Angela Wulff

SHORT-TERM EFFECTS OF UVB RADIATION ON CHLOROPHYLL FLUORESCENCE, BIOMASS, PIGMENTS, AND CARBOHYDRATE FRACTIONS IN A BENTHIC DIATOM MAT

The effects on UVB radiation on a subtidal, cohesive‐sediment biofilm dominated by the diatom Gyrosigma balticum (Ehrenberg) Rabenhorst were investigated. Chlorophyll fluorescence parameters (Fv/Fm, φPSII), pigment concentrations, cell densities, and carbohydrate fractions were measured in four treatments (no UVBR, ambient UVBR, +7%, and +15% enhancement with UVBR). Enhanced UVBR was provided by a computer‐controlled system directly linked to natural diel UVBR levels. Increases in φPSII values in the UVBR‐enhanced treatments and a decrease in the steady‐state fluorescence yield (Fs) from the surface of the biofilms during the middle and latter part of daily exposure periods suggested that G. balticum responded to enhanced UVBR by migrating down into the sediment. Diatoms in the +15% UVBR treatment also had significantly higher concentrations of β‐carotene after 5 days of treatment. Although G. balticum responded to enhanced UVBR by migration and increased β‐carotene concentrations, significant reduction in maximum quantum yield of PSII (Fv/Fm) and in minimal fluorescence (Fo) and decreases in cell densities occurred after 5 days. Concentrations of different carbohydrate fractions (colloidal carbohydrate, glucan, exopolymers [EPS]) associated with diatom biomass and motility also decreased in the UVBR‐enhanced treatments. Short‐term responses (migration) to avoid UVBR appear insufficient to prevent longer‐term decreases in photosynthetic potential and biofilm carbohydrate concentration and biomass.

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.

Effect of Sediment Load on the Microbenthic Community of a Shallow-Water Sandy Sediment

Anthropogenic activities, such as construction work, dredging, and different kinds of recreation activities, can alter sediment loading in shallow coastal areas. The effect of increased load of fine sediment on the microbenthos (benthic microalgae, bacteria, and meiofauna) was studied in two experiments using undisturbed cores of a sandy sediment from a microtidal bay on the Swedish west coast. In each experiment, a total of 24 cores were incubated in an outdoor flow-through set-up. Twelve cores were treated with a 2.5-mm thick layer of autoclaved fine-grained, (silt) carbon-rich surface sediment. In the first experiment, estimates of the impact were based on measurements of chlorophylla, biomass of microalgae, bacteria, and meiofauna, and bacterial production. The main purpose of the second experiment was to study the effect on sediment oxygen profiles using microsensors. Within a week, after being covered by fine sediment, benthic microalgae (particularly diatoms) had migrated upward and the oxygen profiles were restored at the sediment surface by photosynthesis. However, the oxygen-producing layer became thinner and the algal composition changed. Bacterial biomass was restored to the same level as in the sandy sediment. Meiofauna also appeared to move upward and the meiofaunal composition was re-established. The results suggest that the microbenthic community of sandy sediment has an inherent capacity to recover after a moderate deposition of fine-particle sediment. Active upward migration of benthic diatoms appears to be a key mechanism for restoring the oxygenation of the sediment surface. The altered sediment type also implies changed species composition, and hence altered benthic trophic interactions, which may affect, for example, flatfish recruitment.

Grazing and UV radiation effects on an Antarctic intertidal microalgal assemblage: a long-term field study

A 15xa0week field experiment (austral summer Nov–Mar) was carried out in an intertidal hard bottom platform in Antarctica (King George Island). To test whether grazing and ultraviolet radiation (UVR) influenced the succession of a benthic microalgal assemblage, a two-factorial design was used (1) ambient radiation, >280xa0nm; (2) ambient minus UV-B, >320xa0nm; (3) ambient minus UVR, >400xa0nm versus grazer–no grazer). On four sampling occasions microalgae were identified, counted and carbon contents were calculated. The assemblage was dominated by the diatom genera Navicula and Cocconeis. Biomass was generally low in all treatments but was significantly reduced by grazing throughout the experiment. No significant UV effects were found. Grazer absence particularly favoured diatoms of the genus Cocconeis. We conclude that the Antarctic microalgal assemblage was unaffected by present day UVR whereas grazers acted as important drivers on the intertidal microalgal community structure.

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.

Marine benthic diatoms from Potter Cove, King George Island, Antarctica

Abstract Antarctic benthic marine diatoms from the Potter Cove region, King George Island were studied in samples collected during the austral summer 2003. A floristic list was made to add information on the Antarctic benthic diatom distribution. A total of 84 species was identified from four localities in Potter Cove, the majority of which are of cosmopolitan distribution. The most common taxa encountered were Cocconeis spp., Gyrosigma fasciola, Navicula cf. cancellata, N. cf. perminuta, Petroneis plagiostoma and Pleurosigma obscurum. Both G. fasciola and P. obscurum are recorded for the first time from Antarctica with such common occurrence. The overall diatom population in Potter Cove appeared rather different from other diatom populations observed in Antarctic marine habitats.

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.

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

AbstractThe depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m−2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.n

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.