Highly ordered vertical structure of Synechococcus populations within the one-millimeter-thick photic zone of a hot spring cyanobacterial mat

A variety of contemporary techniques were used to investigate the vertical distribution of thermophilic unicellular cyanobacteria, Synechococcus spp., and their activity within the upper 1-mm-thick photic zone of the mat community found in an alkaline siliceous hot spring in Yellowstone National Park in Wyoming. Detailed measurements were made over a diel cycle at a 61 degrees C site. Net oxygenic photosynthesis measured with oxygen microelectrodes was highest within the uppermost 100- to 200-microm-thick layer until midmorning, but as the day progressed, the peak of net activity shifted to deeper layers, stabilizing at a depth of 300 microm from midday throughout the afternoon. Examination of vertical thin sections by bright-field and autofluorescence microscopy revealed the existence of different populations of Synechococcus which form discrete bands at different vertical positions. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene segments from horizontal cryosections obtained at 100-microm-thick vertical intervals also suggested vertical stratification of cyanobacterial, green sulfur bacterium-like, and green nonsulfur bacterium-like populations. There was no evidence of diel migration. However, image analysis of vertical thin sections revealed the presence of a narrow band of rod-shaped Synechococcus cells in which the cells assumed an upright position. These upright cells, located 400 to 800 microm below the surface, were observed only in mat samples obtained around noon. In mat samples obtained at other time points, the cells were randomly oriented throughout the mat. These combined observations reveal the existence of a highly ordered structure within the very thin photic zone of this hot spring microbial mat, consisting of morphologically similar Synechococcus populations that are likely to be differentially adapted, some co-occurring with green sulfur bacterium-like populations, and all overlying green nonsulfur bacterium-like populations.     

Diel interactions of oxygenic photosynthesis and n(2) fixation (acetylene reduction) in a marine microbial mat community

Diel variations in N(2) fixation (acetylene reduction), CO(2) fixation, and oxygen concentrations were measured, on three separate occasions, in a marine microbial mat located on Shackleford Banks, North Carolina. Nitrogenase activity (NA) was found to be inversely correlated with CO(2) fixation and, in two of the three diel periods studied, was higher at night than during the day. Oxygen concentrations within the top 3 mm of the mat ranged from 0 to 400 muM on a diel cycle; anaerobic conditions generally persisted below 4 mm. NA in the mat was profoundly affected by naturally occurring oxygen concentrations. Experimentally elevated oxygen concentrations resulted in a significant depression of NA, whereas the addition of the Photosystem II inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea decreased oxygen concentrations within the mat and resulted in a significant short-term enhancement of NA. Mat N(2)-fixing microorganisms include cyanobacteria and heterotrophic, photoautotrophic, and chemolithotrophic eubacteria. Measured (whole-mat) NA is probably due to a combination of the NA of each of these groups of organisms. The relative contributions of each group to whole-mat NA probably varied during diel and seasonal (successional) cycles. Reduced compounds derived from photosynthetic CO(2) fixation appeared to be an important source of energy for NA during the day, whereas heterotrophic or chemolithotrophic utilization of reduced compounds appeared to be an important source of energy for NA at night, under reduced ambient oxygen concentrations. Previous estimates of N(2) fixation calculated on the basis of daytime measurements may have seriously underestimated diel and seasonal nitrogen inputs in mat systems.     

Spatial and temporal variability in a stratified hypersaline microbial mat community

Hypersaline microbial mat communities have recently been shown to be more diverse than once thought. The variability in community composition of hypersaline mats, both in terms of spatial and temporal dimensions, is still poorly understood. Because this information is essential to understanding the complex biotic and abiotic interactions within these communities, terminal restriction fragment analysis and 16S rRNA gene sequencing were used to characterize the near-surface community of a hypersaline microbial mat in Guerrero Negro, Mexico. Core samples were analyzed to assay community variability over large regional scales (centimeter to kilometer) and to track depth-related changes in population distribution at 250-μm intervals over a diel period. Significant changes in total species diversity were observed at increasing distances across the mat surface; however, key species (e.g. Microcoleus sp.) were identified throughout the mat. The vertical position and abundance of >50% of the 60 peaks detected varied dramatically over a diel cycle, including Beggiatoa sp., cyanobacteria, Chloroflexus sp., Halochromatium sp., Bacteroidetes sp. and several as-yet-identified bacteria. Many of these migrations correlated strongly with diel changes in redox conditions within the mat, contributing to strong day–night community structure differences.     

Highly Ordered Vertical Structure of Synechococcus Populations within the One-Millimeter-Thick Photic Zone of a Hot Spring Cyanobacterial Mat

A variety of contemporary techniques were used to investigate the vertical distribution of thermophilic unicellular cyanobacteria, Synechococcus spp., and their activity within the upper 1-mm-thick photic zone of the mat community found in an alkaline siliceous hot spring in Yellowstone National Park in Wyoming. Detailed measurements were made over a diel cycle at a 61°C site. Net oxygenic photosynthesis measured with oxygen microelectrodes was highest within the uppermost 100- to 200-μm-thick layer until midmorning, but as the day progressed, the peak of net activity shifted to deeper layers, stabilizing at a depth of 300 μm from midday throughout the afternoon. Examination of vertical thin sections by bright-field and autofluorescence microscopy revealed the existence of different populations of Synechococcus which form discrete bands at different vertical positions. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene segments from horizontal cryosections obtained at 100-μm-thick vertical intervals also suggested vertical stratification of cyanobacterial, green sulfur bacterium-like, and green nonsulfur bacterium-like populations. There was no evidence of diel migration. However, image analysis of vertical thin sections revealed the presence of a narrow band of rod-shaped Synechococcus cells in which the cells assumed an upright position. These upright cells, located 400 to 800 μm below the surface, were observed only in mat samples obtained around noon. In mat samples obtained at other time points, the cells were randomly oriented throughout the mat. These combined observations reveal the existence of a highly ordered structure within the very thin photic zone of this hot spring microbial mat, consisting of morphologically similar Synechococcus populations that are likely to be differentially adapted, some co-occurring with green sulfur bacterium-like populations, and all overlying green nonsulfur bacterium-like populations.     

Diel Interactions of Oxygenic Photosynthesis and N2 Fixation (Acetylene Reduction) in a Marine Microbial Mat Community

Diel variations in N₂ fixation (acetylene reduction), CO₂ fixation, and oxygen concentrations were measured, on three separate occasions, in a marine microbial mat located on Shackleford Banks, North Carolina. Nitrogenase activity (NA) was found to be inversely correlated with CO₂ fixation and, in two of the three diel periods studied, was higher at night than during the day. Oxygen concentrations within the top 3 mm of the mat ranged from 0 to 400 μM on a diel cycle; anaerobic conditions generally persisted below 4 mm. NA in the mat was profoundly affected by naturally occurring oxygen concentrations. Experimentally elevated oxygen concentrations resulted in a significant depression of NA, whereas the addition of the Photosystem II inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea decreased oxygen concentrations within the mat and resulted in a significant short-term enhancement of NA. Mat N₂-fixing microorganisms include cyanobacteria and heterotrophic, photoautotrophic, and chemolithotrophic eubacteria. Measured (whole-mat) NA is probably due to a combination of the NA of each of these groups of organisms. The relative contributions of each group to whole-mat NA probably varied during diel and seasonal (successional) cycles. Reduced compounds derived from photosynthetic CO₂ fixation appeared to be an important source of energy for NA during the day, whereas heterotrophic or chemolithotrophic utilization of reduced compounds appeared to be an important source of energy for NA at night, under reduced ambient oxygen concentrations. Previous estimates of N₂ fixation calculated on the basis of daytime measurements may have seriously underestimated diel and seasonal nitrogen inputs in mat systems.     

Diel variation of phytoplankton functional groups in a subtropical reservoir in southern Brazil during an autumnal stratification period

A knowledge of diel variation and the vertical distribution of phytoplankton communities may contribute to a better understanding of the driving factors of key species. Applying functional-group classification provides important information on the causes of species selection in the pelagic community. The diel variation of phytoplankton functional groups was analysed during an autumnal stratification period with the aim of understanding their changes in the vertical position related to light, mixing regime and grazing pressure. Phytoplankton and zooplankton communities were sampled every 4 h during a 24-h period in a vertical profile in a subtropical meso-eutrophic reservoir. Strong stratification during a 24-h cycle and a mixed clear epilimnion with partial atelomixis marked the autumn season in the Faxinal reservoir, southern Brazil. The highest phytoplankton densities and biomass were found during the second part of the day, a general pattern reported in the literature, and may be explained by zooplankton dynamics. During the 24-h cycle, phytoplankton functional groups lacking a self-regulating capacity and those able to regulate their vertical position were vertically segregated in the lake. The diel behaviour of both groups was driven by the mixing regime (including atelomixis), light and zooplankton grazing pressure.     

Diel and seasonal vertical distribution of meiobenthic copepods in muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay)

The vertical distribution of meiobenthic copepods was investigated within muddy sediments of a eutrophic lagoon (fish ponds of Arcachon Bay, France). The aim of the study was to determine if in muddy sediments, as previously established in sandy sediments, meiobenthic copepods migrate vertically according to the seasons or diel periods. Two experimental approaches were used, viz: a three-season comparison was made of the diel vertical distribution of the harpacticoid Canuella perplexa T. & A. Scott (1893) and secondly the depth distribution of a meiobenthic copepod assemblage was followed for a 24 h period, in shallow water subtidal locations. The harpacticoid C. perplexa vertically migrated through the top three centimeters of the sediment, showing diel and seasonal variations in depth distribution. The differential vertical distributions shown by the dominant meiobenthic populations suggest that emergence into the water column may mainly concern surface dwelling copepods. The physical and biological factors affecting seasonal and diel changes in the copepod assemblage of the fish ponds are discussed.     

Nitrogenase Activity and nifH Expression in a Marine Intertidal Microbial Mat

N₂ fixation, diazotrophic community composition, and organisms actively expressing genes for N₂ fixation were examined over at 3−year period (1997–1999) for intertidal microbial mats on a sand flat located in the Rachel Carson National Estuarine Research Reserve (RCNERR) (Beaufort, NC, USA). Specifically, diel variations of N₂ fixation in the mats from the RCNERR were examined. Three distinct diel patterns of nitrogenase activity (NA) were observed. NA responses to short-term inhibitions of photosynthesis corresponded to one of the three patterns. High rates of NA were observed during peak O₂ production periods for diel experiments during summer months. Different types of NA diel variations correspond to different stages of mat development. Chloramphenicol treatments indicated that the mechanism of protein synthesis supporting NA changed throughout the day. Analysis of mat DNA and RNA gave further evidence suggesting that in addition to cyanobacteria, other functional groups were responsible for the NA observed in the RCNERR mats. The role of microbial diversity in the N₂ fixation dynamics of these mats is discussed.     

Growth of elaborate microbial pinnacles in Lake Vanda,Antarctica

Microbial pinnacles in ice‐covered Lake Vanda, McMurdo Dry Valleys, Antarctica, extend from the base of the ice to more than 50 m water depth. The distribution of microbial communities, their photosynthetic potential, and pinnacle morphology affects the local accumulation of biomass, which in turn shapes pinnacle morphology. This feedback, plus environmental stability, promotes the growth of elaborate microbial structures. In Lake Vanda, all mats sampled from greater than 10 m water depth contained pinnacles with a gradation in size from <1‐mm‐tall tufts to pinnacles that were centimeters tall. Small pinnacles were cuspate, whereas larger ones had variable morphology. The largest pinnacles were up to ~30 cm tall and had cylindrical bases and cuspate tops. Pinnacle biomass was dominated by cyanobacteria from the morphological and genomic groups Leptolyngbya, Phormidium, and Tychonema. The photosynthetic potential of these cyanobacterial communities was high to depths of several millimeters into the mat based on PAM fluorometry, and sufficient light for photosynthesis penetrated ~5 mm into pinnacles. The distribution of photosynthetic potential and its correlation to pinnacle morphology suggests a working model for pinnacle growth. First, small tufts initiate from random irregularities in prostrate mat. Some tufts grow into pinnacles over the course of ~3 years. As pinnacles increase in size and age, their interiors become colonized by a more diverse community of cyanobacteria with high photosynthetic potential. Biomass accumulation within this subsurface community causes pinnacles to swell, expanding laminae thickness and creating distinctive cylindrical bases and cuspate tops. This change in shape suggests that pinnacle morphology emerges from a specific distribution of biomass accumulation that depends on multiple microbial communities fixing carbon in different parts of pinnacles. Similarly, complex patterns of biomass accumulation may be reflected in the morphology of elaborate ancient stromatolites.     

The daily integral of photosynthesis by Planktothrix rubescens during summer stratification and autumnal mixing in Lake Zürich

It has been suggested that the populations of planktonic cyanobacteria that occupy the metalimnion of stratified lakes during the summer months may be aestivating between the main periods of growth during entrainment in the epilimnion in spring and summer. We determined the vertical distribution of the biomass and daily integral of photosynthesis of the population of Planktothrix ( Oscillatoria ) rubescens in Lake Zürich for 136 d from July to November 1995. The population showed an 80-fold increase during the stratified period but it only doubled over the subsequent period of entrainment. During the first eight days, part of the increase was attributed to recruitment of filaments floating up from greater depths but all of the subsequent production could be accounted for by photoautotrophic growth. On sunny days the biomass-specific photosynthesis of this population reached some of the highest values over the whole period despite its depth (>13 m). On very cloudy days, however, primary productivity was very low and on 4 days, when the mean depth of the population exceeded 15 m, there was no net production. Over the whole period of the study, the accumulated photosynthetic production exceeded the increase in biomass of the population by a factor of 9·5. Although much of this production occurred during the period of entrainment only a small proportion was translated into growth of the population. It is concluded that the growth that takes place in the period of stratification in the metalimnion is essential to subsequent production.     

Diel variation in feeding and vertical distribution of ten co-occurring fish species: consequences for resource partitioning

The diel variations in feeding behaviour and vertical distribution were determined for ten species of a tropical fish community in a shallow SE Sri Lankan reservoir. The fish community consisted of two introduced exotic tilapias and eight indigenous riverine species including five cyprinids, one clarid, one hemiramphid and one gobid. Multi-mesh gillnets dividing the water column in six depth strata were set with a six-hour interval throughout four 24-hour periods. The study of the diel variation in feeding behaviour was based on analysis of catch data, gut contents and gut fullness per setting time. The diel variation in vertical distribution was determined per species for each of the six depth-strata. Feeding behaviour and vertical distribution differed significantly among species and for most species throughout the 24-hour period. Feeding behaviour and vertical distribution were correlated. The status of whether or not a fish is feeding determines if the trophic and spatial resource dimensions are interrelated. Resource partitioning along the spatial dimension may reduce exploitative competition at the time of feeding, whereas it may limit interference competition or predation during non-feeding periods. This revised version was published online in July 2006 with corrections to the Cover Date.     

Diel Migrations of Microorganisms within a Benthic, Hypersaline Mat Community

We studied the diel migrations of several species of microorganisms in a hypersaline, layered microbial mat. The migrations were quantified by repeated coring of the mat with glass capillary tubes. The resulting minicores were microscopically analyzed by using bright-field and epifluorescence (visible and infrared) microscopy to determine depths of coherent layers and were later dissected to determine direct microscopic counts of microorganisms. Microelectrode measurements of oxygen concentration, fiber optic microprobe measurements of light penetration within the mat, and incident irradiance measurements accompanied the minicore sampling. In addition, pigment content, photosynthesis and irradiance responses, the capacity for anoxygenic photosynthesis, and gliding speeds were determined for the migrating cyanobacteria. Heavily pigmented Oscillatoria sp. and Spirulina cf. subsalsa migrated downward into the mat during the early morning and remained deep until dusk, when upward migration occurred. The mean depth of the migration (not more than 0.4 to 0.5 mm) was directly correlated with the incident irradiance over the mat surface. We estimated that light intensity at the upper boundary of the migrating cyanobacteria was attenuated to such an extent that photoinhibition was effectively avoided but that intensities which saturated photosynthesis were maintained through most of the daylight hours. Light was a cue of paramount importance in triggering and modulating the migration of the cyanobacteria, even though the migrating phenomenon could not be explained solely in terms of a light response. We failed to detect diel migration patterns for other cyanobacterial species and filamentous anoxyphotobacteria. The sulfide-oxidizing bacterium Beggiatoa sp. migrated as a band that followed low oxygen concentrations within the mat during daylight hours. During the nighttime, part of this population migrated toward the mat surface, but a significant proportion remained deep.     

Advective effects of a reservoir flood on zooplankton abundance and dispersion

SUMMARY. 1. The diel vertical distribution and abundance of zooplankton in the Red River arm of Lake Texoma, Oklahoma-Texas, U.S.A., were examined between 25 January and 26 June 1982.
2. During periods of low river inflow, zooplankton vertical patchiness was related to wind stress. Zooplankton did not maintain distinct vertical patches under strong mixing.
3. In mid May heavy rainfall in the drainage basin increased discharge from the Red River and reduced water retention time in the reservoir. The Red River water mass entered the lake as a turbid, surface overflow plume. During this period of flooding most zooplankton populations declined and the distributions of remaining individuals deepened. Greatest population losses occurred between 0 and 7m, depths where most zooplankton had been concentrated prior to the flood.
4. A model of population dynamics based on depth specific rates of water exchange between the lake and the Red River, estimated from conductivity changes through time, accounted for the general pattern of change in zooplankton abundance and vertical dispersion.     

Diel vertical and horizontal distribution of crustacean zooplankton and young of the year fish in a sub-alpine lake: an approach based on high frequency sampling

Understanding the spatial dynamics of predators and their preyis one of the most important goals in aquatic ecology. We studiedspatial and temporal onshore–offshore distribution patternsin young of the year (YOY) Eurasian perch (Perca fluviatilis)and crustacean zooplankton (Daphnia hyalina, Cyclops prealpinus)along a transect in Lake Annecy (France). Our study representsa first attempt at coupling hydroacoustic fish survey and highfrequency zooplankton recording to assess simultaneously thelarge-scale distribution patterns of YOY fish and their zooplanktonprey over a diel cycle (day, dusk and night sampling). We hypothesizedthat the spatial distribution of zooplankton could be shapedby both anti-predator behaviour (horizontal and vertical migrations)and predation losses. Fish biomass, size structure and dietwere assessed from split-beam echosounding and net trawlingsamples, whereas crustacean abundances were estimated with asmall modified Longhurst–Hardy continuous plankton recorder.We evaluated the diel changes in the spatial distribution patternsof fish and zooplankton and determined the overlap between theirdistributions. Fish biomass was dominated by YOY perch in upperwarmer layers and salmonids (Coregonus lavaretus and Salvelinusalpinus) in the colder and oxygenated deep layers. YOY perchwere aggregated in dense schools in the epilimnion during theday and dispersed at night. Fish biomass was distributed alonga strong increasing onshore–offshore gradient at night,whereas crustacean prey showed a decreasing gradient. This onshore–offshorenegative gradient in crustacean distribution, expressed on ashorter scale during the day, shifted toward the surface watersat night. A distinct kinetic of diel vertical migration (DVM)patterns was exhibited by daphnid and cyclopoid populationsand resulted in distinct vulnerability to perch predation. Spatio-temporaldistribution of crustaceans in Lake Annecy during the diel cyclestudy was probably shaped both by predation loss to YOY perchand by anti-predator behaviour (DVM, DHM) by zooplankton. Theimplications for fine-scale studies of fish-zooplankton interactionsare discussed.     

Vertical migration of autotrophic micro-organisms during a vernal bloom at the coastal Baltic Sea – coexistence through niche separation

Vertical migration of two dinoflagellate species (Peridiniellacatenata and Scrippsiella hangoei) and a phototrophic ciliate(Mesodinium rubrum) were studied during the peak and decline of avernal bloom at the SW coast of Finland. During the diel cycle, part of thepopulations of P. catenata and M. rubrum wereobserved in the deeper layers with elevated nutrient concentrations, whileS. hangoei remained in the upper nutrient depleted mixed layer.Using a correspondence analysis the vertical distribution patterns of thespecies and chlorophyll a were examined over a temporal scale of hoursand weeks. The vertical migration was reflected in much higher variabilityin the depth distribution of P. catenata and M. rubrum over a diel scale, compared to S. hangoei. The analysisrevealed also significant differences in species specific depth distributionpatterns over both time scales. It is discussed that the co-existence of thetwo dominant dinoflagellate species during the vernal bloom is due to nicheseparation through behavioural adaptations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental investigations of diel vertical movements by lotic mayflies over substrate surfaces

SUMMARY. 1. The vertical distribution within the substrate of three lotic mayflies (Rhithrogena sp., Paraleptophlebia sp. and Baetis tricaudalus Dodds) was examined in laboratory streams to determine if diel patterns of vertical movement across substrate surfaces existed and whether light intensity, temperature, substrate composition or food availability influenced the observed movement patterns.
2. Rhithrogena sp. and B. tricaudatus exhibited significant diel movement from lower substrate surfaces during the day to upper surfaces at night, although >64% of B. tricaudalus occupied the upper surfaces throughout the 24 h period. In contrast, Parakplophlebia sp. larvae did not change their vertical distribution over the 24 h period.
3. The proportion of mayflies occupying substrate surfaces was influenced by the amount of upper and lower surface area available in the substrate and, possibly, crevice sizes associated with lower surfaces.
4. The movement of Rhiihrogena sp. from lower surfaces during the day to the upper surfaces during the night was influenced by changes in light intensity, but not by changes in food availability or water temperature.
5. The diel patterns of vertical movement exhibited by lotic mayflies in this investigation raise the possibility that at least some biotic interactions may vary over a diel cycle. These could include competitive interactions or temporal cycling of prey availability to predators.     

Short-term dynamics of a Melosira population in the plankton of an impoundment in central Chile

Relationships among the chlorophyll-specific photosyntheticbehaviour of a Melosira-dominated phytoplankton, net variationsin the quantity of Melosira in suspension, the rates of sinkingloss of Melosira cells and did fluctuations in water-columnstability were investigated in Embalse Rapel. central Chile.The short study was carried out during the autumnal equinoxialperiod (March. 1984) of maximal Melosira abundance to determinethe extent to which these various components might contributeto the distinctly bimodal seasonal distribution of phytoplanktonabundance in this reservoir. Diurnal photosynthetic productionwithin the illuminated layer and supposed column respirationwere found to be of a similar order, suggesting that net increasethrough cellular growth was already substantially limited byself-shading. Thus observed net changes in the standing populationmay have depended more on the relative magnitude of sinkingloss rates and recruitment through resuspension, which processesare profoundly influenced by alternations in wind-induced mixingand intense, near-surface, thermal stratification. As the relativefrequency and duration of these episodes vary during the year,so does the capacity of the Melosira to maintain or increaseits standing biomass. High cellular photosynthetic efficiencycoupled with rapid sinking rate appear to be of selective advantagewhen the cycle of alternation is approximately diel. The applicationof these findings to the seasonal distribution of M. granulatain the Rapel reservoir and to the ecology of planktonic Melosirapopulations elsewhere is briefly discussed.     

Impact of carbon metabolism on 13C signatures of cyanobacteria and green non-sulfur-like bacteria inhabiting a microbial mat from an alkaline siliceous hot spring in Yellowstone National Park (USA)

Alkaline siliceous hot spring microbial mats in Yellowstone National Park are composed of two dominant phototropic groups, cyanobacteria and green non-sulfur-like bacteria (GNSLB). While cyanobacteria are thought to cross-feed low-molecular-weight organic compounds to support photoheterotrophic metabolism in GNSLB, it is unclear how this could lead to the heavier stable carbon isotopic signatures in GNSLB lipids compared with cyanobacterial lipids found in previous studies. The two groups of phototrophs were separated using percoll density gradient centrifugation and subsequent lipid and stable carbon isotopic analysis revealed that we obtained fractions with a approximately 60-fold enrichment in cyanobacterial and an approximately twofold enrichment in GNSLB biomass, respectively, compared with the mat itself. This technique was used to study the diel cycling and 13C content of the glucose pools in and the uptake of 13C-bicarbonate by the cyanobacteria and GNSLB, as well as the transfer of incorporated 13C from cyanobacteria to GNSLB. The results show that cyanobacteria have the highest bicarbonate uptake rates and accumulate glucose during the afternoon in full light conditions. In contrast, GNSLB have relatively higher bicarbonate uptake rates compared with cyanobacteria in the morning at low light levels. During the night GNSLB take up carbon that is likely derived through fermentation of cyanobacterial glucose enriched in 13C. The assimilation of 13C-enriched cyanobacterial carbon may thus lead to enriched 13C-contents of GNSLB cell components.