Climate change 2014 - Synthesis Report

Photoinhibition of photosystem II : inactivation, protein damage and turnover

Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

Tracing dissolved organic matter in aquatic environments using a new approach to fluorescence spectroscopy

Increases in ultraviolet radiation at the Earth's surface due to the depletion of the stratospheric ozone layer have recently fuelled interest in the mechanisms of various effects it might have on organisms. DNA is certainly one of the key targets for UV-induced damage in a variety of organisms ranging from bacteria to humans. UV radiation induces two of the most abundant mutagenic and cytotoxic DNA lesions such as cyclobutane–pyrimidine dimers (CPDs) and 6–4 photoproducts (6–4PPs) and their Dewar valence isomers. However, cells have developed a number of repair or tolerance mechanisms to counteract the DNA damage caused by UV or any other stressors. Photoreactivation with the help of the enzyme photolyase is one of the most important and frequently occurring repair mechanisms in a variety of organisms. Excision repair, which can be distinguished into base excision repair (BER) and nucleotide excision repair (NER), also plays an important role in DNA repair
in several organisms with the help of a number of glycosylases and polymerases, respectively. In addition, mechanisms such as mutagenic repair or dimer bypass, recombinational repair, cell-cycle checkpoints, apoptosis and certain alternative repair pathways are also operative in various organisms. This review deals with UV-induced DNA damage and the associated repair mechanisms as well as methods of detecting DNA damage and its future perspectives.

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UV-induced DNA damage and repair: a review

Severe reduction of stratospheric ozone over Antarctica has focused increasing concern on the biological effects of ultraviolet-B (UVB) radiation (280 to 320 nanometers). Measurements of photosynthesis from an experimental system, in which phytoplankton are exposed to a broad range of irradiance treatments, are fit to an analytical model to provide the spectral biological weighting function that can be used to predict the short-term effects of ozone depletion on aquatic photosynthesis. Results show that UVA (320 to 400 nanometers) significantly inhibits the photosynthesis of a marine diatom and a dinoflagellate, and that the effects of UVB are even more severe. Application of the model suggests that the Antarctic ozone hole might reduce near-surface photosynthesis by 12 to 15 percent, but less so at depth. The experimental system makes possible routine estimation of spectral weightings for natural phytoplankton.

https://science.sciencemag.org/content/sci/258/5082/646.full.pdf

Biological weighting function for the inhibition of phytoplankton photosynthesis by ultraviolet radiation.

Photosynthesis of Antarctic phytoplankton is inhibited by ambient ultraviolet (UV) radiation during incubations1,2,3,4, and the inhibition is worse in regions beneath the Antarctic ozone ‘hole’4 But to evaluate such effects, experimental results on, and existing models of, photosynthesis5,6,7 cannot be extrapolated directly to the conditions of the open waters of the Antarctic because vertical mixing of phytoplankton alters UV exposure and has significant effects on the integrated inhibition through the water column2,8,9 Here we present a model of UV-influenced photosynthesis in the presence of vertical mixing, which we constrain with comprehensive measurements from the Weddell-Scotia Confluence during the austral spring of 1993 Our calculations of photosynthesis integrated through the water column (denoted PT) show that photosynthesis is strongly inhibited by near-surface UV radiation This inhibition can be either enhanced or decreased by vertical mixing, depending on the depth of the mixed layer Predicted inhibition is most severe when mixing is rapid, extending to the lower part of the photic zone Our analysis reveals that an abrupt 50% reduction in stratospheric ozone could, in the worst case, lower PT by as much as 85% However, stronger influences on inhibition can come from realistic changes in vertical mixing (maximum effect on PT of about ±37%), measured differences in the sensitivity of phytoplankton to UV radiation (±46%) and cloudiness (±15%)

Interactive effects of ozone depletion and vertical mixing on photosynthesis of Antarctic phytoplankton

Introduction UV radiation is the photochemically most reactive waveband of the incident solar radiation field and causes a broad spectrum of genetic and cytotoxic effects in aquatic organisms. In the natural environment, these responses are offset by various protection strategies such as avoidance, screening, photochemical quenching and repair. The net stress imposed by UV exposure thus reflects a balance between damage, repair and the energetic costs of protection, and may be manifested in terms of increased energy demand, changes in cell composition, and decreased growth and survival rates (Figure 6.1). In this chapter, we focus on the damaging effects of UV exposure, while Chapter 7 examines the protection mechanisms that allow organisms to avoid, reduce or recover from such effects. At the cellular level the toxic effects of UV radiation are initiated by one of two photochemical pathways (Figure 6.1). Firstly, certain biomolecules such as proteins and nucleic acids have chromophores that absorb in the UV region of the spectrum. Under high UV fluxes these molecules are photochemically degraded or transformed, resulting in impairment or even complete loss of biological function. The magnitude of damage caused by these so-called direct or primary mechanisms is determined by the amount of radiation absorbed (optical cross-section in the UV range) and the quantum yield of photodestruction (molecules damaged per photon absorbed). A second class of UV toxicity effects is caused by a series of indirect mechanisms. UV is absorbed by some intermediate compound (photosensitising agent) either inside or outside the cell to produce reactive oxygen species (ROS).

The effects of UV radiation in the marine environment: Mechanisms of UV damage to aquatic organisms

Marine phytoplankton are sensitive to inhibition of photosynthesis by solar ultraviolet (UV) radiation, although sensitivity varies, depending on the growth environment. A mechanism suggested to increase resistance to UV inhibition is the accumulation of UV-absorbing compounds, such as the mycosporine-like amino acids (MAAs) found in many marine organisms. However, the effectiveness of these compounds as direct optical screens in microorganisms has remained unclear. The red-tide dinoflagellate Gymnodinium sanguineum Hirasaka accumulates about 14-fold more MAAs (per unit of chlorophyll) in high (76 W·m−2) than in low (15 W·m−2) growth irradiance. Biological weighting functions were estimated for UV inhibition of photosynthesis and showed that the high-light-grown cultures have lower sensitivity to UV radiation at wavelengths strongly absorbed by the MAAs. The time course of photosynthesis during exposure to UV radiation was measured using pulsed amplitude modulated (PAM) fluorometry and displayed a steady-state level after 15 min of exposure, indicating active repair of damage to the photosynthetic apparatus. Repair was blocked in the presence of the antibiotic streptomycin, yet high-light G. sanguineum remained less sensitive to UV radiation than did low-light cultures. These experiments show that MAAs act as spectrally specific UV sunscreens in phytoplankton.

Ultraviolet sunscreens in Gymnodinium sanguineum (dinophyceae): mycosporine-like amino acids protect against inhibition of photosynthesis

▪ Abstract Ozone depletion by anthropogenic gases has increased the atmospheric transmission of solar ultraviolet-B radiation (UV-B, 280–315 nm) Our understanding of the consequencences of enhanced UV-B levels on primary producers has grown dramatically over the past 20 years, but it has been hampered by how realistically experimental UV-B exposures mimic ozone-depletion scenarios Overcoming these shortcomings will require sophisticated and creative approaches Biological weighting functions and solar spectral irradiance estimates are critical in evaluating effects and require more attention Whereas UV screening compounds in terrestrial and aquatic producers commonly increase with UV-B exposure, the implications, while potentially far reaching, are unclear Photosynthesis is more sensitive to UV-B in phytoplankton than in terrestrial plants, probably owing to less effective screening in phytoplankton Productivity of terrestrial plants is usually unaffected by enhanced UV-B, although reduced growth has

Patrick J. Neale

Papers

Biological weighting function for the inhibition of phytoplankton photosynthesis by ultraviolet radiation.

Interactive effects of ozone depletion and vertical mixing on photosynthesis of Antarctic phytoplankton

The effects of UV radiation in the marine environment: Mechanisms of UV damage to aquatic organisms

Ultraviolet sunscreens in Gymnodinium sanguineum (dinophyceae): mycosporine-like amino acids protect against inhibition of photosynthesis

Effects of UV-B Radiation on Terrestrial and Aquatic Primary Producers*