reported worldwide (Dayton et al., 1999; Steneck et al., 2002; Connell et al., 2008) and recent European monitoring programs indicate substantial losses of Laminaria digitata in France (Morizur, 2001) and of Saccharina latissima (formerly L. saccharina) along the Southwest coast of Norway and Sweden (survey in 1996–2006, Norwegian Institute for Water Research, 2007) and on the German island Helgoland (Pehlke and Bartsch, 2008). For instance, the losses of S. latissima at the Norwegian West and Skagerrak coasts are estimated to be 50% and 90%, respectively. Here, the decline in kelp abundance is most pronounced in sheltered waters, where the kelp forest in large areas has been replaced by a silty turf community dominated by filamentous algae. Anthropogenic influences, such as eutrophication and global climate change, have been postulated as possible causes for the loss of canopy-forming kelps. However, substantial scientific evidence is still lacking.