Low Light Availability Research Articles

Watershed inputs of suspended sediment drive patterns of total phosphorus in Chequamegon Bay, Lake Superior

Lake Superior is considered the least anthropogenically stressed and has the lowest offshore phosphorus (P) concentrations of the five Laurentian Great Lakes. However, nearshore habitats in Lake Superior are showing evidence of nutrient related stress. We examined drivers of total P dynamics in Chequamegon Bay, a shallow embayment in southwestern Lake Superior located in a region of the Laurentian Great Lakes that is primarily forested with low human development. Over a nine-year period (2014–2022) we measured total and soluble reactive phosphorus (TP and SRP, respectively), total suspended solids (TSS), and chlorophyll-a (Chl-a) at 12 locations distributed across Chequamegon Bay. Path analysis revealed that TP in this region of Lake Superior is largely sediment bound and driven by watershed inputs of suspended sediment. SRP and Chl-a make up only a small portion of TP. TP and TSS were highly correlated, with a stronger correlation at the most nearshore locations and following extreme precipitation events in 2016 and 2018. TP and Chl-a had a weak positive correlation at low TP concentrations, and lack of correlation at high TP concentrations. This suggests that despite high TP inputs from runoff events, Chl-a response was minimal, likely due to low light availability and limited bioavailability of sediment-bound P. Understanding conditions where episodic inputs of TP could contribute to the reactive P pool and how hydrodynamics affect biogeochemical processes and algal response to nutrient inputs are critical to understanding how an expected increase in extreme events will influence nearshore water quality in large lakes.

Mesophyll conductance limits photosynthesis and relates to anatomical traits in high-elevation plants in the Andes

Plants face harsher conditions with increasing elevation, including shorter growing seasons, lower temperatures, and reduced gas pressure. This often leads to increased leaf mass per area, suggesting greater limitation to photosynthesis due to decreased mesophyll conductance. However, some species maintain consistent photosynthetic rates at higher elevations, suggesting compensatory mechanisms. In the central Chile Andes, high-elevation habitats present cold temperatures with no soil moisture deficits, whereas low-elevations experience warm temperatures and summer droughts. Zonal plants adapt to these changes, whereas azonal plants grow near water sources and avoid drought. We assessed how elevation affects photosynthesis and its limitations in these plant-types, together with the role of leaf internal anatomy. This was done with gas exchange and chlorophyll fluorescence analyses, along with measurements of leaf inner structure, on zonal and azonal species growing at 2600 and 3550 m a.s.l. Results showed that whilst photosynthesis decreased with elevation in azonal plants, zonal plants showed no change, with mesophyll conductance being a primary limitation, influenced by chloroplast arrangement rather that cell wall thickness. This affects carbon acquisition in high-elevation plants due to low gas pressure and light availability.

Impact of light and nutrient availability on the phagotrophic activity of harmful bloom-forming dinoflagellates

Abstract Phagotrophy is a key nutritional mode for many bloom-forming dinoflagellates that can supplement their carbon and nutrient requirements. However, the environmental drivers and ecological relevance of phagotrophy in algal blooms are still poorly understood. This study evaluates the effect of light and nutrient availability on the phagotrophic activity of three common bloom-forming dinoflagellates (Alexandrium minutum, Heterocapsa triquetra and Prorocentrum micans) using three fluorescently labeled preys: bacteria, Synechococcus and the haptophyte Isochrysis galbana. The three dinoflagellates exhibited distinct responses to light and nutrient availability in terms of growth, cell size, prey ingestion and preference. A. minutum and H. triquetra showed higher cell-specific ingestion rates on bacteria (0.53 ± 0.13 and 1.64 ± 0.39 prey dinoflagellate−1 h−1, respectively) under co-limited nutrient and light availability, whereas P. micans showed higher ingestion on Synechococcus (0.93 ± 0.22 prey dinoflagellate−1 h−1) under low-light availability alone. However, the three dinoflagellates exhibited the highest carbon and nitrogen-specific ingestion rates when feeding on the larger prey I. galbana. Our findings indicate that phagotrophy could be of advantage in short periods of light or nutrient limitation and may play different roles during the development of blooms, likely influencing the energy transfer through the food web.

Synechococcus sp. PCC7335 responses to far-red enriched spectra and anoxic/microoxic atmospheres: Potential for astrobiotechnological applications

Recently, cyanobacteria have gained attention in space exploration to support long-term crewed missions via Bioregenerative Life Support Systems. In this frame, cyanobacteria would provide biomass and profitable biomolecules through oxygenic photosynthesis, uptaking CO2, and releasing breathable O2. Their growth potential and organic matter production will depend on their ability to photoacclimate to different light intensities and spectra, maximizing incident light harvesting. Studying cyanobacteria responses to different light regimes will also benefit the broader field of astrobiology, providing data on the possibility of oxygenic photosynthetic life on planets orbiting stars with emission spectra different than the Sun. Here, we tested the acclimation and productivity of Synechococcus sp. PCC7335 (hereafter PCC7335), capable of Far-Red Light Photoacclimation (FaRLiP) and type III chromatic acclimation (CA3), in an anoxic, CO2-enriched atmosphere and under a spectrum simulating the low energetic light regime of an M-dwarf star, also comparable to a subsuperficial environment. When exposed to the light spectrum, with few photons in the visible (VIS) and rich in far-red (FR), PCC7335 did not activate FaRLiP but acclimated only via CA3, achieving a biomass productivity higher than expected, considering the low VIS light availability, and a higher production of phycocyanin, a valuable pigment, with respect to solar light. Its growth or physiological responses of PCC7335 were not affected by the anoxic atmosphere. In these conditions, PCC7335 efficiently produced O2 and scavenged CO2. Results highlight the photosynthetic plasticity of PCC7335, its suitability for astrobiotechnological applications, and the importance to investigate biodiversity of oxygenic photosynthesis for searching life beyond Earth.

Research Progress of NiS Cocatalysts in Photocatalysis

In recent decades, photocatalysis technology, a typical mild and environmentally friendly technique, can use sunlight to drive the oxidation/reduction reactions on the surface of catalysts. However, single semiconductor materials can hardly meet the requirements of photocatalytic application due to the low availability of visible light, quick e− − h+ recombination, and the insufficiency of active sites. Cocatalysts are often introduced to solve the above problems. Among the numerous cocatalysts, NiS cocatalysts exhibit excellent properties, such as superior chemical stability, suitable band structure, and larger specific surface area, which can improve the photocatalytic efficiency of various semiconductors. Herein, the research progress of NiS cocatalysts in photocatalysis is reviewed. First, the structure of NiS and several common preparation methods are briefly introduced. Second, the NiS cocatalytic principle about how to achieve the effective separation of photogenerated carriers is introduced. Third, the methods to optimize the catalytic activity of NiS are mainly summarized. At the same time, the review summarizes the research progress of composite photocatalysts based on NiS cocatalyst. Finally, the review discusses the existing challenges and obstacles of NiS‐based photocatalytic systems and proposes feasible improvements and ideas to meet the future requirements of practical applications.

Differential photosynthetic plasticity of Amazonian tree species in response to light environments.

To investigate how and to what extent there are differences in the photosynthetic plasticity of trees in response to different light environments, six species from three successional groups (late successional, mid-successional, and pioneers) were exposed to three different light environments [deep shade - DS (5% full sunlight - FS), moderate shade - MS (35% FS) and full sunlight - FS]. Maximum net photosynthesis (Amax), leaf N partitioning, stomatal, mesophile, and biochemical limitations (SL, ML, and BL, respectively), carboxylation velocity (Vcmax), and electron transport (Jmax) rates, and the state of photosynthetic induction (IS) were evaluated. Higher values of Amax, Vcmax, and Jmax in FS were observed for pioneer species, which invested the largest amount of leaf N in Rubisco. The lower IS for pioneer species reveals its reduced ability to take advantage of sunflecks. In general, the main photosynthetic limitations are diffusive, with SL and ML having equal importance under FS, and ML decreasing along with irradiance. The leaf traits, which are more determinant of the photosynthetic process, respond independently in relation to the successional group, especially with low light availability. An effective partitioning of leaf N between photosynthetic and structural components played a crucial role in the acclimation process and determined the increase or decrease of photosynthesis in response to the light conditions.

Mesophotic corals in Hawai'i maintain autotrophy to survive low-light conditions.

In mesophotic coral ecosystems, reef-building corals and their photosynthetic symbionts can survive with less than 1% of surface irradiance. How depth-specialist corals rely upon autotrophically and heterotrophically derived energy sources across the mesophotic zone remains unclear. We analysed the stable carbon (δ13C) and nitrogen (δ15N) isotope values of a Leptoseris community from the 'Au'au Channel, Maui, Hawai'i (65-125 m) including four coral host species living symbiotically with three algal haplotypes. We characterized the isotope values of hosts and symbionts across species and depth to compare trophic strategies. Symbiont δ13C was consistently 0.5‰ higher than host δ13C at all depths. Mean colony host and symbiont δ15N differed by up to 3.7‰ at shallow depths and converged at deeper depths. These results suggest that both heterotrophy and autotrophy remained integral to colony survival across depth. The increasing similarity between host and symbiont δ15N at deeper depths suggests that nitrogen is more efficiently shared between mesophotic coral hosts and their algal symbionts to sustain autotrophy. Isotopic trends across depth did not generally vary by host species or algal haplotype, suggesting that photosynthesis remains essential to Leptoseris survival and growth despite low light availability in the mesophotic zone.

Herbage and sheep production from simple, diverse, and legume pastures established in an agrivoltaic production system

AbstractAgrivoltaic systems combine solar energy and agriculture, promoting dual land use. Although grazing sheep in these systems is common, research on intentionally designed pastures to increase production is lacking. This study seeks to compare the herbage growth and lamb production in simple, diverse, and legume pastures in an agrivoltaic system over 2 years in Oregon. Annual herbage production in diverse pastures (4583 kg DM ha−1) was comparable to simple pastures (4457 kg DM ha−1), but higher than legume pastures (p < .05; 4023 kg DM ha−1) for the 2020/2021 growing season. In the 2021/2022 growing season, diverse pastures had a higher yield (4660 kg DM ha−1, p < .05) than simple pastures (4052 kg DM ha−1), which were similar to legume pastures (3955 kg DM ha−1). Liveweight gain (LWG; g head−1 d−1) tended (p = .06) to be higher from lambs in diverse and legume pastures compared to simple pastures in spring 2021. Liveweight production (LWP; kg ha−1 d−1) of legume pasture were greater (P < 0.05) than other pasture types, while simple pastures had the lowest LWP during this period. In summer 2021, LWG tended to be greater in lambs grazing diverse versus simple pastures. No difference was detected (all p > .05) in LWG or LWP at any grazing period in 2022. Overall, efficiency of pasture production was greatly reduced by lower light availability in all pasture types. Diversification of pastures using forbs in agrivoltaics was a good strategy for greater herbage production, while the legume pastures had poor persistence.

Facultative mycorrhization in a fern (Struthiopteris spicant L. Weiss) is bound to light intensity

BackgroundThe establishment of mycorrhizal relationships between a fungus and a plant typically enhances nutrient and water uptake for the latter while securing a carbon source for the fungus. However, under a particular set of environmental conditions, such as low availability of light and abundant nutrients in the soil, the resources invested in the maintenance of the fungi surpass the benefits obtained by the host. In those cases, facultative mycorrhizal plants are capable of surviving without symbiosis. Facultative mycorrhization in ferns has been overlooked until now. The present study measured the response of Struthiopteris spicant L. Weiss, and its root-associated fungi to different levels of light and nutrient availability in terms of growth, mycorrhizal presence, and leaf nutrient content. This fern species exhibits a great tolerance to variable light, nutrient, and pH conditions, and it has been found with and without mycorrhizae. We conducted a greenhouse experiment with 80 specimens of S. spicant and three factors (Light, Phosphorus, and Nitrogen) resulting in eight treatments.ResultsWe found a significant influence of the factor light on fungal community composition, plant biomass, and nutrient accumulation. Departing from a lack of colonization at the initial stage, plants showed a remarkable increment of more than 80% in the arbuscular mycorrhizal fungi (AMF) richness and abundance in their roots when grown under high light conditions, compared with the ones in low light. We also observed an upward trend of C:P and C:N ratios and the above- and belowground biomass production when AMF abundance increased. Furthermore, the compositional analysis of the whole fungal communities associated with S. spicant roots revealed clear differences among low-light and high-light treatments.ConclusionsThis study is the first to investigate the importance of light and nutrient availability in determining fern-AMF relationships. We confirmed that Struthiopteris spicant is a facultative mycorrhizal plant. The composition and diversity of AMF found in the roots of this fern are strongly influenced by light and less by nutrient conditions. Our study shows that ferns respond very sensitively to changes in environmental factors, leading to shifts in the associated mycorrhizal communities.

Effects of herbivorous fish on shallow lake ecosystems increase at moderate nutrient conditions

Submerged macrophytes are vital in shallow lakes, as they provide critical ecosystem functions and services and can stabilize the clear-water conditions by various mechanisms. Nutrient enrichment reduces the resilience of macrophyte dominance in shallow lakes, thereby making them susceptible to shifts towards phytoplankton dominance following perturbations. Here, we conducted a mesocosm experiment to examine the individual and combined effects of nutrient enrichment and the addition of grass carp (Ctenopharyngodon idella) on the abundance of submerged macrophytes, epiphyton, and phytoplankton. We hypothesized that moderate nutrient enrichment facilitates macrophyte abundance, but also phytoplankton abundance after macrophyte removal by herbivorous fish. Our data showed that herbivory by grass carp could trigger a shift from macrophytes to algal dominance in mesocosms with moderate nutrient concentrations, but not in those with low nutrient concentrations. Moderate nutrient enrichment alone promoted submerged macrophyte growth, whereas the introduction of grass carp induced a collapse of submerged macrophytes regardless of nutrient conditions. Moreover, the introduction of grass carp showed more negative effects on light conditions of the water column in mesocosms with moderate nutrient concentrations compared to those with low nutrient concentrations. A recovery of submerged macrophytes might thus be limited by low light availability in lakes with moderate nutrient conditions suffering grass carp perturbation. Our results suggest that submerged macrophyte-dominated shallow lakes with moderate nutrient conditions are vulnerable to perturbation by herbivorous fish such as grass carp. In turn, managing the abundance of herbivores in these lakes can support the dominance of macrophytes and associated clear water conditions.

Simulated climate warming strongly constrains the seedling establishment of alpine cushion Arenaria oreophila

Population shrinkage of alpine cushion plants with ongoing climate warming has been empirically confirmed. Since cushion plants play important roles in sustaining alpine plant community and ecosystem functions, their population dynamics may directly influence the future alpine ecosystems. However, little is known about how climate warming affect cushion population recruitment, especially at early life-history stages. In this study, we conducted a laboratory simulation of climate warming to detect the effects of warming temperature and associated moisture and light changes on seed germination and seedling growth of the typical alpine cushion plant Arenaria oreophila. Results suggested that increasing temperature indeed exerted strong constraints on the population recruitment processes. Specifically, increased temperatures could quickly initialize seed germination (4-6 days at higher temperatures vs. 29-32 days at low temperature, respectively, after sowing), accelerate them to reach the maximum germination percentage (9-19 days at higher temperatures vs. 57-86 days at low temperature, respectively, after sowing) and significantly accelerate seedling growth rate. However, higher temperatures accelerated seedling mortality (more than 80%). In addition, lower light availability also increased seedling mortality though it could generally increase the final seed germination percentage. The effects of water might be dependent on temperature and light. All results suggested that cushion A. oreophila is quite sensitive to climate warming which strongly constraints its seedling establishment process. We, therefore, speculate that the continuing climate warming in future will exert uncertain risks in the persistence of cushion A. oreophila, possibly by constraining the process of seedling recruitments.

Photoprotective responses of three intertidal Antarctic macroalgae to short-term temperature stress

The Antarctic Peninsula is experiencing one of the highest warming rates globally. In polar regions, macroalgae thrive under extreme environmental conditions, which could worsen because of future climate change scenarios, including increased ultraviolet exposure, extremely low light availability, and fluctuating temperatures, particularly in the intertidal zones. To investigate the potential role of photoprotective and antioxidant mechanisms in response to future increases in sea surface temperatures caused by climate change, we conducted laboratory experiments using three intertidal macroalgae model species: Adenocystis utricularis (Ochrophyta, Phaeophyceae), Pyropia endiviifolia (Rhodophyta, Bangiophyceae), and Monostroma hariotii (Chlorophyta, Ulvophyceae). These algae were collected in Punta Artigas (King George Island, Antarctica) and acclimated at 2°C for 48 h. They were then assessed in laboratory experiments for up to 5 days under two treatments: (1) control conditions at 2°C and (2) elevated tem.perature conditions at 8°C, representing the most negative increment in SSTs estimated by the end of the 21st century. Carbon, nitrogen, pigments (chlorophylls and carotenoids), mycosporine-like amino acids (MAAs), and phenolic compounds were quantified after 3 and 5 days of exposure. For M. hariotii, elevated temperatures led to an increase in the C/N ratio, total antioxidant capacity, and levels of nitrogen, total carotenoids, chlorophyll-a, pigments (chlorophyll-b and violaxanthin), and phenolic compounds. For A. utricularis, elevated temperatures led to elevated C/N ratio and levels of chlorophyll-a and carotenoids (fucoxanthin and β-carotene). For P. endiviifolia, elevated temperatures resulted in elevated levels of carotenoids (lutein and β-carotene), phenolic compounds, and MAAs (porphyra-334, shinorine, and palythine). Thus, our study suggests that increasing water temperatures due to global warming can enhance the photoprotective abilities of three Antarctic intertidal macroalgae (M. hariotii, A. utricularis, and P. endiviifolia), with each species showing specific responses.

Competitive asymmetry in a forest composed of a shade-tolerant species depends on gap formation

Competitive asymmetry is one of the most important determinants of size structure in plant communities. Light competition was believed to be size-asymmetric, as taller individuals preempt light. In fact, many studies indicated that light competition is size-asymmetric in monospecific stands of herbaceous plants. However, competition is size-symmetric in multispecific stands because shorter, shade-adapted species compensate for the low light availability through higher leaf area per aboveground mass (leaf area ratio; LAR) or higher light use efficiency for growth (LUE). Then, a question arises on whether light competition is asymmetric or symmetric if the stand is occupied by a shade-tolerant species. Moreover, it is also interesting how gap dynamics affect competitive asymmetry, given the fact that gap formation can improve light availability in lower layers of the canopy. We investigated the plant size, growth rate, and three-dimensional distribution of foliage and light availability in a natural forest dominated by a shade-tolerant tree, Fagus crenata. We found that competitive asymmetry varied dynamically across the forest. In closed canopies, the relative growth rate (RGR) was higher in taller trees, indicating size-asymmetric growth. Shorter trees partly compensated for their light interception by having higher LAR but did not achieve comparative RGR to taller trees. Conversely, in gaps, the RGR of shorter trees was equivalent to those of taller trees because of the improved light availability for smaller trees, indicating size-symmetric growth. These results suggest that changes in competitive asymmetry driven by gap dynamics can influence the spatiotemporal diversity of size structures in forests.

Induction and Characterisation of Lignocellulolytic Activities from Novel Deep-Sea Fungal Secretomes

Fungi are increasingly recognised as being able to inhabit extreme environments. The deep sea is considered an extreme environment because of its low temperatures, high hydrostatic and lithostatic pressures, 3.5% salinity, and low oxygen, nutrient and light availability. Fungi inhabiting the deep sea may have evolved to produce proteins that allow them to survive these conditions. Investigation and characterisation of fungal lignocellulolytic enzymes from extreme environments like the deep sea is needed, as they may have unusual adaptations that would be useful in industry. This work, therefore, aimed to profile in detail the lignocellulolytic capabilities of fungi isolated from deep-sea sediments in the Atlantic Ocean, and a comparative lignocellulolytic terrestrial isolate. The isolates were strains of Emericellopsis maritima, Penicillium chrysogenum, P. antarcticum and Talaromyces stollii. Lignocellulolytic enzyme induction was achieved using liquid-state fermentation (LSF) with wheat bran as the main carbon source, while enzyme characteristics were evaluated using biochemical assays and gel-based proteomics. This study revealed that the isolates were halotolerant, produced xylanase over wide pH and temperature ranges, and produced a variety of glycoside hydrolase and feruloyl esterase activities. The T. stollii secretome demonstrated remarkable levels of exo-glycoside hydrolase activity, with xylanase activity optimum between pH 1.5–6.0 and temperatures between 1–60 °C, making this isolate an ideal candidate for biotechnological applications. This study is the first to quantitatively characterise xylanase activities and exo-glycoside hydrolase activities secreted by E. maritima, P. antarcticum and a marine T. stollii strain. This study is also the first to quantitatively characterise xylanase activities by a marine strain of P. chrysogenum during LSF.

Eutrophication decreases ecological resilience by reducing species diversity and altering functional traits of submerged macrophytes.

Positive feedback is key to producing alternative stable states and largely determines ecological resilience in response to external perturbations. Understanding the positive feedback mechanisms in macrophyte-dominated lakes is crucial for resilience-based management and restoration. Based on the field investigation of submerged macrophyte communities in 35 lakes in China, we found that morphological complexity (MC) and morphological plasticity (MP) are correlated with the stoichiometric homeostasis of phosphorus (HP ) and are related to ecosystem structure, functioning, and stability. We also found that the positive feedback strength of lakes dominated by macrophytes is biomass- and diversity-dependent. Eutrophication can decrease the community biomass by decreasing community MC, MP, and HP and the species diversity through low-light availability, ultimately decreasing the positive feedback strength and resilience of clear water states. We argue that functional traits and species diversity should be considered to build more resilient ecosystems in future changing environment scenarios.

0648 Retinal responsivity is associated with light and season-related cognitions in older adults with seasonal depression

Abstract Introduction Maladaptive cognitions associated with lower light availability in winter and the changing of the seasons are elevated in winter depression and are treatment targets in psychotherapy. Mechanisms underlying the development of SAD-specific cognitions may be related to seasonal variation in melanopsin-driven retinal responsivity to light. Prior work has shown reduced retinal responsivity to light during the winter in seasonal depression. We hypothesized maladaptive seasonal cognitions would be associated with reduced retinal responses to light. We also tested whether age moderated this relationship, hypothesizing that this learned association may only appear in older individuals who have experienced a greater number of seasonal depressive episodes. Methods Participants ages 19-65 (M=36.13, SD=12.81) with seasonal depression (n=37) and non-depressed controls (n=32) completed assessments in winter months (21st December to 21st March). Maladaptive seasonal cognitions were measured by the Seasonal Beliefs Questionnaire (SBQ). Melanopsin-driven retinal responsivity was assessed using the post illumination pupil response (PIPR). Net PIPR as a percent of baseline was averaged across the 10-30 seconds post-stimulus for use in analyses. Using multiple regression, we tested (1) if seasonal beliefs predict the PIPR and (2) if an interaction between seasonal beliefs and age predicts the PIPR, controlling for gender, the pupillary light reflex (PLR), and circadian time of testing from Dim Light Melatonin Onset. Results Seasonal beliefs were not associated with the PIPR (b= -0.86e-5, SE=0.27e-3, p=0.98). There was a significant interaction between seasonal beliefs and age on the PIPR (b= -0.56e-4, SE=0.23e-4, p=0.02), in which greater seasonal beliefs were associated with reduced PIPR in older individuals but not younger individuals. The Johnson-Neyman interval indicated that in participants above age 49, greater seasonal beliefs were significantly associated with lower retinal responsivity (PIPR; p< 0.05). Conclusion Greater seasonal beliefs were associated with reduced retinal responsivity, but only in older adults with seasonal depression. It is possible that reduced retinal responsivity creates learned maladaptive schemas regarding seasonal changes that are reinforced with annually recurring winter depressive episodes. However, longitudinal data focused on changes in seasonal beliefs resulting from reduced retinal sensitivity is necessary to test whether retinal responsivity is a mechanism in the formation of these seasonal-specific cognitions. Support (if any)

Shading by giant kelp canopy can restrict the invasiveness of Undaria pinnatifida (Laminariales, Phaeophyceae).

The spread of non-indigenous and invasive seaweeds has increased worldwide, and their potential effects on native seaweeds have raised concern. Undaria pinnatifida is considered among the most prolific non-indigenous species. This species has expanded rapidly in the Northeast Pacific, overlapping with native communities such as the iconic giant kelp forests (Macrocystis pyrifera). Canopy shading by giant kelp has been argued to be a limiting factor for the presence of U. pinnatifida in the understory, thus its invasiveness capacity. However, its physiological plasticity under light limitation remains unclear. In this work, we compared the physiology and growth of juvenile U. pinnatifida and M. pyrifera sporophytes transplanted to the understory of a giant kelp forest, to juveniles growing outside of the forest. Extreme low light availability compared to that outside (~0.2 and ~4.4 mol photon ⋅ m-2 ⋅ d-1 , respectively) likely caused a "metabolic energy crisis" in U. pinnatifida, thus restricting its photoacclimation plasticity and nitrogen acquisition, ultimately reducing its growth. Despite M. pyrifera juveniles showing photoacclimatory responses (e.g., increases in photosynthetic efficiency and lower compensation irradiance, Ec ), their physiological/vegetative status deteriorated similarly to U. pinnatifida, which explains the low recruitment inside the forest. Generally, our results revealed the ecophysiological basis behind the limited growth and survival of juvenile U. pinnatifida sporophytes in the understory.

Effect of culture conditions at lab-scale on metabolite composition and antibacterial and antibiofilm activities of Dunaliella tertiolecta.

Dunaliella tertiolecta RCC6 was cultivated indoors in glass bubble column photobioreactors operated under batch and semi-continuous regimens and using two different conditions of light and temperature. Biomass was harvested by centrifugation, frozen, and then lyophilized. The soluble material was obtained by sequential extraction of the lyophilized biomass with solvents with a gradient of polarity (hexane, ethyl acetate, and methanol) and its metabolic composition was investigated through nuclear magnetic resonance (NMR) spectroscopy. The effect of light on chlorophyll biosynthesis was clearly shown through the relative intensities of the 1 H NMR signals due to pheophytins. The highest signal intensity was observed for the biomasses obtained at lower light intensity, resulting in a lower light availability per cell. Under high temperature and light conditions, the 1 H NMR spectra of the hexane extracts showed an incipient accumulation of triacylglycerols. In these conditions and under semi-continuous regimen, an enhancement of β-carotene and sterols production was observed. The antibacterial and antibiofilm activities of the extracts were also tested. Antibacterial activity was not detected, regardless of culture conditions. In contrast, the minimal biofilm inhibitory concentrations (MBICs) against Escherichia coli for the hexane extract obtained under semi-continuous regimen using high temperature and irradiance conditions was promising.

Nitrogen fixation responds to soil nitrogen at low but not high light in two invasive understory species

Abstract Light and soil nitrogen availability can be strong controls of plant nitrogen (N) fixation, but data on how understory N‐fixing plants respond to these drivers are limited despite their important role in ecosystem N cycling. Furthermore, ecosystem N cycling can be altered by the introduction of species with nutrient use patterns that differ from natives. We assessed how N fixation of two exotic, understory species responded to varying light and soil N environments. We sampled leaf tissue from Mimosa pudica L., Desmodium triflorum (L.) DC., and a nonfixing reference plant (Axonopus) growing in control and two N fertilization treatments under either N‐fixing or non‐N‐fixing trees, which may alter local soil nutrient cycling, across a range of light conditions. We measured N fixation with 15N isotope dilution, and ensured that N‐fixing neighbour trees were in fact fixing N. All understory plants were wild‐growing species not native to the study location. Desmodium and Mimosa acquired 82.6% and 71.6% of their nitrogen from fixation (%Ndfa) in the control, compared to 66.8% and 58.1% in the +10 g N m−2 year−1 treatment and 73.1% and 64.7% in the +15 g N m−2 year−1 treatment. These subtle %Ndfa differences across fertilization treatments were more apparent at low light availability and disappeared at high light availability. The amount of N fixed by neighbouring trees did not influence %Ndfa in the understory species. Synthesis. Our study shows some differences in N fixation across different nutrient environments at low light for two N‐fixing species, though the changes were small, and both species derived most of their N from fixation. These findings imply that introduced N‐fixing species could exacerbate ecosystem N enrichment, particularly under high soil N conditions.

Tree growth periodicity in the ever‐wet tropical forest of the Americas

Abstract The occurrence of annual growth rings in tropical trees—the result of the seasonal activity of vascular cambium—has been explained by seasonal water deficit or flooding periods. However, little is known about the drivers of annual tree‐ring formation under tropical hyper‐humid conditions without clear seasonal dry periods or flooding (ever‐wet conditions). Shelford's law states that the deficit or the excess of environmental resources limits plant growth. Accordingly, we hypothesize that excess soil moisture, a slight seasonal reduction of precipitation and a reduction in light availability determine rhythmic growth in ever‐wet tropical forests. We first assessed the occurrence of rhythmic growth in 14 tree species from the Biogeographic Chocó Region (annual rainfall 7200 mm) using three methods: Radiocarbon (14C) dating (all studied species), tree‐ring synchronization (4 species that have replicates) and automatic dendrometers (two species). Then, we assessed the effect of environmental drivers (rainfall, short‐wave radiation, temperature and soil moisture) on tree growth based on tree ring and dendrometer observations. We present evidence of annual tree‐ring formation in all 14 studied tree species. Depending on the tree species, we observed positive and negative correlations between growth, water availability and light availability. These relationships suggest that both excess or deficit of environmental resources may explain the seasonal pattern of tree growth. Although we cannot differentiate between excess soil water and low light availability by high cloudiness, we suggest that cloudiness frequency could affect tree growth in these forests. Synthesis. We reveal the annual formation of growth rings in the unexplored wetter‐end tropical forests, where seasonal growth depends on either high soil moisture and hypoxia or light limitations by cloudiness and photosynthesis constraints.