Adaptation versus plastic responses to temperature, light, and nitrate availability in cultured snow algal strains.

Snow algal blooms are widespread, dominating low temperature, high light, and oligotrophic melting snowpacks. Here, we assessed the photophysiological and cellular stoichiometric responses of snow algal genera Chloromonas spp. and Microglena spp. in their vegetative life stage isolated from the Arctic and Antarctic to gradients in temperature (5 - 15°C), nitrate availability (1 - 10 µmol L-1), and light (50 and 500 µmol photons m-2 s-1). When grown under gradients in temperature, measured snow algal strains displayed Fv/Fm values increased by ∼115% and electron transport rates decreased by ∼50% at 5°C compared to 10 and 15°C, demonstrating how low temperatures can mimic high light impacts to photophysiology. When using carrying capacity as opposed to growth rate as a metric for determining the temperature optima, these snow algal strains can be defined as psychrophilic, with carrying capacities ∼90% higher at 5°C than warmer temperatures. All strains approached Redfield C:N stoichiometry when cultured under nutrient replete conditions regardless of temperature (5.7 ± 0.4 across all strains), whereas significant increases in C:N were apparent when strains were cultured under nitrate concentrations that reflected in situ conditions (17.8 ± 5.9). Intra-specific responses in photophysiology were apparent under high light with Chloromonas spp. more capable of acclimating to higher light intensities. These findings suggest that in situ conditions are not optimal for the studied snow algal strains, but they are able to dynamically adjust both their photochemistry and stoichiometry to acclimate to these conditions.

Photoacclimation by Arctic cryoconite phototrophs.

Cryoconite is a matrix of sediment, biogenic polymer and a microbial community that resides on glacier surfaces. The phototrophic component of this community is well adapted to this extreme environment, including high light stress. Photoacclimation of the cryoconite phototrophic community on Longyearbreen, Svalbard, was investigated using in situ variable chlorophyll fluorescence. Rapid light curves (RLCs) and induction-recovery curves were used to analyse photosystem II quantum efficiency, relative electron transport rate and forms of downregulation including non-photochemical quenching (NPQ) and state transitions in cyanobacteria. Phototrophs used a combination of behavioural and physiological photochemical downregulation. Behavioural downregulation is hypothesised to incorporate chloroplast movement and cell or filament positioning within the sediment matrix in order to shade from high light, which resulted in a lack of saturation of RLCs and hence overestimation of productivity. Physiological downregulation likely consisted of biphasic NPQ, comprising a steadily induced light-dependent form and a light-independent NPQ that was not reversed with decreasing light intensity. State transitions by cyanobacteria were the most likely physiological downregulation employed by cyanobacteria within the mixed phototroph community. These findings demonstrate that cryoconite phototrophs combine multiple forms of physiological and behavioural downregulation to optimise light exposure and maximise photosynthetic productivity. This plasticity of photoacclimation enables them to survive productively in the high-light stress environment on the ice surface.

Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK).

Phoslock(®) is increasingly used worldwide to control sediment phosphorus (P) release and cyanobacterial blooms despite the fact that little is known about its mode of action in lake bed sediments. This study quantified the effects of Phoslock(®) on sediment elemental composition and P fractionation (one year pre- and post-application of 170 g Phoslock(®) m(-2)) in an attempt to address these knowledge gaps. Post-application, sediment La content was significantly higher in the top 10 cm of the sediment compared to pre-application conditions. Mass balance calculations indicated that the applied mass of La had the potential to bind 25% of potentially release-sensitive P (Pmobile; sum 'labile P', 'reductant-soluble P' and 'organic P' fraction) present in the top 4 cm or 10% of Pmobile present in the top 10 cm of the sediment. Assessing variation in sediment P partitioning indicated that the application caused a significant increase in the mass of P present in the more refractory 'apatite bound P' fraction between post-application month 4 and 7 compared to Pmobile. This suggests that Phoslock(®) controls sediment P release by increasing the mass of P permanently bound in the sediment. To address uncertainty in estimating product dose required to control sediment P release we conducted laboratory assays using intact sediment cores to which we added serial additions of Phoslock(®) under either aerobic or anaerobic conditions. The laboratory experiment indicated that the original dose to Loch Flemington was sufficient to control sediment P release under aerobic conditions but that significant P release will occur should prolonged anaerobic conditions persist. However, Phoslock(®) may be a viable option to control sediment P-release under anaerobic conditions which would require an estimated additional application of up to 510 g Phoslock(®) m(-2). A conceptual model is proposed for the use of P-capping agents in lake remediation projects which is likely to increase cost-effectiveness and reduce non-target effects by applying multiple smaller doses compared to a single high dose.

Antifouling action of polyisoprene-based coatings by inhibition of photosynthesis in microalgae.

Previous studies have demonstrated that ionic and non-ionic natural rubber-based coatings inhibit adhesion and growth of marine bacteria, fungi, microalgae, and spores of macroalgae. Nevertheless, the mechanism of action of these coatings on the different micro-organisms is not known. In the current study, antifouling activity of a series of these rubber-based coatings (one ionic and two non-ionic) was studied with respect to impacts on marine microalgal photosynthesis using pulse-amplitude-modulation (PAM) fluorescence. When grown in contact with the three different coatings, an inhibition of photosynthetic rate (relative electron transport rate, rETR) was observed in all of the four species of pennate diatoms involved in microfouling, Cocconeis scutellum, Amphora coffeaeformis, Cylindrotheca closterium, and Navicula jeffreyi. The percentage of inhibition ranged from 44% to 100% of the controls, depending on the species and the coating. The ionic coating was the most efficient antifouling (AF) treatment, and C. scutellum and A. coffeaeformis are the most sensitive and tolerant diatoms tested, respectively. Photosynthetic inhibition was reversible, as almost complete recovery of rETR was observed 48 h post exposure, after detachment of cells from the coatings. Thus, the antifouling activity seemed mostly due to an effect of contact with materials. It is hypothesized that photosynthetic activity was suppressed by coatings due to interference in calcium availability to the microalgal cells; Ca(2+) has been shown to be an essential micro/macro nutrient for photosynthesis, as well as being involved in cell adhesion and motility in pennate diatoms.

Photophysiology and albedo-changing potential of the ice algal community on the surface of the Greenland ice sheet.

Darkening of parts of the Greenland ice sheet surface during the summer months leads to reduced albedo and increased melting. Here we show that heavily pigmented, actively photosynthesising microalgae and cyanobacteria are present on the bare ice. We demonstrate the widespread abundance of green algae in the Zygnematophyceae on the ice sheet surface in Southwest Greenland. Photophysiological measurements (variable chlorophyll fluorescence) indicate that the ice algae likely use screening mechanisms to downregulate photosynthesis when exposed to high intensities of visible and ultraviolet radiation, rather than non-photochemical quenching or cell movement. Using imaging microspectrophotometry, we demonstrate that intact cells and filaments absorb light with characteristic spectral profiles across ultraviolet and visible wavelengths, whereas inorganic dust particles typical for these areas display little absorption. Our results indicate that the phototrophic community growing directly on the bare ice, through their photophysiology, most likely have an important role in changing albedo, and subsequently may impact melt rates on the ice sheet.

Modern stromatolite phototrophic communities: a comparative study of procaryote and eucaryote phototrophs using variable chlorophyll fluorescence.

Stromatolites are laminated organosedimentary structures formed by microbial communities, principally cyanobacteria although eucaryote phototrophs may also be involved in the construction of modern stromatolites. In this study, productivity and photophysiology of communities from stromatolites (laminated) and thrombolites (nonlaminated) were analysed using fluorescence imaging. Sub-samples of mats were excised at Highborne Cay, Bahamas, and cross-sectioned to simultaneously analyse surface, near-surface (1-2 mm), and deeper (2-10 mm) communities. Rapid light curve parameters and nonphotochemical downregulation showed distinct differences between phototroph communities, consistent with the reported quasi-succession of classic stromatolite mat types. Greater productivity was shown by cyanobacteria in Type 1 and Type 3 mats (first and final stage of the succession, Schizothrix gebeleinii and Solentia sp. respectively) and lower productivity within Type 2 mats (intermediate mat type). Eucaryote mat types, dominated by stalked (Striatella sp. and Licmophora sp.) and tube-dwelling (e.g. Nitzschia and Navicula spp.) diatoms, showed greater productivity than cyanobacteria communities, with the exception of Striatella (low productivity) and an unidentified coccoid cyanobacterium (high productivity). Findings indicate comparative variability between photosynthetically active procaryote and eucaryote sub-communities within stromatolites, with a pattern logically following the succession of 'classic' mat types, and lower than the productivity of eucaryote dominated 'nonclassic' mat types.

Sediment amendment with Phoslock® in Clatto Reservoir (Dundee, UK): Investigating changes in sediment elemental composition and phosphorus fractionation.

Lanthanum-modified bentonite clay (Phoslock(®) is a lake remediation tool designed to strip dissolved phosphorus (P) from the water column and increase the sediment P-sorption capacity. This study investigated short term alterations in sediment elemental composition and sediment P-fractions based on sediment cores taken 2 days before and 28 days following the application of 24 t of Phoslock® to a 9 ha, man-made reservoir. Following the application, sediment lanthanum (La) content increased significantly (p < 0.05; n = 4) in the top 8 cm of the sediment, thereby theoretically increasing sediment P-binding capacity on the whole reservoir scale by 250 kg. Mass balance calculations were used to estimate the theoretical binding of release-sensitive P (P(mobile); sum of 'labile P', 'reductant-soluble P' and 'organic P' fraction) by La across the top 4 cm and 10 cm depth of sediment. The amended mass of La in the sediment had the potential to bind 42% of P(mobile) present in the top 4 cm or 17% of P(mobile) present in the top 10 cm. However, with the exception of a significant increase (p<0.05; n=4) in the 'residual P' fraction in the top 2 cm, sediment P-fractions, including P(mobile,) did not differ significantly following the Phoslock® application. Experimental P-adsorption studies indicated P-saturation values for Phoslock® of 21,670 mg P kg⁻¹ Phoslock®. Sequential extraction of P from saturated Phoslock® under laboratory conditions indicated that around 21% of P bound by Phoslock® was release-sensitive, while around 79% of bound P was unlikely to be released under reducing or common pH (5-9) conditions in shallow lakes. Applying Phoslock® is, therefore, likely to increase the P-sorption capacity of sediments under reducing conditions.

DETECTION OF DIATOM XANTHOPHYLL CYCLE USING SPECTRAL REFLECTANCE(1).

Analysis of reflectance spectra was used to monitor the conversion of diadinoxanthin (DD) into diatoxanthin (DT) in two benthic diatom species, Amphora coffeaeformis (C. Agardh) Kütz. and Cylindrotheca closterium (Ehrenb.) J. C. Lewin et Reiman, cultured at high light (HL, 400 µmol · m(-2) · s(-1) PAR) and low light (LL, 25 µmol · m(-2) · s(-1) PAR). Cultures were exposed to saturating light for 32 min. HL cultures of both species showed higher (DT + DD) content, whereas LL cultures exhibited higher chl a and fucoxanthin content. DD to DT conversion, measured by HPLC, occurred mainly in the first 2 min (LL) or 5 min (HL) after exposure to saturating light. Nonphotochemical quenching (NPQ), measured by PAM fluorescence, showed the same pattern as DT/(DD + DT), resulting in a linear relationship between these parameters. Addition of dithiothreitol (DTT) blocked the conversion of DD into DT and significantly reduced NPQ induction. Reflectance spectra showed no obvious change after light exposure. However, second derivative spectra (δδ) showed a shift in reflectance from 487 to 508 nm, which was not present for DTT-treated samples. Changes in δδ487 were strongly correlated with changes in DD (r = 0.76), while changes in δδ508 were strongly correlated with changes in DT (r = 0.94). The best index to estimate DD to DT conversion was δδ508 /δδ630 (r = 0.87). This index was very sensitive to minute changes that occurred immediately after exposure to light and was species insensitive. Good relationships were observed between indices for xanthophyll cycle activation (DD to DT conversion and NPQ induction) and the second derivative spectra. With further in situ validation, this index may prove to be highly useful for investigation into aquatic global photoregulation mechanisms in diatom-dominated samples.

In-line laser holography and video analysis of eroded floc from engineered and estuarine sediments.

Microphytobenthic polymers mediate intertidal sediment erosion processes, through biostabilization and modifying the nature of eroded floc material. The latter is of key importance with respect to sediment transport dynamics, including floc aggregation and particle deposition. In this study, eroded floc material was analyzed by video imaging, alongside novel application of in-line laser holography (ILH). The erosion of engineered sediment was compared to that of natural estuarine sediments. Both video and holography showed an increase in floc size eroded from engineered cohesive clay sediment as a function of sediment dewatering and sediment polymer content. Estuarine sediment showed a curvilinear increase in floc size as a function of both microphytobenthic biomass and sediment colloidal polymer content when measured by video analysis. Holography did not show these functions for floc size due to temporal limitations of the current ILH methodology. An interaction of sediment polymer binding and sediment desiccation was observed for engineered sediments and, most notably, for estuarine cohesive sediments. In conclusion, engineered sediments were not accurate analogues for natural intertidal sediments, failing to reproduce eroded floc material similar to that from estuarine cohesive sediment. The size of eroded floc from estuarine sediments is a function of the complex interaction between biological and physicochemical processes, primarily algal colloidal polymer and desiccation. Holography demonstrated an excellent potential for the high-resolution imaging of eroded material but is limited by temporal constraints; the solution to this would be the development of real-time holographic video.