Antarctic snow algae: unraveling the processes underlying microbial community assembly during blooms formation.

BACKGROUND AND AIMS: At the West Antarctic Peninsula, snow algae blooms are composed of complex microbial communities dominated by green microalgae and bacteria. During their progression, the assembly of these microbial communities occurs under harsh environmental conditions and variable nutrient content due to fast snow melting. To date, it is still unclear what are the ecological mechanisms governing the composition and abundance of microorganisms during the formation of snow algae blooms. In this study, we aim to examine the main ecological mechanisms governing the assembly of snow algae blooms from early stages to colorful stages blooms. METHODS: The composition of the microbial communities within snow algae blooms was recorded in the West Antarctic Peninsula (Isabel Riquelme Islet) during a 35-day period using 16S rRNA and 18S rRNA metabarcoding. In addition, the contribution of different ecological processes to the assembly of the microbial community was quantified using phylogenetic bin-based null model analysis. RESULTS: Our results showed that alpha diversity indices of the eukaryotic communities displayed a higher variation during the formation of the algae bloom compared with the bacterial community. Additionally, in a macronutrients rich environment, the content of nitrate, ammonium, phosphate, and organic carbon did not play a major role in structuring the community. The quantification of ecological processes showed that the bacterial community assembly was governed by selective processes such as homogenous selection. In contrast, stochastic processes such as dispersal limitation and drift, and to a lesser extent, homogenous selection, regulate the eukaryotic community. CONCLUSIONS: Overall, our study highlights the differences in the microbial assembly between bacteria and eukaryotes in snow algae blooms and proposes a model to integrate both assembly processes. Video Abstract.

Functional filtering and random processes affect the assembly of microbial communities of snow algae blooms at Maritime Antarctic.

The increasing temperatures at the West Antarctic Peninsula (Maritime Antarctic) could lead to a higher occurrence of snow algal blooms which are ubiquitous events that change the snow coloration, reducing albedo and in turn exacerbating melting. However, there is a limited understanding of snow algae blooms biodiversity, composition, and their functional profiles, especially in one of the world's areas most affected by climate change. In this study we used 16S rRNA and 18S rRNA metabarcoding, and shotgun metagenomics to assess the diversity, composition, and functional potential of the snow algae blooms bacterial and eukaryotic communities at three different sites of Maritime Antarctic, between different colors of the algae blooms and between seasonal and semi-permanent snowfields. We tested the hypothesis that the functional potential of snow algae blooms is conserved despite a changing taxonomic composition. Furthermore, we determined taxonomic co-occurrence patterns of bacteria and eukaryotes and assessed the potential for the exchange of metabolites among bacterial taxa. Here, we tested the prediction that there are co-occurring taxa within snow algae whose biotic interactions are marked by the exchange of metabolites. Our results show that the composition of snow algae blooms vary significantly among sites. For instance, a higher abundance of fungi and protists were detected in Fildes Peninsula compared with Doumer Island and O'Higgins. Likewise, the composition varied between snow colors and snow types. However, the functional potential varied only among sampling sites with a higher abundance of genes involved in tolerance to environmental stress at O'Higgins. Co-occurrence patterns of dominant bacterial genera such as Pedobacter, Polaromonas, Flavobacterium and Hymenobacter were recorded, contrasting the absence of co-occurring patterns displayed by Chlamydomonadales algae with other eukaryotes. Finally, genome-scale metabolic models revealed that bacteria within snow algae blooms likely compete for resources instead of forming cooperative communities.

Photobiology of the giant kelp Macrocystis pyrifera in the land-terminating glacier fjord Yendegaia (Tierra del Fuego): A look into the future?

The channel and fjord region of southern Chilean Patagonia hosts giant kelp forests (Macrocystis pyrifera) that have little known site-specific responses to diverse physical gradients. In this study, the functionality of the bio-optical, morphological and biochemical features of the kelps, that determine their light trapping and acclimation, were studied along a gradient of varying turbidity and light conditions at the land-terminating glacier of fjord Yendegaia in the Beagle Channel. These habitats are marked by glacial retreat, and M. pyrifera has successfully colonized new areas due to the effects of warming. Results indicated that under a sharp gradient of turbidity and light availability, the kelps have adapted shading characteristics. The photobiological traits (e.g. light absorption, pigment concentration, photochemistry and blade optics) of algae from depths between 6 and 13 m varied in relation to the degree of turbidity along the fjord. However, these populations did not show obvious intra-thallus variation along the longitudinal profile e.g. blades located at different depths showed relatively similar acclimation potential to the prevailing light field. Only basal sporophylls showed general differences in comparison with the vegetative fronds. Otherwise, the high phenolic (phlorotannin) content, which was reflected in the massive presence of intracellular physodes, suggests that these organisms could be biochemically well-equipped to cope with changes in physical conditions or the presence of herbivore invertebrates (e.g. sea urchins).

Revealing the Characteristics of the Antarctic Snow Alga Chlorominima collina gen. et sp. nov. Through Taxonomy, Physiology, and Transcriptomics.

Snow algae play crucial roles in cold ecosystems, however, many aspects related to their biology, adaptations and especially their diversity are not well known. To improve the identification of snow algae from colored snow, in the present study we used a polyphasic approach to describe a new Antarctic genus, Chlorominima with the species type Chlorominima collina. This new taxon was isolated of colored snow collected from the Collins Glacier (King George Island) in the Maritime Antarctic region. Microscopy revealed biflagellated ellipsoidal cells with a rounded posterior end, a C-shaped parietal chloroplast without a pyrenoid, eyespot, and discrete papillae. Several of these characteristics are typical of the genus Chloromonas, but the new isolate differs from the described species of this genus by the unusual small size of the cells, the presence of several vacuoles, the position of the nucleus and the shape of the chloroplast. Molecular analyzes confirm that the isolated alga does not belong to Chloromonas and therefore forms an independent lineage, which is closely related to other unidentified Antarctic and Arctic strains, forming a polar subclade in the Stephanosphaerinia phylogroup within the Chlamydomonadales. Secondary structure comparisons of the ITS2 rDNA marker support the idea that new strain is a distinct taxon within of Caudivolvoxa. Physiological experiments revealed psychrophilic characteristics, which are typical of true snow algae. This status was confirmed by the partial transcriptome obtained at 2°C, in which various cold-responsive and cryoprotective genes were identified. This study explores the systematics, cold acclimatization strategies and their implications for the Antarctic snow flora.

Uptake of microalgae as sublethal biomarker reveals phototoxicity of oxytetracycline to the crustacean Daphnia magna.

Antibiotics are considered emerging pollutants as their presence in the environment is increasingly common. Although their environmental concentrations are generally low, they can pose risk to organisms through bioaccumulation, causing sublethal effects. Furthermore, solar radiation can trigger reactions in certain compounds after their accumulation within organisms or in the environment. Toxicity and photoinduced toxicity of oxytetracycline (OTC, widely used antibiotic in salmon aquaculture) on Daphnia magna (Crustacea, Cladocera) and microalgae Raphidocelis subcapitata (Chlorophyceae) as its food source was assessed via aqueous exposure. Also, the impact via diet (microalga) to the crustacean was examined. In addition to lethal (immobility) effect, in vivo chlorophyll fluorescence techniques were used to determine food ingestion (gut content as a biomarker of physiological health) in D. magna and physiological status of microalgae. OTC (≤10 mg L - 1) was not acutely (24 h) toxic to R. subcapitata when measured as maximum quantum yield (Fv/Fm) in darkness. However, under short (1 h) UV exposure OTC caused irreversible decrease of Fv/Fm (50%) at ≥0.5 mg L - 1. OTC was not acutely lethal to D. magna (≤10 mg L - 1), however, sublethal effects (43% decrease in food ingestion) at 10 mg L - 1 were demonstrated. UV exposure (4.5 h) strongly exacerbated toxicity of OTC, leading to lethal (87% immobility) and sublethal (81% decrease of feeding in survived individuals) effects. Uptake of OTC (aqueous exposure) and its photosensitization in tissues of D. magna under UV exposure was confirmed. On the other hand, rapid bioadsorption of OTC on cell surface was evident in R. subcapitata. Uptake of OTC in D. magna through diet could not be confirmed at short-term. Photomodification of OTC under UV exposure was observed through changes in its absorption spectrum. The results show that short exposure to summer UV levels of southern Chile can rapidly induce phototoxicity of OTC, suggesting a potential risk to aquatic organisms.

Satellite-derived mapping of kelp distribution and water optics in the glacier impacted Yendegaia Fjord (Beagle Channel, Southern Chilean Patagonia).

Giant kelp Macrocystis pyrifera is a brown alga with extensive global distribution, however, recent evidence suggests that its dynamics presents high degree of regional variability. In southern Chilean fjord region, largely unexplored kelp forests are currently being threatened by global change and human impacts. High-resolution satellite (Sentinel-2) imagery was used to describe temporal and spatial distribution patterns of kelp beds in Yendegaia Fjord (Beagle Channel) using Spectral Mixture Analysis (SMA), and to characterize water optical gradients of this habitat strongly influenced by river runoff from a melting glacier. The suitability of SMA for kelp classification was contrasted with other vegetation indices (NDVI, EVI, FAI). Validation was made using drone aerial photographs of kelp canopies. Different analysis tools resulted in up to 35% difference in kelp coverage estimation. The overall accuracy (66-82%) of kelp classification followed an order FAI < EVI < NDVI < SMA. Omission error of SMA and lower coincidence with vegetation indices occurred in pixels with low kelp pixel abundance (<0.50). Based on SMA, the lowest kelp abundance was observed in the river mouth with high turbidity, increasing towards the Beagle Channel. The highest kelp abundance was observed in late summer, but otherwise no clear seasonal patterns could be observed. Water turbidity presented both spatial and seasonal variation. Strong particle sedimentation (leading to light attenuation, interference with remote detection of kelps, and even to their detachment due to substrate quality) and tidal fluctuations in glacier-impacted fjord-type environments can be identified as key features affecting both the kelp population dynamics as well as their remote sensing. Also, low sun elevation at high latitudes in mid winter produces uncertainties in image analyses. In all, the remote sensing approach used in the present study can be regarded as a useful tool to map and monitor kelps forests from a remote region.

Antarctic intertidal macroalgae under predicted increased temperatures mediated by global climate change: Would they cope?

The Antarctic Peninsula is one of the regions to be most affected by increase in sea surface temperatures (SSTs) mediated by Global Climate Change; indeed, most negative predictions imply an up to 6 °C increment by the end of the XXI century. Temperature is one of the most important factors mediating diversity and distribution of macroalgae, although there is still no consensus as to the likely effects of higher SSTs, especially for polar seaweeds. Some available information suggests that potential strategies to withstand future increases in SSTs will be founded upon the glutathione-ascorbate cycle and the induction of chaperone-functioning heat shock proteins (HSPs); however, their eventual role, even for general stress responses, is unclear. The intertidal green, brown and red macroalgae species Monostroma hariotii, Adenocystis utricularis and Pyropia endiviifolia, respectively, from King George Island, Antarctic Peninsula, were exposed to 2 °C (control) and 8 °C (climate change scenario) for up to 5 days (d). Photosynthetic activity (αETR and ETRmax, and EkETR), photoinhibition (Fv/Fm) and photoprotection processes (αNPQ, NPQmax, and EkNPQ) provided no evidence of negative ecophysiological effects. There were moderate increases in H2O2 production and levels of lipid peroxidation with temperature, results supported by stable levels of total glutathione and ascorbate pools, with mostly higher levels of reduced ascorbate and glutathione than oxidized forms in all species. Transcripts of P. endiviifolia indicated a general upregulation of all antioxidant enzymes and HSPs genes studied under warmer temperature, although with different levels of activation with time. This pioneering investigation exploring different levels of biological organization, suggested that Antarctic intertidal macroalgae may be able to withstand future rise in SSTs, probably slightly altering their latitudinal distribution and/or range of thermal tolerance, by exhibiting robust glutathione-ascorbate production and recycling, as well as the induction of associated antioxidant enzymatic machinery and the syntheses of HSPs.

Mapping of spatial and temporal variation of water characteristics through satellite remote sensing in Lake Panguipulli, Chile.

Central-southern Chile is characterized by a series of large lakes that originate in the Andes Mountains. This region is facing increasing anthropogenic impact, which threatens the oligotrophic status of these lakes. While monitoring programs are often based on a limited spatial and temporal coverage, remote sensing offers promising tools for large-scale observations improving our capacity to study comprehensively indicators of lake properties. Seasonal trends (long-term means) and intra-lake variation of surface water temperature (SWT), turbidity and chlorophyll a in Lake Panguipulli were studied through satellite imagery from Landsat 5 TM, 7 ETM+ and 8 OLI (1998-2018; SWT, turbidity), and Sentinel-2A/B MSI (2016-2017; chlorophyll). Remotely sensed data were validated against in situ data from monitoring database. Satellite-derived SWT (representing the surface skin layer of water, so-called skin temperature) showed good similarity with in situ (bulk) temperature (RRMSD 0.17, R2 = 0.86), although was somewhat lower (RMSD of 2.77 °C; MBD of -2.10 °C). Seasonal long-term means of turbidity from satellite imagery corresponded to those from in situ data, while satellite-derived predictions (based on OC2v2 algorithm) overestimated chlorophyll a levels slightly in summer-spring. SWT ranged from 8.0 °C in winter to 17.5 °C in summer. Mean turbidity (1.6 FNU) and chlorophyll a (1.1 µg L-1) levels were at their lowest in summer. Spatial and seasonal patterns reflected the bathymetry and previously described mixing patterns of this monomictic lake: warming of shallow bays in spring extended to wider area along with summer stratification period, while mixing of the water column was reflected in spatially more homogenous SWT in fall-winter. Spatial heterogeneity in summer was confirmed by a clear separation of different lake areas based on SWT, turbidity and chlorophyll a using 3-D plot. Mapping of spatial and seasonal variation using satellite imagery allowed identifying lake areas with different characteristics, improving strategies for water resource management.

A five-year study of solar ultraviolet radiation in southern Chile (39 degrees S): potential impact on physiology of coastal marine algae?

This study reports 5 years of (1998-2003) data on continuous solar-irradiation measurements from a scanning spectroradiometer (SUV-100) in Valdivia, Chile (39 degrees S), accompanied by evaluation of the impact of ultraviolet radiation (UVR) on marine macroalgae of this site. UVR conditions showed a strong seasonal variation, which was less pronounced toward longer wavelengths. Daily maximum dose rates (clear days) averaged in winter-summer: UV-B(290-315 nm) 0.30-2.1, UV-B(290-320 nm) 0.70-3.7, UV-A(315-400 nm) 20.6-62.1, UV-A(320-400 nm) 20.2-60.5 W m(-2), and photosynthetically active radiation (PAR) 969-2423 micromol m(-2) s(-1). The corresponding daily doses (all the days) ranged: UV-B(290-315 nm) 2.6-40.7, UV-B(290-320 nm) 6.7-78.5, UV-A(315-400 nm) 228-1539, UV-A(320-400 nm) 224-1501, and PAR 2008-13308 kJ m(-2) d(-1). Taking into consideration action spectra of a biological interest, the risk of UV exposure could be up to 37 times higher in summer than in winter. The photosynthetic activity (as maximum quantum yield of chlorophyll fluorescence, F(v)/F(m)) of the brown alga Lessonia nigrescens from the infralittoral zone was markedly more sensitive to UVR than of the green alga Enteromorpha intestinalis from the upper midlittoral, and the UV-B wave band increased markedly photoinhibition. In L. nigrescens, maximal photoinhibition (40%) took place at weighted (the action spectrum for photoinhibition of photosynthesis) UVR doses of 800 kJ m(-2), irrespective of the season (corresponding midsummer daily dose in Valdivia is 480 kJ m(-2)). In winter, when this alga was at its most sensitive, the weighted UV dose causing 35-40% photoinhibition was around 200 kJ m(-2). In E. intestinalis, weighted doses of 800 kJ m(-2) resulted in low photoinhibition (<10 %) and no clear seasonal patterns could be inferred. These results confirm that midday summer levels of UV-B and their daily doses in southern Chile are high enough to produce stress to intertidal macroalgae.

Underwater Optics in Sub-Antarctic and Antarctic Coastal Ecosystems.

Understanding underwater optics in natural waters is essential in evaluating aquatic primary production and risk of UV exposure in aquatic habitats. Changing environmental conditions related with global climate change, which imply potential contrasting changes in underwater light climate further emphasize the need to gain insights into patterns related with underwater optics for more accurate future predictions. The present study evaluated penetration of solar radiation in six sub-Antarctic estuaries and fjords in Chilean North Patagonian region (39-44°S) and in an Antarctic bay (62°S). Based on vertical diffuse attenuation coefficients (Kd), derived from measurements with a submersible multichannel radiometer, average summer UV penetration depth (z1%) in these water bodies ranged 2-11 m for UV-B (313 nm), 4-27 m for UV-A (395 nm), and 7-30 m for PAR (euphotic zone). UV attenuation was strongest in the shallow Quempillén estuary, while Fildes Bay (Antarctica) exhibited the highest transparency. Optically non-homogeneous water layers and seasonal variation in transparency (lower in winter) characterized Comau Fjord and Puyuhuapi Channel. In general, multivariate analysis based on Kd values of UV and PAR wavelengths discriminated strongly Quempillén estuary and Puyuhuapi Channel from other study sites. Spatial (horizontal) variation within the estuary of Valdivia river reflected stronger attenuation in zones receiving river impact, while within Fildes Bay a lower spatial variation in water transparency could in general be related to closeness of glaciers, likely due to increased turbidity through ice-driven processes. Higher transparency and deeper UV-B penetration in proportion to UV-A/visible wavelengths observed in Fildes Bay suggests a higher risk for Antarctic ecosystems reflected by e.g. altered UV-B damage vs. photorepair under UV-A/PAR. Considering that damage repair processes often slow down under cool temperatures, adverse UV impact could be further exacerbated by cold temperatures in this location, together with episodes of ozone depletion. Overall, the results emphasize the marked spatial (horizontal and vertical) and temporal heterogeneity of optical characteristics, and challenges that these imply for estimations of underwater optics.

Stress Tolerance of the Endemic Antarctic Brown Alga Desmarestia anceps to UV Radiation and Temperature is Mediated by High Concentrations of Phlorotannins.

The endemic Antarctic brown macroalga Desmarestia anceps is strongly shade-adapted, but shows also a high capacity to cope with different environmental stressors, e.g. UV radiation and temperature. Therefore, this species colonizes wide depth gradients, which are characterized by changing environmental conditions. In this study, we examine whether the different physiological abilities allowing D. anceps to grow across a wide depth range is determined by high levels of phlorotannins. Photosynthesis, measured by PAM-fluorometry, the contents of soluble phlorotannins, antioxidant capacities of field grown were analyzed in response to different conditions of radiation (PAR and PAR + UV) and temperature (2, 7 and 12°C). The results show that maximal quantum of fluorescence (Fv /Fm ) decreased with increasing doses of UV radiation, but remained unaffected by temperature. High levels of soluble phlorotannins were detected and confirmed by microscopic observation revealing the abundance of large physodes. Exposure to UV radiation and elevated temperature showed that phlorotannins were not inducible by UV but increased at 12°C. ROS scavenging capacity was positively correlated with the contents of phlorotannins. In general, highest contents of phlorotannins were correlated with the lowest inhibition of Fv /Fm in all experimental treatments, highlighting the UV-protective role of these compounds in D. anceps.

Lack of Physiological Depth Patterns in Conspecifics of Endemic Antarctic Brown Algae: A Trade-Off between UV Stress Tolerance and Shade Adaptation?

A striking characteristic of endemic Antarctic brown algae is their broad vertical distribution. This feature is largely determined by the shade adaptation in order to cope with the seasonal variation in light availability. However, during spring-summer months, when light penetrates deep in the water column these organisms have to withstand high levels of solar radiation, including UV. In the present study we examine the light use characteristics in parallel to a potential for UV tolerance (measured as content of phenolic compounds, antioxidant activity and maximum quantum yield of fluorescence) in conspecific populations of four Antarctic brown algae (Ascoseira mirabilis, Desmarestia menziesii, D. anceps and Himantothallus grandifolius) distributed over a depth gradient between 5 and 30 m. The main results indicated that a) photosynthetic efficiency was uniform along the depth gradient in all the studied species, and b) short-term (6 h) exposure to UV radiation revealed a high tolerance measured as chlorophyll fluorescence, phlorotannin content and antioxidant capacity. Multivariate analysis of similarity indicated that light requirements for photosynthesis, soluble phlorotannins and antioxidant capacity are the variables determining the responses along the depth gradient in all the studied species. The suite of physiological responses of algae with a shallower distribution (A. mirabilis and D. menziesii) differed from those with deeper vertical range (D. anceps and H. grandifolius). These patterns are consistent with the underwater light penetration that defines two zones: 0-15 m, with influence of UV radiation (1% of UV-B and UV-A at 9 m and 15 m respectively) and a zone below 15 m marked by PAR incidence (1% up to 30 m). These results support the prediction that algae show a UV stress tolerance capacity along a broad depth range according to their marked shade adaptation. The high contents of phlorotannins and antioxidant potential appear to be strongly responsible for the lack of clear depth patterns in light demand characteristics and UV tolerance.

UV Sensitivity of Vegetative and Reproductive Tissues of Two Antarctic Brown Algae is Related to Differential Allocation of Phenolic Substances.

UV sensitivity of the vegetative and reproductive tissues of two Antarctic brown macroalgae was compared. Photosynthesis as well as the content and localization of phenolic substances were determined. Responses to UV radiation were quantified as chlorophyll fluorescence (Fv /Fm ). Ascoseira mirabilis showed high UV tolerance, while in Cystosphaera jacquinotii Fv /Fm decreased by 15-21%, the receptacles being more tolerant than the vegetative blades. The phlorotannin contents showed an opposite pattern: the soluble fraction dominated in C. jacquinotii while in A. mirabilis the insoluble fraction was more abundant. Soluble phlorotannins were higher in the reproductive than in vegetative tissues in both species. Images of tissue cross-sections under violet-blue light excitation confirmed a high allocation of phenolic compounds (as blue autofluorescence) in C. jacquinotii, both in reproductive and vegetative blades. The allocation and proportions of the soluble and insoluble phlorotannins could be related with the observed UV tolerance of the vegetative and reproductive tissues.

Up, down, and all around: scale-dependent spatial variation in rocky-shore communities of Fildes Peninsula, King George Island, Antarctica.

Understanding the variation of biodiversity along environmental gradients and multiple spatial scales is relevant for theoretical and management purposes. Hereby, we analysed the spatial variability in diversity and structure of intertidal and subtidal macrobenthic Antarctic communities along vertical environmental stress gradients and across multiple horizontal spatial scales. Since biotic interactions and local topographic features are likely major factors for coastal assemblages, we tested the hypothesis that fine-scale processes influence the effects of the vertical environmental stress gradients on the macrobenthic diversity and structure. We used nested sampling designs in the intertidal and subtidal habitats, including horizontal spatial scales ranging from few centimetres to 1000s of metres along the rocky shore of Fildes Peninsula, King George Island. In both intertidal and subtidal habitats, univariate and multivariate analyses showed a marked vertical zonation in taxon richness and community structure. These patterns depended on the horizontal spatial scale of observation, as all analyses showed a significant interaction between height (or depth) and the finer spatial scale analysed. Variance and pseudo-variance components supported our prediction for taxon richness, community structure, and the abundance of dominant species such as the filamentous green alga Urospora penicilliformis (intertidal), the herbivore Nacella concinna (intertidal), the large kelp-like Himantothallus grandifolius (subtidal), and the red crustose red alga Lithothamnion spp. (subtidal). We suggest that in coastal ecosystems strongly governed by physical factors, fine-scale processes (e.g. biotic interactions and refugia availability) are still relevant for the structuring and maintenance of the local communities. The spatial patterns found in this study serve as a necessary benchmark to understand the dynamics and adaptation of natural assemblages in response to observed and predicted environmental changes in Antarctica.

Phlorotannin and antioxidant responses upon short-term exposure to UV radiation and elevated temperature in three south Pacific kelps.

Rapid adjustments of the photosynthetic machinery and efficient antioxidant mechanisms to scavenge harmful ROS are physiologic adaptions exhibited by intertidal seaweeds to persist in temperate regions. This study examines short-term (3 h) responses of three large kelps from the cold-temperate coast of Chile, normally adapted to water temperatures <16°C, but exposed abruptly to simultaneous high temperatures and UV radiation during low tide in summer. The kelps were exposed in the laboratory to three temperatures (10, 20 and 28°C) with and without UV radiation, and photochemical reactions, concentration of phlorotannins and antioxidant activity were examined. The exposure to elevated temperature (slightly exacerbated by the presence of UV radiation) decreased photochemical processes (measured as fluorescence kinetics) in the three studied species and increased lipid peroxidation in two of them. The concentration of total soluble phlorotannins was variable and correlated with the antioxidant activity in the presence of UV radiation. Insoluble phlorotannins did not change during the exposure. In all, the downregulation of the photochemical machinery, which was expressed as dynamic photoinhibition, and the rapid induction of soluble phlorotannins triggered by UV radiation minimized the effects of oxidative stress and maintained the operation of photochemical processes during short-term thermal stress.

Induction of phlorotannins during UV exposure mitigates inhibition of photosynthesis and DNA damage in the kelp Lessonia nigrescens.

Phlorotannins of brown algae are multifunctional compounds with putative roles in herbivore deterrence, antioxidation and as primary cell wall components. Due to their peripheral localization and absorption at short wavelengths, a photoprotective role is suggested. We examined the induction of phlorotannins by artificial UV radiation in the intertidal kelp Lessonia nigrescens and whether they attenuate the inhibition of photosynthesis and DNA damage, two major detrimental effects of UV. The soluble and cell wall-bound fractions of phlorotannins were quantified in blades collected in summer and winter. Major findings were that (1) the synthesis of phlorotannins (both forms) was induced by UV only in summer; (2) the induction was fast (within 3 days); and (3) there was a positive relationship between of the contents of insoluble phlorotannins and the suppression of photoinhibition and DNA damage, measured as formation of cyclobutane pyrimidine dimers and 6-4 photoproducts. Overall, the photoprotective role of phlorotannins appears to respond to an interplay between the external UV stimulus, seasonal acclimation and intrinsic morpho-functional processes. In summer, when algae are naturally exposed to high UV irradiances, soluble phlorotannins are induced, while their transition to insoluble phlorotannins could be related with the growth requirements, as active blade elongation occurs during this season.