Chlorophyll Maximum Research Articles

Remote sensing of the subsurface chlorophyll maximum depth in the North Pacific Ocean

ABSTRACT Estimation of the subsurface chlorophyll maximum (SCM) depth is critical to constructing the vertical profile of chlorophyll a, which in turn is important in accurately assessing phytoplankton distribution, especially in the extensive tropical oceans. Current ocean colour algorithms generally detect a limited area only or use a suite of environmental variables, part of which may be difficultly quantified. By using field in situ observations, a two-step approach, in which the first step used a relationship between sea surface temperature and surface chlorophyll to detect the occurrence of the SCM and the second step used an algorithm based on the ratio of remote sensing reflectance, was developed to remotely estimate the SCM depth in the North Pacific. Although this approach, with the inputs of easily available ocean colour data, is simple, the deviation in the remotely sensed SCM depth was only about ± 17%. Our study may provide a promising approach to assess the stratification status of the water column and to help profiling the vertical variation of chlorophyll in the North Pacific and even in the global oceans.

Effects of Irrigation with Treated Slaughterhouse Effluent and Bradyrhizobium spp. Inoculation on Soybean Development and Productivity: Strategies for Sustainable Management

Water scarcity challenges in agriculture are prompting the exploration of alternative irrigation sources, including treated effluents. This study investigates the effects of irrigation with treated slaughterhouse effluent on soybean productivity and development, with and without inoculation, over two cropping cycles. Plant performance was significantly influenced by environmental factors and the interaction between effluent and inoculation. Plant height and leaf area were greater in the second cycle, with effluent enhancing growth and foliar development. Fresh and dry shoot biomass showed significant interactions among cycle, inoculation, and effluent, with higher effluent concentrations combined with inoculation being most effective in the first cycle. Foliar nitrogen concentrations were higher in the first cycle, particularly at elevated effluent doses, while foliar sodium showed a positive correlation with nitrogen and a negative correlation with magnesium. Chlorophyll indices varied across developmental stages, with maximum chlorophyll b estimated at 85.35% effluent irrigation. Soybean yield did not vary significantly with effluent dilutions, suggesting environmental factors had a greater influence. In conclusion, treated effluent irrigation represents a sustainable strategy for soybean production, optimizing water and nutrient use while reducing reliance on chemical fertilizers.

Effect of Irrigation Intervals and Plastic Mulching On Flower Production of Carnation

Calyx splitting is a physiological disorder that results in poor flower production and vase life in carnation. Several scientific reports and literature suggests that this physiological die back and curly tip of carnation flowers might be due to the water stress and potassium deficiency. Keeping in view these facts a field experiment was carried out to study the effect of irrigation intervals and plastic mulching on flower production and control of calyx split in carnation at Ornamental Horticulture Nursery, The University of Agriculture Peshawar, in 2020. Randomized Complete Block Design with a split plot arrangement having two factors was used. Treatments were repeated three times. The plants of carnation were irrigated at different interval (3, 6, 9 and 12 days) allotted to main plots and plastic mulches (transparent, black and green and control / no mulching) were assigned to subplots. Data pertaining to irrigation intervals revealed that least days to flowering (89.7), least calyx splitting (6.1%) and maximum vase life (11.33 days) were recorded in plants irrigated with 12 days’ interval that were statistically similar to the effect of irrigation at 9 days of interval. Findings regarding plastic mulches showed that maximum chlorophyll content (62.8 SPAD), days to flowering (100.5), dry flower weight (2.4g) and vase life (10.95 days) with minimum calyx splitting (7.4%) were recorded in carnation plants of plots covered with green plastic mulch that were statistically similar to the effect of black and transparent plastic mulches. It is concluded that irrigation intervals of 9 and 12 days resulted in early flowering, least calyx splitting and extended vase life. While use of plastic (Transparent, black or green) as mulching material proved superior as compared to control/ no mulching.

Horizontal distribution of marine microbial communities in the North Pacific Subtropical Front.

Microbial communities are crucial for important ecosystem functions in the open ocean, such as primary production and nutrient cycling. However, few studies have addressed the distribution of microplankton communities in the remote oligotrophic region of the Pacific Ocean. Moreover, the biogeochemical and physical drivers of microbial community structure are not fully understood in these areas. This research aims to investigate the patterns of prokaryotic and protists communities' distribution in the North Pacific Subtropical Front (NPSF). The NPSF is a vast oligotrophic region with layered surface water and strong ocean currents. Despite its considerable size, its community distribution and function are poorly studied. We used a 16S and 18S rRNA gene sequencing approach to identify and characterize the water column microbial communities at two depths, the surface (3-5 m) and the deep chlorophyll maximum (DCM, 108-130 m). We aimed to elucidate the horizontal distribution patterns of these communities and to dissect the factors intricately shaping their distribution in the NPSF. Results showed that the community structure of both prokaryotes and protists was significantly influenced by depth, temperature, and longitude. Regarding alpha diversity, both communities presented a higher diversity at the surface. The prokaryotes also demonstrated to have a higher diversity in samples placed further east. The prokaryotes were dominated by Proteobacteria and Cyanobacteria, and the eukaryotic communities were dominated by Syndiniales. Combining biological and hydrographic data analysis showed the influence of vertical currents near the frontal jet in shaping the vertical distribution of both prokaryotic and protist communities. Even though most studies do not consider anomalies that emerge at each depth, these occurrences are capable of having a strong impact and influence on community structure. This study marks a significant advance in unraveling the intricate community structure and distribution dynamics of marine microbial communities within the North Pacific Ocean.

Co-Inoculation of Azospirillum brasilense and Bacillus sp. Enhances Biomass and Photosynthetic Efficiency in Urochloa brizantha

The synergism between plant growth-promoting bacteria species (PGPB) was evaluated regarding the effect of inoculation on productivity and the physiological aspects of Urochloa brizantha. The study included seven experimental groups arranged in a 3 × 2 + 1 factorial design consisting of three inoculants (Azospirillum brasilense, Bacillus sp. isolate EB-40, and Bacillus sp. isolate EB-40 + A. brasilense mixture), two application methods (seed and foliar spray), and controls. The MIX conjugate inoculation significantly increased plant height in all three harvests, with gains of 57%. At 60 and 90 days, MIX increased the number of tillers by 47% and the number of leaves by 61% compared to other treatments in all harvests. MIX also increased shoot dry mass in the second and third harvests, with improvements of 57–60% compared to the control. MIX improved the quantum efficiency of photosystem II and the ratio between variable and maximum chlorophyll fluorescence. Maximum fluorescence (Fm) was 11% higher in MIX-treated plants compared to the control, indicating increased potential photosynthesis. Variable fluorescence (Fv) efficiency improved by 22% for inoculation with A. brasilense and Bacillus sp. Our study reveals that A. brasilense plus the Bacillus sp. isolate EB-40 (MIX) has the potential to improve the resilience and productivity of U. brizantha.

Interaction of Meloidogyne incognita and Pseudomonas syringae pv. aptata in different types of soil on plant growth, photosynthetic pigments and proline contents of beetroot (Beta vulgaris L.).

Effect of Meloidogyne incognita and Pseudomonas syringae pv. aptata (Psa) was observed singly, together and pre and post inoculations in 4 soil types on plant growth, parameters, chlorophyll, carotenoid and proline contents of beetroot (Beta vulgaris L). Plant growth, chlorophyll and carotenoid contents were greater in loam soil followed by 20% fly ash soil, 10% fly ash plus 10% sand amended soil and least in 20 % sand mix soil. However, proline contents were high in 20% sand mix soil and least in loam soil. Plant growth (root dry weight), chlorophyll and carotenoid contents were reduced in plants inoculated with any test pathogen while proline contents were increased in plants inoculated with pathogens under study. Inoculation of both pathogens together caused a greater reduction of plant growth, chlorophyll and carotenoid contents than their individual inoculation. Inoculation of M. incognita 20 days prior to Psa resulted in greatest reduction in plant growth, chlorophyll and carotenoid and maximum proline contents. Inoculation of Psa with M. incognita reduced galling and nematode multiplication while prior inoculation of Psa caused maximum reduction in galling and nematode multiplication. Galling and nematode multiplication was high in 20% sand mix soil followed by loam soil and least in 20% fly ash amended soil. Bacterial leaf spot indices by Psa was 3 when alone. Disease indices were 5 when Psa was inoculated with M. incognita. Prior inoculation of M. incognita predisposed beetroots to Psa and aggravates the disease. Influence of M. incognita, Psa and their interactions in different soil types on various studied parameters in diseased plants was demonstrated by Principal component analysis.

Evaluating MULTIOBS Chlorophyll-a with Ground-Truth Observations in the Eastern Mediterranean Sea

Satellite-derived observations of ocean colour provide continuous data on chlorophyll-a concentration (Chl-a) at global scales but are limited to the ocean’s surface. So far, biogeochemical models have been the only means of generating continuous vertically resolved Chl-a profiles on a regular grid. MULTIOBS is a multi-observations oceanographic dataset that provides depth-resolved biological data based on merged satellite- and Argo-derived in situ hydrological data. This product is distributed by the European Union’s Copernicus Marine Service and offers global multiyear, gridded Chl-a profiles within the ocean’s productive zone at a weekly temporal resolution. MULTIOBS addresses the scarcity of observation-based vertically resolved Chl-a datasets, particularly in less sampled regions like the Eastern Mediterranean Sea (EMS). Here, we conduct an independent evaluation of the MULTIOBS dataset in the oligotrophic waters of the EMS using in situ Chl-a profiles. Our analysis shows that this product accurately and precisely retrieves Chl-a across depths, with a slight 1% overestimation and an observed 1.5-fold average deviation between in situ data and MULTIOBS estimates. The deep chlorophyll maximum (DCM) is adequately estimated by MULTIOBS both in terms of positioning (root mean square error, RMSE = 13 m) and in terms of Chl-a (RMSE = 0.09 mg m−3). The product accurately reproduces the seasonal variability of Chl-a and it performs reasonably well in reflecting its interannual variability across various depths within the productive layer (0–120 m) of the EMS. We conclude that MULTIOBS is a valuable dataset providing vertically resolved Chl-a data, enabling a holistic understanding of euphotic zone-integrated Chl-a with an unprecedented spatiotemporal resolution spanning 25 years, which is essential for elucidating long-term trends and variability in oceanic primary productivity.

Metabolites reflect variability introduced by mesoscale eddies in the North Pacific Subtropical Gyre

Mesoscale eddies significantly alter open ocean environments such as those found in the subtropical gyres that cover a large fraction of the global ocean. Previous studies have explored eddy effects on biogeochemistry and microbial community composition but not on the molecular composition of particulate organic matter. This study reports the absolute concentration of 67 metabolites and relative abundances for 640 molecular features, measured using liquid chromatography-mass spectrometry (LC-MS) following both targeted and untargeted approaches. This approach allowed us to better understand how mesoscale eddies impact the metabolome of the North Pacific Subtropical Gyre during two cruises in 2017 and 2018. We find that many metabolites track biomass trends, but metabolites like isethionic acid, homarine, and trigonelline linked to eukaryotic phytoplankton were enriched at the deep chlorophyll maximum of the cyclonic features, while degradation products such as arsenobetaine were enriched in anticyclones. In every analysis, the metabolites with the strongest responses were identified using LC-MS through untargeted metabolomics approaches, highlighting that the molecules most sensitive to environmental perturbation were not among the previously characterized metabolome. By analyzing depth variability (accounting for 20-40% of metabolomic variability across ~150 meters) and the vertical displacement of isopycnal surfaces (explaining 10-20% of variability across a sea level anomaly range of 40 centimeters and a spatial distance of 300 kilometers), this analysis constrains the importance of mesoscale eddies in shaping the chemical composition of particulate matter in the largest biomes on the planet.

Amino Acids as Indicators of Organic Matter Sources and Degradation in Suspended Matter off the Pearl River: Indications for Resuspension in the Northern South China Sea

AbstractSuspended matter (SPM) was sampled in a grid off the Pearl River mouth in the northern South China Sea (SCS). SPM concentrations and the content of total organic carbon (TOC), total nitrogen (TN), amino acids (AA) and hexosamines (HA) and derived biogeochemical indicators were used to study organic matter sources on the shelf and slope. A surface SPM maximum curtailed the water mass of mixed riverine and marine origin off the Pearl River mouth with salinities <30. SPM in this river plume was rich in organic matter of fresh planktic origin. In areas outside the river plume chlorophyll (Chl‐a) maxima were found at the subsurface nutricline. The AA composition shows that the degradation state of organic matter is very similar in all samples except in bottom water samples. Rather than degradation indicators, an indicator of SPM residence time in the ocean shows differences between samples from the upper 200 m and the deeper SPM samples. On the shelf and the shelf break a distinct SPM maximum was found above the sea floor. AA and HA spectra revealed that its organic matter was more degraded than the other SPM samples and that part of the organic matter in the bottom water turbidity maximum originated from the fine fraction of sediments. The state of organic matter implies that degradation of this resuspended material possibly adds to bottom water hypoxia; furthermore, contaminants originally deposited in shelf sediments can be redistributed into distal areas of the South China Sea.

Application of functional metagenomics in the evaluation of microbial community dynamics in the Arabian Sea: Implications of environmental settings

Ocean microbial communities form the base of marine food webs, facilitating energy transfer and nutrient cycling, thereby supporting higher trophic levels. We investigated their composition and functional profiles across depths (surface waters 0, 29, and 63 m and bottom waters 100, 150, and 200 m) in the central-eastern Arabian Sea (CEAS) using next-generation sequencing. It was hypothesized that the composition and functional diversity of these communities would be influenced by depth and environmental parameters. Our research showed that microbial communities vary with depth and are shaped by environmental factors like irradiance, temperature, dissolved oxygen, suspended particulate matter, chlorophyll a, and ammonia concentrations. Cyanobacteria (Prochlorococcus sp) and Mamiellaceae, belonging to picoeukaryotes, exhibited distinct depth-specific distributions up to subsurface chlorophyll maxima (SCM) at 63 m. On the other hand, a community shift in the microbial communities comprising Firmicutes, Bacteroidetes, and Actinobacteria phyla was observed at the deeper water depths. The profiling of functional genes pointed out the expression of carbon fixation by photosynthetic organisms at the surface (0, 29, and 63 m), which shifted to prokaryotic carbon fixation in deeper waters (0, 150, and 200 m). Microcosm experiments (mixing of surface water with water from the SCM) carried out simulating disturbances such as climate change forced mixing (cyclones), revealed shifts in microbial structure and function. It was observed that within 48 h, the carbon fixation activity changed from photosynthetic organisms to prokaryotes and indicated an increase in stress-related biosynthetic pathways such as expression of quorum sensing, biosynthesis of antibiotics, lipopolysaccharides, and secondary metabolites. These findings have implications for predictive modelling of food web dynamics and fisheries management in the context of climate change.

Sodium chloride-induced metabolic shifts and morpho-physiological responses in Bacopa monnieri (L.) Wettst. for enhanced bacoside-A production

This study investigated the impact of NaCl concentrations (50–200 mM) on Bacopa monnieri (BM) grown under natural environmental conditions. The results revealed significant enhancements (p < 0.05) in various traits under NaCl exposure. Maximum increase of up to 1.4× (plant height), 2.6× (leaf number), 6.6× (shoot no.), and 2× (root no.) was recorded in morphological parameters at 100 mM. Similarly, maximum chlorophyll content (1.8×), catalase (2.2×), and guaiacol peroxidase (GPX) (2.4×) enzyme activities were also recorded at 100 mM NaCl. Furthermore, HPLC analysis showed a substantial increase (∼1.5×) in bacoside-A content after treatment with 100 mM. GC–MS analysis revealed stimulation of specific metabolites influencing bacoside production. This study offers a practical approach to enhance biomass and bacoside-A synthesis in BM plants, eliminating the need for costly in vitro methods and benefiting both farmers and pharmaceutical sector. Further investigation at molecular level for mapping changes due to different salt levels in metabolic pathways could also be performed.

Influence of Sowing Media and Variety on Physiological Performance and Yield of Early Cauliflower (Brassica oleracea L. var. botrytis) and Correlation Study with Yield

An experiment was conducted during 2018 and 2019 at the Experimental Farm, Department of Horticulture, College of Agriculture, Assam Agricultural University, Jorhat and at Farmer’s field to assess the physiological performance of early cauliflower varieties in different sowing media and to evaluate their performance in the field as well as to study the correlation of different physiological parameters with curd yield. The experiment was laid out in Randomized Block Design with eight treatments comprising of four sowing media [M1-cocopeat (60): vermiculite (20): perlite (20), M2-cocopeat (50): vermicompost (50),M3-cocopeat (50): vermicompost (50): microbial consortium @1:100 and M4-Conventional nursery]and two varieties [V1 (White Diamond) and V2 (CFL1522)] replicated thrice. The results revealed that growing medium had significant influence on different physiological parameters of early cauliflower seedling. Maximum chlorophyll content was recorded in M3 (0.98 mg g-1fw), maximum number of stomata in upper and lower surface (83.47and 150.97), relative water content (90.06 %) and dry matter accumulation (11.41 %) were recorded in M2. Between the two varieties, V1 recorded higher stomata number (145.06) in the lower surface. In the main field, the plants grown from M3 showcased superior performance with highest relative water content (93.62%), chlorophyll content at 30 days after transplanting (1.40 mg g-1fw) and at harvest (2.16 mg g-1fw), membrane stability index (65.32%), minimum lipid peroxidation (0.85 n mol g-1fw) at harvest and maximum curd yield (200.22 q/ha). Between the two varieties White Diamond(V1) recorded the higher relative water content (89.48 %) while V2 (CFL 1522) recorded the maximum membrane stability index (55.10%) and non-significant influence of variety was noticed on yield. Correlation coefficient analysis indicated chlorophyll content at seedling stage, crop growth and at harvest stage, relative leaf water content and membrane stability index at harvest have positive and significant correlation with curd yield of cauliflower. The study indicates sowing media had significant influence on physiological parameters of seedlings of early cauliflower which is again reflected in the field condition influencing yield as well and can be concluded that sowing media cocopeat (50): vermicompost (50): microbial consortium @1:100 can be used for nursery raising of early cauliflower seedling in Assam condition.

Key processes controlling the variability of the summer marine CO2 system in Fram Strait surface waters

The aim of this study was to decouple and quantify the influence of various biological and physical processes on the structure and variability of the marine carbonate system in the surface waters of the eastern part of the Fram Strait area. This productive region is characterized by its complex hydrographic and sea ice dynamics, providing an ideal set up to study their influence on the variability of the marine carbonate system. Different variables of the marine CO2 system: Total Alkalinity (TA), Dissolved Inorganic Carbon (DIC), partial pressure of CO2 (pCO2), and pH, were analysed together with temperature, salinity, sea ice extension, and chlorophyll a distribution during three consecutive summers (2019, 2020 and 2021), each of them having a unique oceanographic setting. The data revealed that TA and DIC are mostly controlled by the mixing of Atlantic water and sea ice meltwater. The combined effects of organic matter production/remineralization, calcium carbonate precipitation/dissolution, and air/sea CO2 gas exchange cause deviations from this salinity-related mixing. The scale of these deviations and the proportion between the effects observed for TA and DIC suggest interannual shifts in net primary production and dominant phytoplankton species in the area. These shifts are correlated with the sea ice extent and the spread of the Polar Surface Waters in the region. Net primary production is the main factor controlling the temporal and spatial variability of pH and pCO2 in the study area followed by the influence of temperature and, mixing of water masses expressed with salinity (seawater freshening). Surface waters of the Fram Strait area were generally undersaturated in CO2. The lowest pCO2 values, coinciding with an increase in oxygen saturation, were observed in areas of mixing of Arctic and Atlantic-derived water masses. However, as shown for 2021, a reduction of the sea ice extent may induce a westward shift of the chlorophyll maximum, resulting in pCO2 increase and pH decrease in the eastern part. This indicates that sea ice extent and associated spread of Polar Surface Waters may be important factors shaping primary production, and thus pCO2 and pH, in the Fram Strait area.

Parasitic taxa are key to the vertical stratification and community variation of pelagic ciliates from the surface to the abyssopelagic zone.

An increase in upper-ocean thermal stratification is being observed worldwide due to global warming. However, how ocean stratification affects the vertical profile of plankton communities remains unclear. Understanding this is crucial for assessing the broader implications of ocean stratification. Pelagic ciliates cover multiple functional groups, and thus can serve as a model for studying the vertical distribution and functional strategies of plankton in stratified oceans. We hypothesize that pelagic ciliate communities exhibit vertical stratification caused by shifts in functional strategies, from free-living groups in the photic zone to parasitic groups in deeper waters. 306 samples from the surface to the abyssopelagic zone were collected from 31 stations in the western Pacific and analyzed with environmental DNA (the V4 region of 18S rDNA) metabarcoding of pelagic ciliates. We found a distinct vertical stratification of the entire ciliate communities, with a boundary at a depth of 200m. Significant distance-decay patterns were found in the photic layers of 5m to the deep chlorophyll maximum and in the 2,000m, 3000m and bottom layers, while no significant pattern occurred in the mesopelagic layers of 200m - 1,000m. Below 200m, parasitic Oligohymenophorea and Colpodea became more prevalent. A linear model showed that parasitic taxa were the main groups causing community variation along the water column. With increasing depth below 200m, the ASV and sequence proportions of parasitic taxa increased. Statistical analyses indicated that water temperature shaped the photic communities, while parasitic taxa had a significant influence on the aphotic communities below 200m. This study provides new insights into oceanic vertical distribution, connectivity and stratification from a biological perspective. The observed shift of functional strategies from free-living to parasitic groups at a 200m transition layer improves our understanding of ocean ecosystems in the context of global warming.

Effects of Topography on Nutrient Variations in the Western South China Sea

AbstractA topography change in the continental plain plays an important role in nutrient replenishment mechanisms in oligotrophic oceans. Effects of the topography on the nutrient distribution in the western South China Sea (WSCS) have been overlooked since most studies have focused on the dipole‐induced upwelling and downwelling processes of nutrients. We hypothesize that the seamount topography in the northwestern side of the WSCS contributes to the upward distribution of nutrients. We conducted a cruise to investigate the vertical distribution of nutrients in a large area where there is a gradient in the topography: shallow in the north to deep in the south. Our results showed that the depth contours of nutrients, temperature, and salinity shoaled upward from deep to shallow with their isolines being parallel to the bottom depth. The depth of mixed layer, pycnocline, nutricline, and deep chlorophyll maximum showed the similar topographic effect. In the deep water column of 4,308 m deep, integrated NO3− and PO43– over 0–200 m were 879.60 and 81.78 mmol m−2, but increased to 2010.17 and 143.79 mmol m−2 in the shallow water column of 930 m deep, respectively. The increased supply of nutrients enhanced 0–200 m integrated chlorophyll from 21.71 mg m−2 in the deep water column to 51.51 mg m−2 in the shallow water column. These results demonstrate that topographic elevations such as seamounts induce deep‐to‐shallow shoaling and upwelling that lead to enhanced nutrients and biological production in the euphotic zone of oligotrophic oceans.

Temporal insights into deep chlorophyll maxima dynamics in the Indian sector of the Southern Ocean: a Bio-Argo float study

Temporal insights into deep chlorophyll maxima dynamics in the Indian sector of the Southern Ocean: a Bio-Argo float study

Periodically asymmetric responses of deep chlorophyll maximum to light and thermocline in a clear monomictic lake: Insights from monthly and diel scale observations

Deep chlorophyll maximum (DCM), a chlorophyll peak in the water column, has important implications for biogeochemical cycles, energy flow and water surface algal blooms in deep lakes. However, how an observed periodically asymmetric DCM response to environmental variables remains unclear, limiting our in-depth understanding and effective eco-environmental management of deep lakes. Based on both monthly field investigations in 2021 and diel continuous observations in 2021–2023 in clear, monomictic Lake Fuxian, Southwest China, the temporal dynamics and drivers of DCM were examined and periodic features of DCM were found, with a formation period (FP, February–July) and a weakening period (WP, August–December). On the monthly scale, although DCM dynamics were partly attributed to thermocline structures, the role of light penetration depths varied with period. In the FP, the influence of light on DCM was direct, i.e., increased depth and thickness but decreased magnitude. Differently, the influence of light mainly occurred by affecting thermocline structures in the WP, where water quality was another important driver. On the diel scale, light was a major reason for a thicker and lower (magnitude) DCM during day than at night, and the response of DCM to environmental factors between the FP and WP differed also more during day. This periodically asymmetric response of daytime DCM not only being caused by light but possibly also related to other physical factors such as lake surface water temperature, wind speed and precipitation. Bayesian network modelling suggested that water darkening and stratification intensification may promote a shallower, thinner and larger (magnitude) DCM in both FP and WP, but achieving such changes in DCM requires different light and thermocline thresholds. Our findings provide new information valuable for modelling DCM and for predicting the related surface algal blooms in deep lakes under climate change and eutrophication.

Novel isolates expand the physiological diversity of Prochlorococcus and illuminate its macroevolution.

Prochlorococcus is a diverse picocyanobacterial genus and the most abundant phototroph on Earth. Its photosynthetic diversity divides it into high-light (HL)- or low-light (LL)-adapted groups representing broad phylogenetic grades-each composed of several monophyletic clades. Here, we physiologically characterize four new Prochlorococcus strains isolated from below the deep chlorophyll maximum in the North Pacific Ocean. We combine these physiological properties with genomic analyses to explore the evolution of photosynthetic antennae and discuss potential macroevolutionary implications. The isolates belong to deeply branching low-light-adapted clades that have no other cultivated representatives and display some unusual characteristics. For example, despite its otherwise low-light-adapted physiological characteristics, strain MIT1223 has low chl b2 content similar to high-light-adapted strains. Isolate genomes revealed that each strain contains a unique arsenal of pigment biosynthesis and binding alleles that have been horizontally acquired, contributing to the observed physiological diversity. Comparative genomic analysis of all picocyanobacteria reveals that Pcb, the major pigment carrying protein in Prochlorococcus, greatly increased in copy number and diversity per genome along a branch that coincides with the loss of facultative particle attachment. Collectively, these observations support a recently developed macroevolutionary model, in which niche-constructing radiations allowed ancestral lineages of picocyanobacteria to transition from a particle-attached to planktonic lifestyle and broadly colonize the euphotic zone.IMPORTANCEThe marine cyanobacterium, Prochlorococcus, is among the Earth's most abundant organisms, and much of its genetic and physiological diversity remains uncharacterized. Although field studies help reveal the scope of diversity, cultured isolates allow us to link genomic potential to physiological processes, illuminate eco-evolutionary feedbacks, and test theories arising from comparative genomics of wild cells. Here, we report the isolation and characterization of novel low-light (LL)-adapted Prochlorococcus strains that fill in multiple evolutionary gaps. These new strains are the first cultivated representatives of the LLVII and LLVIII paraphyletic grades of Prochlorococcus, which are broadly distributed in the lower regions of the ocean euphotic zone. Each of these grades is a unique, highly diverse section of the Prochlorococcus tree that separates distinct ecological groups: the LLVII grade branches between monophyletic clades that have facultatively particle-associated and constitutively planktonic lifestyles, whereas the LLVIII grade lies along the branch that leads to all high-light (HL)-adapted clades. Characterizing strains and genomes from these grades yields insights into the large-scale evolution of Prochlorococcus. The new LLVII and LLVIII strains are adapted to growth at very low irradiance levels and possess unique light-harvesting gene signatures and pigmentation. The LLVII strains represent the most basal Prochlorococcus group with a major expansion in photosynthetic antenna genes. Furthermore, a strain from the LLVIII grade challenges the paradigm that all LL-adapted Prochlorococcus exhibit high ratios of chl b:a2. These findings provide insights into the photophysiological evolution of Prochlorococcus and redefine what it means to be a low- vs high-light-adapted Prochlorococcus cell.

Oceanographic features boost latitudinal patterns in copepod body size distribution in the South Atlantic

AbstractThe South Atlantic Ocean (SAO) is an under‐sampled ocean where the influence of environmental drivers on copepod body size is poorly understood. This study investigated the body size distribution of copepods from 13°S to 64°S to test Bergmann's rule, which predicts the occurrence of smaller organisms in warmer areas. We hypothesized that additional influence of oceanographic features strengthens this pattern. Zooplankton were sampled during the austral summer from the chlorophyll maximum depth up to the surface, at approximately 100 m depth, using a 200‐μm net. The samples were analyzed using the ZooScan imaging system and were classified using the Ecotaxa tool. We estimated copepod family abundance, biomass, and size structure, and their relationships with environmental variables were assessed through generalized linear mixed models (GLMM). Although most copepods increased in size at higher latitudes, not all families followed Bergmann's rule. Small adults of Clausocalanidae, Paracalanidae, Corycaeidae, and Oncaeidae contributed mostly in the Brazil Current (BC), while Calanidae copepodites disproportionately contributed to overall biomass, and Oithonidae adults were the most abundant in high latitudes. Copepod abundance or biomass hotspots were present in the southern limit of the warm BC, at the Subtropical Confluence Zone, and in the subantarctic waters at the Drake passage. Our results suggest that oceanographic features strengthen latitudinal body size relationships due to food availability, and the importance of different life history strategies.

Diverse Responses of Upper Ocean Temperatures to Chlorophyll‐Induced Solar Absorption Across Different Coastal Upwelling Regions

AbstractChlorophyll in phytoplankton absorbs solar radiation (SR) and affects the thermal structure and dynamics within upwelling regions. However, research on this process across global‐scale coastal upwelling systems is still lacking. Here, we use a coupled ocean‐biogeochemical model to investigate differing responses to chlorophyll‐induced solar absorption between Pacific and Atlantic coastal upwelling regions. Chlorophyll‐induced solar absorption leads to colder Pacific coastal upwelling but warmer Atlantic coastal upwelling. In the Pacific, the shading effect of the surface chlorophyll maximum leads to colder subsurface water, which is then upwelled, contributing to cooling. The more stratified upper ocean leads to shallower mixed layer depth, intensifying offshore transport and upwelling. In the Atlantic, the absorption of SR by the subsurface chlorophyll maximum causes warmer and weaker upwelling. The processes described, in turn, trigger positive feedback to ocean biogeochemistry and potentially interact with climate dynamics, underscoring the necessity to incorporate them into Earth system models.