The role of solar UV radiation in the ecology of alpine lakes.

Solar ultraviolet radiation (UVR, 290-400 nm) is a crucial environmental factor in alpine lakes because of the natural increase of the UVR flux with elevation and the high water transparency of these ecosystems. The ecological importance of UVR, however, has only recently been recognized. This review, examines the general features of alpine lakes regarding UVR, summarizes what is known about the role of solar UVR in the ecology of alpine lakes, and identifies future research directions. Unlike the pattern observed in most lowland lakes, variability of UV attenuation in alpine lakes is poorly explained by differences in dissolved organic carbon (DOC) concentrations, and depends mainly on optical characteristics (absorption) of the chromophoric dissolved organic matter (CDOM). Within the water column of lakes with low DOC concentrations (0.2-0.4 mg l(-1)), UV attenuation is influenced by phytoplankton whose development at depth (i.e. the deep chlorophyll maximum) causes important changes in UV attenuation. Alpine aquatic organisms have developed a number of strategies to minimize UV damage. The widespread synthesis or bioaccumulation of different compounds that directly or indirectly absorb UV energy is one such strategy. Although most benthic and planktonic primary producers and crustacean zooplankton are well adapted to high intensities of solar radiation, heterotrophic protists, bacteria, and viruses seem to be particularly sensitive to UVR. Understanding the overall impact of UVR on alpine lakes would need to consider synergistic and antagonistic processes resulting from the pronounced climatic warming, which have the potential to modify the UV underwater climate and consequently the stress on aquatic organisms.     

Impact of solar ultraviolet radiation on marine phytoplankton of Patagonia, Argentina

Patagonia area is located in close proximity to the Antarctic ozone "hole" and thus receives enhanced ultraviolet B (UV-B) radiation (280-315 nm) in addition to the normal levels of ultraviolet A (UV-A; 315-400 nm) and photosynthetically available radiation (PAR; 400-700 nm). In marine ecosystems of Patagonia, normal ultraviolet radiation (UVR) levels affect phytoplankton assemblages during the three phases of the annual succession: (1) prebloom season (late summer-fall), (2) bloom season (winter-early spring) and (3) postbloom season (late spring-summer). Small-size cells characterize the pre- and postbloom communities, which have a relatively high photosynthetic inhibition because of high UVR levels during those seasons. During the bloom, characterized by microplankton diatoms, photosynthetic inhibition is low because of the low UVR levels reaching the earth's surface during winter; this community, however, is more sensitive to UV-B when inhibition is normalized by irradiance (i.e. biological weighting functions). In situ studies have shown that UVR significantly affects not only photosynthesis but also the DNA molecule, but these negative effects are rapidly reduced in the water column because of the differential attenuation of solar radiation. UVR also affects photosynthesis versus irradiance (P vs E) parameters of some natural phytoplankton assemblages (i.e. during the pre- but not during the postbloom season). However, there is a significant temporal variability of P vs E parameters, which are influenced by the nutrient status of cells and taxonomic composition; taxonomic composition is in turn associated with the stratification conditions (e.g. wind speed and duration). In Patagonia, wind speed is one of the most important variables that conditions the development of the winter bloom by regulating the depth of the upper mixed layer (UML) and hence the mean irradiance received by cells. Studies on the interactive effects of UVR and mixing show that responses of phytoplankton vary according to the taxonomic composition and cell structure of assemblages; therefore cells use UVR if >90% of the euphotic zone is being mixed. In fact, cell size plays a very important role when estimating the impact of UVR on phytoplankton, with large cells being more sensitive when determining photosynthesis inhibition, whereas small cells are more sensitive to DNA damage. Finally, in long-term experiments, it was determined that UVR can shape the diatom community structure in some assemblages of coastal waters, but it is virtually unknown how these changes affect the trophodynamics of marine systems. Future studies should consider the combined effects of UVR on both phytoplankton and grazers to establish potential changes in biodiversity of the area.     

Ultraviolet Radiation Induces Filamentation in Bacterial Assemblages from North Andean Patagonian Lakes

Through laboratory experiments, we tested whether UV radiation (UVR) induces filamentation in natural bacteria assemblages from North Andean Patagonian lakes. We incubated water from three different lakes for 72 h in four separate treatments: (1) UVR + PAR (photosynthetically active radiation), (2) 50% UVR + PAR, (3) PAR and (4) 50% PAR. The irradiance levels used in the experiments were equivalent to those registered at the epilimnion of the lakes. In the UVR treatments filamentation was induced after the first 24 h and the proportion continued to increase for the next 48–72 h. A comparison of the gross composition and diversity of the entire community (cells >0.2 μm) with bacterial filaments alone (>5.0 μm) showed that UVR-induced filamentation is not a feature of any particular cluster. By sequencing part of the 16S rRNA gene of the taxonomic units obtained using denaturing gels, we observed that strains in the β-Proteobacteria group were of relatively high importance in filament formation, followed by Cytophaga–Flavobacterium–Bacteroides, γ-Proteobacteria and α-Proteobacteria, whereas Actinobacteria were almost nonexistent in the filaments. We propose that UVR doses equivalent to those of Andean lakes produce bacterial morphological changes, and that all bacterial groups except Actinobacteria can potentially form filaments.     

Photodegradation of natural organic matter exposed to fluctuating levels of solar radiation

Irradiation of natural water samples with natural or artificial UVR typically results in a progressive loss of color and decreased absorbance; a process often referred to as photobleaching. In a typical photobleaching experiment, samples are exposed to a relatively constant level of artificial or natural UVR. However, under most natural situations, the vertical mixing of the water within the upper mixed layer results in strong and periodic fluctuations in UV irradiance. In this paper, we present the results of an experiment in which natural lake water was exposed to solar radiation in quartz tubes that were incubated either at fixed depths or rotating within the water column. We found differences between rotating and fixed samples in (i) photobleaching, (ii) nutrient release, and (iii) subsequent use by algae and bacteria. The evidence presented in this study demonstrated that photochemical processes might be affected by vertical water motion. The reasons for such differences remain largely unknown. Although we offer a potential explanation for such differences, our proposed mechanism is based on a post-hoc analysis of the data and should be taken solely as a working hypothesis for future research.     

Effect of UVR on Lake Water and Macrophyte Leachates in Shallow Andean-Patagonian Lakes: Bacterial Response to Changes in Optical Features

The aim of this study was to identify bacterial responses in two shallow lakes from Patagonia to UV-irradiated dissolved organic matter (DOM) coming from different sources. We carried out laboratory experiments in which natural lake water and Potamogeton linguatus leachates were irradiated (UVA-340 fluorescent tubes Q-Panel) or kept in darkness. Natural bacterial assemblages were then incubated in four treatments: natural lake water, irradiated lake water, macrophyte leachate and irradiated macrophyte leachate. We estimated bacterial abundance, composition and activity, and changes in the optical features of DOM. Our results showed that the addition of leachates caused an increase in the DOM mean molecular size. After UV exposure, a high bacterial activity was observed in lake water treatments. On the contrary, carbon uptake by bacteria was reduced in the irradiated leachate treatment. The degree of aromatization in the leachate treatments increased and thus may contribute to a dissolved carbon less available for bacterial activity. Regarding the bacteria assemblage we observed that β -Proteobacteria outcompete the other groups in the leachate treatments, this group being more efficient at utilizing the high molecular weight DOM. These results highlight the importance of UVR interacting with different DOM sources in bacteria responses of shallow lakes.     

Vertical mixing and ecological effects of ultraviolet radiation in planktonic communities

We present a mathematical model for a phytoplankton-zooplankton system, based on a predator-prey scheme. The model considers the effects of sinking in the phytoplankton, vertical mixing and attenuation of photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) in the water column. In a first approach, the model was studied under conditions of average PAR irradiance and shows fluctuations and stable equilibrium points. Secondly, we introduced the effects of photoperiod and photoinhibition by UVR and vertical mixing. Under these conditions, the phytoplankton biomass oscillates depending on the combined effects of UVR and mixing. Higher inhibition by UVR and longer mixing periods can induce strong fluctuations in the system but can also produce higher plankton peaks.     

Penetration of ultraviolet radiation in the marine environment. A review

UV radiation (UVR) is a significant ecological factor in the marine environment that can have important effects on planktonic organisms and dissolved organic matter (DOM). The penetration of UVR into the water column is likely to change in the near future due to interactions between global warming and ozone depletion. In this study we report underwater instruments employed for the measurement of UVR and we review data dealing with the depth of UVR penetration in different oceanic areas including the open ocean, Antarctic waters and coastal waters. We provide the 10% irradiance depth (Z10%) for UV-A and UV-B as well as for DNA damage effective dose (DNA), which we calculated from the values of diffuse attenuation coefficients or vertical profiles reported in the literature. We observe a clear distinction between open ocean (high Z10%, no variation in the ratio UV-B/UV-A), Antarctic waters (increase in the ratio UV-B/UV-A during ozone hole conditions) and coastal waters (low Z10%, no variation in the ratio UV-B/UV-A). These variations in the penetration of UVR could lead to differences in the relative importance of photobiological/photochemical processes. We also compare in this study the penetration of UV-B (unweighted and weighted by the Setlow action spectrum) and DNA damage effective dose.     

Modeling the effects of ultraviolet radiation on estuarine phytoplankton production: impact of variations in exposure and sensitivity to inhibition.

Spectral ultraviolet (UV) irradiance, water column attenuation and biological weighting functions for inhibition of phytoplankton photosynthesis have been measured for the Rhode River, a subestuary of the Chesapeake Bay. Together, these measurements can be used to estimate UV effects on water column production, but each factor shows a significant range of variability even just considering summer time conditions. A sensitivity analysis of UV inhibition is described which assesses the effect of this variation for different combinations of 28 irradiance spectra, 8 biological weighting functions (BWFs) and 16 water column irradiance profiles. Over all combinations, production averaged about 84% relative to potential production in the absence of UV effects. For a few combinations, relative production was as low as 67%, or as high as 97%, but for most combinations the range was 75-95%. Variations in the sensitivity of the phytoplankton assemblage, i.e. the BWF, and optical properties, represented by a transparency ratio of biologically effective UV to photosynthetically available radiation (PAR), had large effects on water column production. A simple relationship for UV inhibition of water column production is developed based on inhibition at the surface and the ratio of UV and PAR transparency.     

Variability of UVR effects on photosynthesis of summer phytoplankton assemblages from a tropical coastal area of the South China Sea

From June to September 2005, we carried out experiments to determine the ultraviolet radiation (UVR) -induced photoinhibition of summer phytoplankton assemblages from a coastal site of the South China Sea. Variability in taxonomic composition was determined throughout the summer, with a peak chlorophyll a (chl a approximately 20 microg chl a L(-1)) dominated by the diatom Skeletonema costatum that was detected early in the study period; the rest of the time samples were characterized by monads and flagellates, with low chl a values (1-5 chl a microg L(-1)). Surface water samples were placed in quartz tubes, inoculated with radiocarbon and exposed to solar radiation for 2-3 h to determine photosynthetic rates under three quality radiation treatments (i.e. PAB, 280-700 nm; PA, 320-700 nm and P, 400-700 nm) using different filters and under seven levels of ambient irradiance using neutral density screens (P vs E curves). UVR inhibition of samples exposed to maximum irradiance (i.e. at the surface) varied from -12.2% to 50%, while the daytime-integrated UVR-related photoinhibition in surface seawater varied from -62% to 7%. The effects of UVR on the photosynthetic parameters P(B)(max) and E(k) were also variable, but UV-B accounted for most of the observed variability. During sunny days, photosynthesis of microplankton (>20 microm) and piconanoplankton (<20 microm) were significantly inhibited by UVR (mostly by UV-B). However, during cloudy days, while piconanoplankton cells were still inhibited by UVR, microplankton cells used UVR (mostly UV-A) as the source of energy for photosynthesis, resulting in higher carbon fixation in samples exposed to UVR than the ones exposed only to photosynthetically active radiation (PAR). Our results indicate that size structure and cloudiness clearly condition the overall impact of UVR on phytoplankton photosynthesis in this tropical site of South China. In addition, model predictions for this area considering only PAR for primary production might have underestimated carbon fixation due to UVR contribution.     

Standard ultraviolet daylight for nonextreme exposure conditions

The skin is exposed to ultraviolet radiation (UVR) from natural or artificial sources on a daily basis. The effects of chronic low dose exposure merit investigation, even when these effects are neither conspicuous nor clinically assessable. The purpose of the present study was to define a relative spectral UV irradiance that is representative of frequent nonextreme sun exposure conditions and therefore more appropriate for studies of the long-term and daily effects of solar UV on the skin. Solar spectral UV irradiance values were calculated for different dates and locations by using a radiative transfer model. The spectral irradiance values obtained when the solar elevation is lower than 45 degrees were averaged. An important feature is the dUVA (320-400 nm) to dUVB (290-320 nm) irradiance values ratio, which was found to be 27.3 for the overall average. When the months corresponding to extreme irradiance values (low or high) were excluded from the calculations, the dUVA to dUVB ratio ranged from 27.2 to 27.5. The mean spectral irradiance of the model presented here represents environmental UV exposure conditions and can be used both as a standard to investigate the biological effects of a nonextreme UVR and to assess the effectiveness of products for daily skin protection.     

Influence of ice and snow covers on the UV exposure of terrestrial microbial communities: dosimetric studies

Bacillus subtilis spore biological dosimeters and electronic dosimeters were used to investigate the exposure of terrestrial microbial communities in micro-habitats covered by snow and ice in Antarctica. The melting of snow covers of between 5- and 15-cm thickness, depending on age and heterogeneity, could increase B. subtilis spore inactivation by up to an order of magnitude, a relative increase twice that caused by a 50% ozone depletion. Within the snow-pack at depths of less than approximately 3 cm snow algae could receive two to three times the DNA-weighted irradiance they would receive on bare ground. At the edge of the snow-pack, warming of low albedo soils resulted in the formation of overhangs that provided transient UV protection to thawed and growing microbial communities on the soils underneath. In shallow aquatic habitats, thin layers of heterogeneous ice of a few millimetres thickness were found to reduce DNA-weighted irradiances by up to 55% compared to full-sky values with equivalent DNA-weighted diffuse attenuation coefficients (K(DNA)) of >200 m(-1). A 2-mm snow-encrusted ice cover on a pond was equivalent to 10 cm of ice on a perennially ice covered lake. Ice covers also had the effect of stabilizing the UV exposure, which was often subject to rapid variations of up to 33% of the mean value caused by wind-rippling of the water surface. These data show that changing ice and snow covers cause relative changes in microbial UV exposure at least as great as those caused by changing ozone column abundance.     

Vertical migration and motility responses in three marine phytoplankton species exposed to solar radiation

Diurnal vertical migration in the water column and the impact of solar radiation on motility were investigated in three marine phytoplankton species: Tetraselmis suecica, Dunaliella salina and Gymnodinium chlorophorum. Cells were exposed to solar radiation either in ultraviolet radiation (UVR, 280-400 nm) transparent Plexiglas tubes (45 cm length, 10 cm diameter) or in quartz tubes under three radiation treatments: PAB (280-700 nm), PA (320-700 nm) and P (400-700 nm). The three species displayed different behavior after exposure to solar radiation. Tetraselmis suecica was insensitive to UVR and under high solar radiation levels, cells accumulated preferentially near the surface. Exposure experiments did not indicate any significant changes in swimming speed nor in the percentage of motile cells after 5 h of exposure. On the other hand, D. salina was sensitive to UV-B displaying a significant decrease in swimming speed and percentage of motile cells after 2-3 h of exposure. Moreover, D. salina cells migrated deep in the water column when irradiance was high. The response of G. chlorophorum was in between that of the other two species tested, with a slight (but significant) decrease in swimming speed and percentage of motile cells in all radiation treatments after 5 h of exposure. While G. chlorophorum cells were more or less homogenously distributed in the water column, a slight (but significant) avoidance response to high radiation was observed at local noon, with cells migrating deep in the water column. Our data clearly indicate that these sub-lethal effects of solar radiation are species-specific and they might have important implications for the aquatic ecosystem.     

Seasonal Impacts of Solar UV Radiation on Photosynthesis of Phytoplankton Assemblages in the Coastal Waters of the South China Sea

We carried out experiments to evaluate seasonal changes in the impacts of UV radiation (UVR, 280–400 nm) on photosynthetic carbon fixation of phytoplankton assemblages. Surface water samples were obtained in the coastal area of the South China Sea, where chlorophyll a ranged 0.72–3.82 μg L⁻¹. Assimilation numbers (photosynthetic carbon fixation rate per chl a) were significantly higher during summer 2005 than those in spring and winter 2004. The mean values obtained under photosynthetically active radiation (PAR) were 2.83 (spring 2004), 4.35 (winter 2004) and 7.29 μg C (μg chl a)⁻¹ h⁻¹ (summer 2005), respectively. The assimilation numbers under PAR + UVR were 1.58, 2.71 and 5.28 μg C (μg chl a)⁻¹ h⁻¹, for spring, winter and summer, respectively. UVR induced less inhibition of photosynthesis during summer 2005 than during the other seasons, in spite of the higher UVR during summer. The seasonal differences in the productivity and photosynthetic response to UV were mainly due to changes in water temperature, while irradiance and vertical mixing explained >80% of the observed variability. Our data suggest that previous studies in the SCS using UV-opaque vessels might have overestimated the phytoplankton production by about 80% in spring, 61% in winter and 38% in summer.     

Occurrence of photoprotective compounds in yeasts from freshwater ecosystems of northwestern Patagonia (Argentina)

In this paper we present the results of research on the occurrence, induction and role of photoprotective compounds (PPCs) present in native aquatic yeasts from freshwater Patagonian ecosystems. We focus on the effect of UV radiation (UVR) as a factor that controls the level of photoprotection of yeasts, and explore its potential significance in shaping yeast distributional patterns. The research presented here combines field surveys and laboratory work, including the isolation and culture of native yeasts strains, and laboratory assays under different radiation conditions. The results obtained suggest that yeasts are common dwellers of oligotrophic Patagonian water bodies, and provide the first evidence of the distribution of PPC (carotenoid and mycosporine)-producing yeasts in temperate freshwaters. A greater proportion of carotenogenic yeasts were observed in high-elevation lakes. The yeast strains isolated from these environments were found to produce higher amounts of mycosporines (MYCs), and to present higher tolerance to UVB exposure than those from piedmont lakes. Patagonian yeasts have only one type of MYC, mycosporine-glutaminol-glucoside (myc-glu-glu), which seems common to all other yeasts. By analyzing the production of myc-glu-glu in a large number of yeasts belonging to different taxonomic groups, we propose that this compound may have potential use as a chemotaxonomic marker in yeast systematics. Collectively, our work reveals that in Patagonian freshwater yeasts there is an apparent relationship between the ability to produce PPCs, their tolerance to UV exposure and their success in colonizing habitats highly exposed to UVR.     

Sensitivity of the early life stages of macroalgae from the Northern Hemisphere to ultraviolet radiation

The reproductive cells of macroalgae are regarded as the life history stages most susceptible to various environmental stresses, including UV radiation (UVR). UVR is proposed to determine the upper depth distribution limit of macroalgae on the shore. These hypotheses were tested by UV-exposure experiments, using spores and young thalli of the eulittoral Rhodophyceae Mastocarpus stellatus and Chondrus crispus and various sublittoral brown macroalgae (Phaeophyceae) with different depth distribution from Helgoland (German Bight) and Spitsbergen (Arctic). In spores, the degree of UV-induced inhibition of photosynthesis is lower in eulittoral species and higher in sublittoral species. After UV stress, recovery of photosynthetic capacity is faster in eulittoral compared to sublittoral species. DNA damage is lowest while repair of DNA damage is highest in eulittoral compared to sublittoral species. When the negative impact of UVR prevails, spore germination is inhibited. This is observed in deep water kelp species whereas the same UVR doses do not inhibit germination of shallow water kelp species. A potential acclimation mechanism to increase UV tolerance of brown algal spores is the species-specific ability to increase the content of UV-absorbing phlorotannins in response to UV-exposure. Growth rates of young Mastocarpus and Chondrus gametophytes exposed to experimental doses of UVR are not affected while growth rates of all young kelp sporophytes exposed to UVR are significantly lowered. Furthermore, morphological UV damage in Laminaria ochroleuca includes tissue deformation, lesion, blistering and thickening of the meristematic part of the lamina. The sensitivity of young sporophytes to DNA damage is correlated with thallus thickness and their optical characteristics. Growth rate is an integrative parameter of all physiological processes in juvenile plants. UV inhibition of growth may affect the upper distribution depth limit of adult life history stages. Juveniles possess several mechanisms to minimize UVR damage and, hence, are less sensitive but at the expense of growth. The species-specific susceptibility of the early life stages of macroalgae to UVR plays an important role for the determination of zonation patterns and probably also for shaping up community structure.     

Variability of bio-optical factors influencing the seasonal attenuation of ultraviolet radiation in temperate coastal waters of Japan

The study identifies the relative contribution of various bio-optical factors to the total attenuation of ultraviolet radiation (UVR) wavelengths and photosynthetically active radiation (PAR) in temperate coastal waters of Japan by surveying the physical properties of the water column, UVR and PAR penetration, and the absorption characteristics of dissolved and particulate material. Spectral absorbance properties of pigment (aph), detritus (ad) and chromophoric dissolved organic material (aCDOM) displayed both seasonal and wavelength specific variability. On an annual basis, absorbance by aCDOM was the highest absorbing fraction (47-59%) for the UVR wavelengths measured (305, 320, 340 and 380 nm) but decreased (32%) at 450 nm. Contribution of pigments to total absorbance was highest (40-60%) during a spring bloom for both UVR and PAR. A large variability (C.V. > 42%) for annual average attenuation coefficients (Kd[lambda]) at respective wavelengths observed suggests that the spectral composition of the water column changes throughout the year in this region. A significant relationship was observed between Kd(lambda) and aCDOM at 305, 320, 340 and 380 nm only (P < 0.01) but not for 450 nm (PAR) indicating the role of CDOM in regulating variations in Kd(lambda), particularly in the UVR range. The slope S, obtained from a natural-log plot of the absorption coefficient of CDOM against wavelength, ranged between 0.014 and 0.036 nm-1 annually (average = 0.020 +/- 0.007, C.V. = 35%) and suggests seasonal changes in the origin of CDOM between terrestrial (low S) and biogenous (high S) CDOM.     

Spatial and temporal variations of the inherent and apparent optical properties in a shallow coastal lake

Apparent and inherent optical properties in the coastal lagoon of Fogliano were measured in three seasonal surveys in 2002. Irradiance data from in situ measurements of ultraviolet and visible wavebands permitted to estimate the related attenuation coefficients. Laboratory extinction measurements on filtered (0.22 microm) and unfiltered water samples were also performed. The integrated approach between in situ and laboratory measurements allowed the determination of the role of the suspended and dissolved matter in the attenuation and extinction of ultraviolet and visible radiation within the water column. As noted, the impact of the suspended and dissolved matter on the lake optical quality was influenced by wind resuspension of particulate matter: the relative role of dissolved matter in the absorption of UV and visible radiation was prevailing at low wind velocity conditions (less than 2.2 m s(-1)), while at high wind velocities (3.9 m s(-1)), particulate matter resuspension strongly influenced the attenuation and the extinction measurements. The extinction in the analyzed wavelengths of filtered and unfiltered water samples and the in situ irradiance measurements allowed us to define new optical parameters and important correlations with limnological and classical optical measurements. By sampling at high spatial resolution (18 stations in 4 km2), it was possible to evidence a spatial gradient of the optical and limnological properties, these distributions showed a consistent pattern in all three surveys, and were important for the characterization of the chromophoric dissolved organic matter that was estimated with the spectral slope of the extinction curve spectra. A relatively higher spectral slope was found in the southern basin with respect to the northern, where the maximum values of the attenuation coefficients and limnological parameters were found. These results suggest different sources of dissolved organic matter and/or a different level of photobleaching.     

Heavy metals in lake water: a review on occurrence and analytical determination

Many lakes especially in Asia are source of livelihood for the surrounding communities. With increased urbanisation and industrialisation, however, these lakes are threatened with emerging environmental contaminants, including heavy metals. Some heavy metals are harmful to human health and the environment. This review aims to describe the different sampling, sample preparation and pretreatment, and instrumental methods of analysis for heavy metals in lake water. Filtration and acid digestion are common sample treatment methods used prior to analytical determination. Atomic absorption spectroscopy and inductively-coupled plasma – mass spectrometry (ICP-MS) are typical analytical techniques but nowadays ICP-MS is frequently used. This review also describes the sources and extent of heavy metals contamination in different lakes. Although some lakes still have natural levels of heavy metals in the water, many have elevated concentrations due to anthropogenic sources, such as vehicular, household, agricultural, industrial and mining activities.     

Differential Responses of Growth and Photochemical Performance of Marine Diatoms to Ocean Warming and High Light Irradiance

Diatoms have relatively high biomass in mid- to high-latitude oceans, which is also the most sensitive region to climate change. Photoautotrophs are thus predicted to become exposed to both higher temperatures and increased solar irradiance. In this study, we examined the consequences of such changes for the growth and photo-physiology of two diatoms by mimicking the scenarios that correspond to present day and that predicted for the end of this century. Elevated light induced higher rates of damage to photosystem II (PSII) that significantly reduced photochemical yields of both diatoms. Treatments including UV radiation induced ~ 50% inhibition of PSII under present PAR levels. Generally, warming alleviated UVR inhibition, resulting in higher photochemical yields, and faster recovery during dim light exposure. Therefore, concurrent increase of irradiance and temperature mitigated UV inhibition of PSII by 8–15%. The growth was stimulated by warming under PAR treatment, while less stimulation, or even decreased growth rates were found under the PAR + UVR treatment. Results suggest that ocean warming could fully offset the inhibition of high light on PSII. However, under the latter higher UVR stress scenario, the energetic expenditure required by the diatoms to repair damage could lead to their lower overall growth in future oceans.