Light-limiting Conditions Research Articles

Effects of nutrient availability and light intensity on the sterol content of Saccharina latissima (Laminariales, Phaeophyceae)

Seaweed phytosterols are associated with potential health benefits, affording them and the seaweeds that produce them commercial interest. However, little is known about how their production is affected by the cultivation environment, limiting the efficiency with which these compounds can be exploited. Therefore, we performed a pilot study on the effect of nutrient availability and light stress on the sterol content of Saccharina latissima, a rapid growing brown alga of increasing interest in western mariculture. Individuals of S. latissima were subjected to a nutrient-replete and nutrient-depleted regime for 5 weeks, followed by the introduction of light-limited and light-saturated conditions in the sixth week; sampling occurred each week. No significant inter-treatment differences were found in the sterol content in week 1–5. However, significant intra-treatment differences were found in weeks 3–5 regardless of nutrient treatment, wherein the fucosterol, 24-methylenecholesterol, and squalene contents of both treatment groups were found to correlate inversely with photosynthetic performance. Factorial treatment of differential nutrient availability and light stress resulted in marked differences between the sterol content of all groups in week 6. Here, squalene and cycloartenol increased in concentration with increasing irradiance regardless of nutrient treatment. Concentrations of all other sterolic components increased with increasing irradiance and low nutrient conditions while decreasing or remaining unchanged with increasing irradiance and high nutrient conditions. Our data shows that within our cultivation conditions and time frame, the sterol content of S. latissima is unaffected by nutrient availability alone but changes with combined alterations in irradiance and nutrient availability.Graphical abstract

Characterization of CYCLOPHILLIN38 shows that a photosynthesis-derived systemic signal controls lateral root emergence.

Photosynthesis in leaves generates fixed-carbon resources and essential metabolites that support sink tissues, such as roots. Two of these metabolites, sucrose and auxin, promote growth in root systems, but the explicit connection between photosynthetic activity and control of root architecture has not been explored. Through a mutant screen to identify pathways regulating root system architecture, we identified a mutation in the Arabidopsis thaliana CYCLOPHILIN 38 (CYP38) gene, which causes accumulation of pre-emergent stage lateral roots. CYP38 was previously reported to stabilize photosystem II (PSII) in chloroplasts. CYP38 expression is enriched in shoots, and grafting experiments show that the gene acts non-cell-autonomously to promote lateral root emergence. Growth of wild-type plants under low-light conditions phenocopies the cyp38 lateral root emergence defect, as does the inhibition of PSII-dependent electron transport or Nicotinamide adenine dinucleotide phosphate (NADPH) production. Importantly, these perturbations to photosynthetic activity rapidly suppress lateral root emergence, which is separate from their effects on shoot size. Supplementary exogenous sucrose largely rescued primary root (PR) growth in cyp38, but not lateral root growth. Auxin (indole-3-acetic acid (IAA)) biosynthesis from tryptophan is dependent on reductant generated during photosynthesis. Consistently, we found that wild-type seedlings grown under low light and cyp38 mutants have highly diminished levels of IAA in root tissues. IAA treatment rescued the cyp38 lateral root defect, revealing that photosynthesis promotes lateral root emergence partly through IAA biosynthesis. These data directly confirm the importance of CYP38-dependent photosynthetic activity in supporting root growth, and define the specific contributions of two metabolites in refining root architecture under light-limited conditions.

Modelling nonlinear dynamics of Crassulacean acid metabolism productivity and water use for global predictions.

Crassulacean acid metabolism (CAM) crops are important agricultural commodities in water-limited environments across the globe, yet modelling of CAM productivity lacks the sophistication of widely used C3 and C4 crop models, in part due to the complex responses of the CAM cycle to environmental conditions. This work builds on recent advances in CAM modelling to provide a framework for estimating CAM biomass yield and water use efficiency from basic principles. These advances, which integrate the CAM circadian rhythm with established models of carbon fixation, stomatal conductance and the soil-plant-atmosphere continuum, are coupled to models of light attenuation, plant respiration and biomass partitioning. Resulting biomass yield and transpiration for Opuntia ficus-indica and Agave tequilana are validated against field data and compared with predictions of CAM productivity obtained using the empirically based environmental productivity index. By representing regulation of the circadian state as a nonlinear oscillator, the modelling approach captures the diurnal dynamics of CAM stomatal conductance, allowing the prediction of CAM transpiration and water use efficiency for the first time at the plot scale. This approach may improve estimates of CAM productivity under light-limiting conditions when compared with previous methods.

A fine-tuned global distribution dataset of marine forests

Species distribution records are a prerequisite to follow climate-induced range shifts across space and time. However, synthesizing information from various sources such as peer-reviewed literature, herbaria, digital repositories and citizen science initiatives is not only costly and time consuming, but also challenging, as data may contain thematic and taxonomic errors and generally lack standardized formats. We address this gap for important marine ecosystem-structuring species of large brown algae and seagrasses. We gathered distribution records from various sources and provide a fine-tuned dataset with ~2.8 million dereplicated records, taxonomically standardized for 682 species, and considering important physiological and biogeographical traits. Specifically, a flagging system was implemented to signal potentially incorrect records reported on land, in regions with limiting light conditions for photosynthesis, and outside the known distribution of species, as inferred from the most recent published literature. We document the procedure and provide a dataset in tabular format based on Darwin Core Standard (DwC), alongside with a set of functions in R language for data management and visualization.

Involvement of Salicylic Acid and Other Phenolic Compounds in Light-Dependent Cold Acclimation in Maize.

The exposure of plants to non-lethal low temperatures may increase their tolerance to a subsequent severe chilling stress. To some extent, this is also true for cold-sensitive species, including maize. In the present work, based on our previous microarray experiment, the differentially expressed genes with phenylpropanoid pathways in the focus were further investigated in relation to changes in certain phenolic compounds and other plant growth regulators. Phenylalanine ammonia lyase (PAL) was mainly activated under limited light conditions. However, light-induced anthocyanin accumulation occurred both in the leaves and roots. Chilling stress induced the accumulation of salicylic acid (SA), but this accumulation was moderated in the cold-acclimated plants. Acclimation also reduced the accumulation of jasmonic acid (JA) in the leaves, which was rather induced in the roots. The level of abscisic acid (ABA) is mainly related to the level of the stress, and less indicated the level of the acclimation. The highest glutathione (GSH) amount was observed during the recovery period in the leaves of plants that were cold acclimated at growth light, while their precursors started to accumulate GSH even during the chilling. In conclusion, different light conditions during the cold acclimation period differentially affected certain stress-related mechanisms in young maize plants and changes were also light-dependent in the root, not only in the leaves.

PGR5 and NDH-1 systems do not function as protective electron acceptors but mitigate the consequences of PSI inhibition

Avoidance of photoinhibition at photosystem (PS)I is based on synchronized function of PSII, PSI, Cytochrome b6f and stromal electron acceptors. Here, we used a special light regime, PSI photoinhibition treatment (PIT), in order to specifically inhibit PSI by accumulating excess electrons at the photosystem (Tikkanen and Grebe, 2018). In the analysis, Arabidopsis thaliana WT was compared to the pgr5 and ndho mutants, deficient in one of the two main cyclic electron transfer pathways described to function as protective alternative electron acceptors of PSI. The aim was to investigate whether the PGR5 (pgr5) and the type I NADH dehydrogenase (NDH-1) (ndho) systems protect PSI from excess electron stress and whether they help plants to cope with the consequences of PSI photoinhibition. First, our data reveals that neither PGR5 nor NDH-1 system protects PSI from a sudden burst of electrons. This strongly suggests that these systems in Arabidopsis thaliana do not function as direct acceptors of electrons delivered from PSII to PSI – contrasting with the flavodiiron proteins that were found to make Physcomitrella patens PSI resistant to the PIT. Second, it is demonstrated that under light-limiting conditions, the electron transfer rate at PSII is linearly dependent on the amount of functional PSI in all genotypes, while under excess light, the PGR5-dependent control of electron flow at the Cytochrome b6f complex overrides the effect of PSI inhibition. Finally, the PIT is shown to increase the amount of PGR5 and NDH-1 as well as of PTOX, suggesting that they mitigate further damage to PSI after photoinhibition rather than protect against it.

Habitat change and alteration of plant and invertebrate communities in waterbodies dominated by the invasive alien macrophyte Lemna minuta Kunth

The free-floating American duckweed, Lemna minuta, is an invasive species now widespread in Europe. Yet, its impact on freshwater ecosystems has been poorly investigated. In this study, the effects of the presence of this invasive duckweed on water quality, and aquatic plant and invertebrate communities were evaluated in sites in Central Italy. Water chemical and physical factors and community descriptors were analyzed to identify these effects. Surveys were carried out across 17 paired aquatic sites. Site pairs were similar in microclimate, hydrogeology and water quality, but differed in relation to the presence/absence of L. minuta floating mats. In sites with mats, light and dissolved oxygen in water were negatively correlated with increasing mat coverage and thickness. The limited light and hypoxic conditions under mats inhibited plant growth and had a selective impact on the invertebrate community. Sites with L. minuta had aquatic communities with a lower plant taxa richness and a contrasting composition, compared with those in sites without. At sites with mats some plants were unaffected, but the majority of plant taxa documented at sites without Lemna were no present at sites with Lemna or were very rare (macroalgae, submerged rhizophytes). As for invertebrates, hypoxic-tolerant taxa dominated under mats (Ostracoda, Copepoda, Isopoda), whilst those more sensitive to oxygen depletion, or obligate herbivores, or those with a winged stage or swimming on water surface, were rare or absent (Ephemeroptera, Amphipoda, Chironomus, Notonecta). Lemna minuta mats presence was associated with alterations in the underlying aquatic ecosystem, severely threatening the conservation of these habitats. Active management strategies, including spread-prevention techniques, or mechanical removal combined with biological control, are required to conserve these habitats.

Light-Harvesting in Mesophotic Corals is Powered by a Spatially Efficient Photosymbiotic System between Coral Host and Microalgae

The coral-algal photosymbiosis fuels global coral-reef primary productivity, extending from sea level to as deep as 150 m (i.e., mesophotic). Currently, it is largely unknown how such mesophotic reefs thrive despite extremely limited light conditions. Here, we show that corals exhibit a plastic response to mesophotic conditions that involves a spatially optimized regulation of the bio-optical properties by coral host and symbiont. In contrast to shallow corals, mesophotic corals absorbed up to three-fold more light, resulting in excellent photosynthetic response under light conditions of only ~3% of the incident surface irradiance. The enhanced light harvesting capacity of mesophotic corals is regulated by average refractive index fluctuations in the coral skeleton that give rise to optical scattering and facilitate light transport and absorption by densely pigmented host tissue. The results of this study provide fundamental insight into the energy efficiency and light-harvesting mechanisms underlying the productivity of mesophotic coral reef ecosystems, yet also raise concerns regarding their ability to withstand prolonged environmental disturbances.

Macrophyte performance in a low arctic lake: effects of temperature, light and nutrients on growth and depth distribution

Submerged macrophytes are important contributors to primary production in clear water arctic and low arctic lakes. However, their production and potential coverage are hampered by the harsh climate conditions. In this study, we investigated the potential for increased macrophyte production and coverage in arctic lakes in a future warmer climate. In situ growth experiments with Callitriche hamulata were performed at 2, 4, 8 and 12 m depth in combination with nutrient assay experiments at 2 m depth. In addition, growth experiments were performed in the laboratory at four temperatures (5, 10, 15 and 20 °C) under saturated and light-limited conditions (150 and 25 µmol m−2 s−1). The results show that macrophyte growth in the low arctic lake, Badeso, is phosphorus limited, but they also indicate that nutrients are not the limiting factor for the macrophyte depth distribution. Rather the short growing season combined with low summer temperatures may limit the expansion of C. hamulata. Our study also shows that C. hamulata is a very temperature-sensitive plant, particularly around 10 °C. In a future warmer climate in the arctic, the thermocline in clear lakes is expected to expand deeper into the water column. Thus, at the present light conditions we can expect an expansion of both colonisation depth and coverage, which may affect the overall primary production in arctic lakes and the carbon flux and carbon cycling in the lake systems. Our findings strongly support recent predictions of increased growth and a more northerly distribution range of cold-temperature submerged macrophytes.

Flood disturbance and shade stress shape the population structure of açaí palm Euterpe precatoria, the most abundant Amazon species

Euterpe precatoria Mart. is the most abundant plant species in the Amazon basin, and one of the main non-timber forest products on the continent. A thorough understanding of the ecology of this species is needed to support sustainable management initiatives. Resource availability, disturbance regime, and human management are some of the main factors influencing population structure. We described the species’ life stages, evaluated its allometric relationships, and assessed the effects of habitat type (floodplain and upland) and proximity to human settlements on population size distribution in the Central Amazon near the Purus River. The height:diameter ratio increased from Seedlings to Juvenile 2, but decreased from Juvenile 2 to Reproductive 2, indicating changing height investment for any given diameter along these life stages. There was a marked habitat dependency in both the density and population size distribution, with populations in upland forests dominated by juveniles, whereas populations in the floodplains were dominated by reproductive palms. Proximity to human settlements was not related to population structure parameters. Our results suggest that the disturbance regime may have opposite meanings in várzea forests, where it limits recruitment under increased light levels, and in terra firme forests, where it may stimulate recruitment under limited light conditions.

Is Karenia brevis really a low-light-adapted species?

Despite nearly annual blooms of the neurotoxic dinoflagellate Karenia brevis (Davis) G. Hansen and Moestrup in the Gulf of Mexico, defining the suite of biological traits that explain its proliferation has remained challenging. Studies have described K. brevis as a low-light-adapted species, incapable of sustaining growth under high light, which is at odds with observed surface aggregations sometimes within centimeters of the sea surface and also with short-term experiments showing photosynthetic machinery accommodating high irradiances. Here, growth and photophysiology of three K. brevis isolates were evaluated under a range of environmentally relevant irradiances (10–1500 μmol photons m−2 s−1) in the laboratory. No differences in growth–irradiance curves were observed among isolates; all sustained maximum growth rates at the highest irradiances examined, even in exposures as long as three weeks. The growth efficiency α of K. brevis under light-limiting conditions appeared mediocre among dinoflagellates, and poorer than that of other phytoplankton (e.g., diatoms, cyanobacteria), implying that K. brevis is not a low-light specialist. This finding substantially alters earlier parameterizations of K. brevis growth–irradiance curves. Therefore, a model was developed to contextualize how these new growth–irradiance curves might affect bottom growth rates. This model was subsequently applied to a case study comparing seasonal light forcing offshore of Pinellas County, FL, USA, with a single empirical value for light attenuation, and seasonal bottom water temperatures. Predictions suggested that light may limit bottom growth as close as 1 km from shore in winter, but would only begin limiting growth 20 km from shore in summer. Population maintenance (no net growth) was possible as far offshore as 90 km in summer and 68 km in winter. These ranges intercept areas thought to be important for bloom initiation.

Aggressive communication in aquatic environments

Abstract Aggressive interactions are ubiquitous among animals. They are either directed towards heterospecifics, like predators or competitors, or conspecifics. During intraspecific encounters, aggression often serves to establish hierarchies within the social group. Thus, in order to understand the mechanisms mediating social organization, it is important to comprehend the escalation and avoidance of aggressive behaviour. Overt aggressive interactions are costly not only in terms of increased risk of injury or death, but also due to opportunity costs and energy expenditure. In order to reduce these costs, animals are expected to communicate their strength and aggressive motivation prior to fights. For this purpose, they use different means of communication in various sensory modalities, that is visual, acoustic, chemical, mechanosensory and electric cues. These different modalities can convey different or similar information, underlining the importance of understanding the multimodal communication of aggression. Thus far, most studies on signalling during aggressive encounters have focussed on visual or acoustic cues, most likely as these are the two modalities predominantly used by humans. However, depending on the species’ ecology, visual or acoustic cues might play a minor role for many species. Especially in aquatic systems, visual communication is often hampered due to high levels of turbidity or limited light conditions. Here, alternative modalities such as chemical, mechanical or electrical cues are expected to play a prominent role. In this review, I provide an overview of different modalities used during aggressive communication in aquatic organisms. I highlight the importance of studying the role of multimodal communication during aggressive encounters in general and discuss the importance of understanding aquatic communication in the light of conservation and animal welfare issues.

Remodeling of excitation energy transfer in extremophilic red algal PSI-LHCI complex during light adaptation

Photosynthetic PSI-LHCI complexes from an extremophilic red alga C. merolae grown under varying light regimes are characterized by decreasing size of LHCI antenna with increasing illumination intensity [1]. In this study we applied time-resolved fluorescence spectroscopy to characterize the kinetics of energy transfer processes in three types of PSI-LHCI supercomplexes isolated from the low (LL), medium (ML) and extreme high light (EHL) conditions. We show that the average rate of fluorescence decay is not correlated with the size of LHCI antenna and is twice faster in complexes isolated from ML-grown cells (~25–30 ps) than from both LL- and EHL-exposed cells (~50–55 ps). The difference is mainly due to a contribution of a long ~100-ps decay component detected only for the latter two PSI samples. We propose that the lack of this phase in ML complexes is caused by perfect coupling of this antenna to PSI core and lack of low-energy chlorophylls in LHCI. On the other hand, the presence of the slow, ~100-ps, fluorescence decay component in LL and EHL complexes may be due to the weak coupling between PSI core and LHCI antenna complex, and due to the presence of particularly low-energy or red chlorophylls in LHCI. Our study has revealed the remarkable functional flexibility of light harvesting strategies that have evolved in the extremophilic red algae in response to harsh or limiting light conditions involving accumulation of low energy chlorophylls that exert two distinct functions: as energy traps or as far-red absorbing light harvesting antenna, respectively.

Interplay and Targetome of the Two Conserved Cyanobacterial sRNAs Yfr1 and Yfr2 in Prochlorococcus MED4

The sRNA Yfr1 and members of the Yfr2 sRNA family are almost universally present within cyanobacteria. The conserved motifs of these sRNAs are nearly complementary to each other, suggesting their ability to participate in crosstalk. The conserved motif of Yfr1 is shared by members of the Yfr10 sRNA family, members of which are otherwise less conserved in sequence, structure, and synteny compared to Yfr1. The different structural properties enable the discrimination of unique targets of Yfr1 and Yfr10. Unlike most studied regulatory sRNAs, Yfr1 gene expression only slightly changes under the tested stress conditions and is present at high levels at all times. In contrast, cellular levels of Yfr10 increase during the course of acclimation to darkness, and levels of Yfr2 increase when cells are shifted to high light or nitrogen limitation conditions. In this study, we investigated the targetomes of Yfr2, Yfr1, and Yfr10 in Prochlorococcus MED4, establishing CRAFD-Seq as a new method for identifying direct targets of these sRNAs that is applicable to all bacteria, including those that are not amenable to genetic modification. The results suggest that these sRNAs are integrated within a regulatory network of unprecedented complexity in the adjustment of carbon and nitrogen-related primary metabolism.

Fatty acids and proteins from marine cold adapted microalgae for biotechnology

Cold-adapted microalgae display unexpectedly high biomass production, pointing to their potential to produce high-value bioproducts under cold and light-limited conditions. From culture collections, we screened eight cold-adapted strains of different genera (Chlamydomonas, Chlorella, Tetraselmis, Pseudopleurochloris, Nannochloropsis and Phaeodactylum) for the production of fatty acids and proteins under low temperature and light regimes (T = 8, 15 °C; I = 50, 100 μmol s−1 m−2). Among the strains, the Arctic isolate Chlamydomonas sp. (RCC 2488) had better growth at 8 °C compared to 15 °C (up to 0.5 gDW L−1 d−1) and highest productivities of protein and polyunsaturated fatty acids (PUFA) (70 and 65 mg L−1 d−1, respectively). Two tested Tetraselmis strains (SAG 1.96, RCC 2604) achieved highest biomass productivities (0.7–1 gDW L−1 d−1), containing up to 50 mg PUFA gDW−1 and 15% proteins. Pseudopleurochloris antarctica (SAG 39.98) grew well at 15 °C (0.4 g L−1 d−1), with 23% proteins in biomass and the highest eicosapentaenoic acid (EPA) productivity (7.6 mg L−1 d−1). Chlorella stigmatophora (RCC 661) achieved productivities of 0.4 gDW L−1 d−1 at 15 °C and produced extracellular polymeric substances (EPS). The major cause for the observed shifts in biochemical profiles was biomass concentration, which is an indicator for the prevailing growth stage. Based on the current experimental design, Chlamydomonas sp. (RCC 2488), T. chuii and P. antarctica can be suggested as the most promising strains for the production of protein and (polyunsaturated-) fatty acids at low temperatures. However, additional strain-specific studies are necessary to statistically validate these findings.

Strawberry, black medic (Medicago lupulina), and Carolina geranium (Geranium carolinianum) growth under light-limiting conditions

AbstractBroadleaf infestations interfere with Florida strawberry production. Broadleaf POST herbicide options applied atop the crop are limited to synthetic auxins and not suitable for conventional multi-cropping and organic systems. Reducing light access and interception during weed emergence may reduce interference. Light-limited growth of two problematic broadleaves, black medic and Carolina geranium, and the most commonly grown strawberry cultivar (‘Florida Radiance’), were examined in the greenhouse. The experimental design was completely randomized, and the trial was repeated. Black medic was susceptible to reductions in incoming solar radiation, wherein reducing the daily maximum available light from 331 to 94 µmol m−2s−1reduced leaf number and area by 93% and 89%, respectively. Carolina geranium growth was less susceptible to reduced-light treatments, with leaf area and number each reduced by 66% when light was reduced from 331 to 94 µmol m−2s−1. Belowground, Carolina geranium biomass was similarly reduced between the 331 and 94 µmol m−2s−1treatments. Strawberry was relatively tolerant to shading at 155 µmol m−2s−1, but further reductions did increase mortality. Shade-induced weed suppression is a promising alternative strategy for conventional and organic Florida strawberry production. Targeted application during periods of weed emergence may play a role within integrated pest management strategies. This approach is most feasible for black medic management but may be useful for Carolina geranium in concert with other strategies.

The ratio of single-turnover to multiple-turnover fluorescence varies predictably with growth rate and cellular chlorophyll in the green alga Dunaliella tertiolecta.

Marine phytoplankton experience a wide range of nutrient and light conditions in nature and respond to these conditions through changes in growth rate, chlorophyll concentration, and other physiological properties. Chlorophyll fluorescence is a non-invasive and efficient tool for characterizing changes in these physiological properties. In particular, the introduction of fast repetition rate fluorometry (FRRf) into studies of phytoplankton physiology has enabled detailed studies of photosynthetic components and kinetics. One property retrieved with an FRRf is the 'single-turnover' maximum fluorescence (FmST) when the primary electron acceptor, Qa, is reduced but the plastoquinone (PQ) pool is oxidized. A second retrieved property is the 'multiple-turnover' fluorescence (FMT) when both Qa and PQ are reduced. Here, variations in FmST and FMT were measured in the green alga Dunaliella tertiolecta grown under nitrate-limited, light-limited, and replete conditions. The ratio of FmST to FMT (ST/MT) showed a consistent relationship with cellular chlorophyll in D. tertiolecta across all growth conditions. However, the ST/MT ratio decreased with growth rate under nitrate-limited conditions but increased with growth rate under light-limited conditions. In addition, cells from light-limited conditions showed a high accumulation of Qb-nonreducing centers, while cells from nitrate-limited conditions showed little to none. We propose that these findings reflect differences in the reduction and oxidation rates of plastoquinone due to the unique impacts of light and nitrate limitation on the stoichiometry of light-harvesting components and downstream electron acceptors.

Light limitation increases multidimensional trait evenness in phytoplankton populations

Individual-level variation arising from responses to environmental gradients influences population and community dynamics. How such responses empirically relate to the mechanisms that govern species coexistence is, however, poorly understood. Previous results from l ake phytoplankton communities suggested that the evenness of organismal traits in multiple dimensions increases with resource limitation, possibly due to resource partitioning at the individual level. Here we experimentally tested the emergence of this pattern by growing two phytoplankton species (Pseudokirchneriella subcapitata and Microcystis aeruginosa) under a gradient of light intensity, in monoculture and jointly. Under low light (resource) conditions, the populations diversified into a wide range of phenotypes, which were evenly distributed in multidimensional trait space (defined by four pigment-related trait dimensions), consistent with the observed field pattern. Our interpretation is that under conditions of light limitation, individual phytoplankton cells alter photosynthetic traits to reduce overlap in light acquisition, acquiring unexploited resources and thereby likely maximising individual success. Our results provide prime experimental evidence that resource limitation increases the evenness of conspecific and heterospecific microbial phenotypes along trait axes, advancing our understanding of trait-based coexistence.

Average cosine coefficient and spectral distribution of the light field under sea ice: Implications for primary production

The Arctic spring phytoplankton bloom has been reported to commence under a melting sea ice cover as transmission of photosynthetically active radiation (PAR; 400–700 nm) suddenly increases with the formation of surface melt ponds. Spatial variability in ice surface characteristics, i.e., snow thickness or melt pond distributions, and subsequent impact on transmitted PAR makes estimating light-limited primary production difficult during this time of year. Added to this difficulty is the interpretation of data from various sensor types, including hyperspectral, multispectral, and PAR-band irradiance sensors, with either cosine-corrected (planar) or spherical (scalar) sensor heads. To quantify the impact of the heterogeneous radiation field under sea ice, spectral irradiance profiles were collected beneath landfast sea ice during the Green Edge ice-camp campaigns in May–June 2015 and June–July 2016. Differences between PAR measurements are described using the downwelling average cosine, μd, a measure of the degree of anisotropy of the downwelling underwater radiation field which, in practice, can be used to convert between downwelling scalar, E0d, and planar, Ed, irradiance. A significantly smallerμd(PAR) was measured prior to snow melt compared to after (0.6 vs. 0.7) when melt ponds covered the ice surface. The impact of the average cosine on primary production estimates, shown in the calculation of depth-integrated daily production, was 16% larger under light-limiting conditions when E0d was used instead of Ed. Under light-saturating conditions, daily production was only 3% larger. Conversion of underwater irradiance data also plays a role in the ratio of total quanta to total energy (EQ/EW, found to be 4.25), which reflects the spectral shape of the under-ice light field. We use these observations to provide factors for converting irradiance measurements between irradiance detector types and units as a function of surface type and depth under sea ice, towards improving primary production estimates.

Clonal integration increases growth performance and expansion of Eichhornia crassipes in littoral zones: A simulation study

Clonal integration can improve the spread and growth of invasive plants in response to various disturbances. However, little is known about its role in floating aquatic clonal plants that expand from aquatic into terrestrial habitats in littoral zones. Thus, in this study, we simulated the expansion of the invasive clonal aquatic plant Eichhornia crassipes from aquatic to terrestrial habitats through two modes of clonal integration. We subjected E. crassipes parent plants and offspring ramets to three levels of natural light in terrestrial habitats: 100%, 60%, and 10%. The stolon connections were either severed or kept intact. Our findings showed that clonal integration had positive effects on plants exposed to shade in the terrestrial habitats and produced negative effects on plants in the aquatic habitats. Overall, clonal integration significantly increased whole-plant growth performance. Parent plants and offspring ramets in the terrestrial environments can enhance their adaptability to shade by increasing the maximum quantum yield of photosystem II and chlorophyll content. Clonal integration can support the expansion of E. crassipes from aquatic into terrestrial habitats with limited light conditions through significantly elevated growth traits. Thus, E. crassipes has a high ability for clonal integration and may be a potential threat to littoral zone ecosystems.