Katharina Zacher

The abiotic environment of polar marine benthic algae.

Due to different oceanographic and geological characteristics, benthic algal communities of Antarctica and the Arctic differ strongly. Antarctica is characterized by high endemism, whereas in the Arctic only few endemic seaweeds occur. In contrast to the Antarctic region, where nutrient levels never limit algal growth, nutrient levels in the Arctic regions are depleted during the summer season. Both regions have a strong seasonally changing light regime, fortified by an ice covering throughout the winter months. After months of darkness algae are suddenly exposed to high light caused by the breaking up of sea ice. Simultaneously, harmful ultraviolet radiation (UVR) entersthe water column and can significantly affect algal growth and community structure. In the intertidal zone fluctuations of temperature and salinity can be very large. Ice scours can further influence growth and settlement of intertidal algae. The subtidal zone offers a more stable habitat than the intertidal,permitting the growth of larger perennial algae and microbial mats. Polar regions are the areas most affected by global climate change, i.e. glacier retreat, increasing temperature and sedimentation, with yet unknown consequences for the polar ecosystem.

Drivers of colonization and succession in polar benthic macro- and microalgal communities

Information on succession in marine benthic primary producers in polar regions is very scarce, particularly with regard to effects of abiotic and biotic drivers of community structure. Primary succession begins with rapid colonizers, such as diatoms and ephemeral macroalgae, whereas slow, highly seasonal recruitment and growth are characteristic of annual or perennial seaweed species. Colonization of intertidal and subtidal assemblages on polar rocky shores is severely affected by physical disturbance and by seasonal changes in abiotic conditions. Biotic factors, such as grazing, can strongly affect colonization patterns and also alter competitive interactions among benthic algae. Ambient UV radiation affects the diversity of macroalgal communities during early and later stages of succession. In contrast, microalgal assemblages have high tolerance to UV stress. Climate warming could alter algal latitudinal distribution and favor invasion of polar regions by cold-temperate species. Reduced sea ice cover and retreating glaciers could expand colonization areas but alter light, salinity, sedimentation and disturbance processes. Although the key role of macroalgae in coastal systems and, to a much reduced extent, the importance of microphytobenthos have been documented for polar regions, information on the successional process is incomplete and will benefit from further ecological studies.

Susceptibility of spores of different ploidy levels from Antarctic Gigartina skottsbergii (Gigartinales, Rhodophyta) to ultraviolet radiation

Abstract Haploid tetraspores and diploid carpospores from Antarctic Gigartina skottsbergii were exposed in the laboratory to photosynthetically active radiation (400–700 nm  =  P), P + ultraviolet (UV)-A radiation (320–700 nm  =  PA) and P + UV-A + UV-B radiation (280–700 nm  =  PAB). Photosynthetic performance, DNA damage and repair, spore mortality, and an initial characterization of the UV-absorbing mycosporine-like amino acids (MAAs) were studied. Rapid photosynthesis vs irradiance (E) curves of freshly released spores showed that both tetraspores and carpospores were low-light adapted (Ek  =  44 ± 2 and 54 ± 2 µmol photons m−2 s−1, respectively). The light-harvesting and photosynthetic conversion efficiencies were similar (α  =  0.13), whereas photosynthetic capacity in terms of optimum quantum yield (Fv/Fm) and relative electron transport rate (rETRmax) were significantly higher in carpospores. Photoinhibition and recovery of photosynthesis were not significantly different between spore ploidy but w...

Responses of Antarctic Iridaea cordata (Rhodophyta) tetraspores exposed to ultraviolet radiation

In Antarctica ozone depletion is highest during spring, coinciding with the reproduction of many seaweed species. Propagules are the life‐stage of an alga most susceptible to environmental perturbations. Therefore, fertile thalli of Iridaea cordata (Turner) Bory (Rhodophyta) were collected in the eulittoral of King George Island (Antarctica) to examine spore susceptibility to ultraviolet radiation (UVR). In the laboratory, freshly released tetraspores were exposed to photosynthetically active radiation (PAR) (400–700 nm), PAR+UV‐A (320–700 nm) or PAR+UV‐A+UV‐B (280–700 nm). Photosynthetic efficiency was measured during 1–8 h of exposure and after 48 h of recovery. Additionally, mycosporine‐like amino acids (MAAs) and DNA damage were determined. Saturating irradiance of photosynthesis of freshly released tetraspores was 57 µmol photons m−2 s−1. Exposure to increasing fluence of PAR reduced photosynthetic efficiency. UVR further decreased the photosynthetic efficiencies of the tetraspores but spores were able to recover completely after UVR exposure and 2 days post‐cultivation under low PAR. DNA damage was minimal and lesions were effectively repaired under photoreactivating light. Concentrations of the MAAs shinorine and palythine were higher in tetraspores treated with UVR than in spores only exposed to PAR. Generally, the tetraspores show a good UV tolerance. This flexible response of the tetraspores of this species to changing radiation conditions enables the alga to grow along a considerable depth gradient from the sublittoral to the eulittoral where they can be exposed to enhanced UVBR under conditions of stratospheric ozone depletion.

UV radiation : a threat to Antarctic benthic marine diatoms?

Abstract This investigation was motivated by the lack of ultraviolet radiation (UVR, 280–400 nm) studies on Antarctic benthic marine microalgae. The objective was to estimate the impact of UV-B (280–315 nm) and UV-A (315–400 nm), on photosynthetic efficiency, species composition, cell density and specific growth rate in a semi-natural soft-bottom diatom community. In both experiments, cell density increased over time. The most frequently observed species were Navicula cancellata, Cylindrotheca closterium, Nitzschia spp., and Petroneis plagiostoma. For both experiments, a shift in species composition and a decreased photosystem II (PSII) maximum efficiency (Fv/Fm) over time was observed, irrespective of treatment. UVR significantly reduced Fv/Fm on days 3 and 10 (Expt 1), disappearing on the last sampling date. A similar trend was found in Expt 2. A significant UV effect on cell density was observed in Expt 1 (day 10) but not in Expt 2. No treatment effects on species composition or specific growth rate were found. Thus, the UV effects were transient (photosynthetic efficiency and cell density) and the growth of the benthic diatoms was generally unaffected. Overall, according to our results, UVR does not seem to be a threat to benthic marine Antarctic diatoms.

EXPOSURE TO SUDDEN LIGHT BURST AFTER PROLONGED DARKNESS—A CASE STUDY ON BENTHIC DIATOMS IN ANTARCTICA

In polar areas, benthic diatoms are regarded to play a major role in supplying energy to the benthic fauna, particularly prior to the release of microalgae from sea ice and the phytoplankton bloom. As phototrophs, benthic polar diatoms have to contend not only with dark polar nights but also with darkness due to sea-ice and snow cover that can prevail in the littoral zone for additional months. Upon sea ice break-up the autotrophs are suddenly exposed to high light intensities including ultraviolet radiation. The aim of our study was to mimic a sudden spring-time sea ice break-up, focusing on the ultraviolet part of the solar spectrum. We therefore exposed a semi-natural community of benthic diatoms to light burst after a period of total darkness. We studied the effects of different spectral qualities: photosynthetically active radiation (PAR, 400–700 nm; P treatment), PAR+ UV-A (UV-A 320–400 nm; PA treatment), and PAR+UV-A+UV-B (UV-B 280–320 nm; PAB treatment) on cell number (growth), species composition and optimum quantum yield (Fv/Fm) in 2 separate experiments where diatoms were kept in darkness for 15 and 64 days, respectively. In both experiments, the most frequently (>50%) observed species were Gyrosigma fasciola and G. obscururn. No growth was observed and no resting spores were found. In both experiments, the initial optimum quantum yield of the PSII prior to dark treatment was comparable (Fv/Fm = 0.70). The Fv/Fm was not affected after 15 days dark incubation but a significant decrease in photosynthetic efficiency was observed after 64 days in the dark (Fv/Fm = 0.39). Exposure to different light treatments (P, PA, PAB) immediately after different dark incubation periods showed higher reduction in Fv/Fm (PAB > PA > P) after the longer dark period. Estimated P-E curve parameters showed an efficient light harvesting and photosynthetic conversion capacity (α= 0.20; rETRmax=14) that was significantly reduced after 64 days in the dark (α= 0.06; rETRmax=8). The reduction in these photo-physiological indices (a and rETRmax) after dark incubation was compensated with higher saturating irradiance (Ek), which we suspect to be a mechanism to optimize photochemical processes. But the PSII antenna was relatively light-sensitive because photosynthesis was already photoinhibited at half the photon flux density (≥ 585 μmol photons m–2 s–1) relative to light-adapted (≥ 972 μmol photons m–2 s–1) diatoms. We conclude that the benthic diatoms in our study were able to resume photosynthetic activity after 64 days in darkness and they were able to cope with relatively high intensities of UV radiation compared with their natural habitat.

Understanding the link between sea ice, ice scour and Antarctic benthic biodiversity - the need for cross-station and international collaboration

The western Antarctic Peninsula (WAP) is a hotspot of rapid recent regional ‘climate change’. This has resulted in a 0.4°C rise in sea temperature in the last 50 years, five days of sea ice lost per decade and increased ice scouring in the shallows. The WAP shallows are ideal for studying the biological response to physical change because most known Antarctic species are benthic, physical change occurs mainly in the shallows and most research stations are coastal. Studies at Rothera Station have found increased benthic disturbance with losses of winter sea ice and assemblage-level changes coincident with this ice scouring. Such studies are difficult to scale up as they depend on SCUBA diving – a very spatially limited technique. Here we report attempts to broaden the understanding of benthic ecosystem responses to physical change by replicating the Rothera experimental grids at Carlini Station through collaboration between the UK, Argentina and Germany across Signy, Rothera and Carlini stations. We argue that such collaborations are the way forward towards understanding the big picture of biota responses to physical climate changes at a regional scale.

Species composition, zonation and biomass of seaweeds in Kongsfjorden, Svalbard - a basline study

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