PHOTOSYNTHESIS IN AN EXTREME SHADE ENVIRONMENT: BENTHIC MICROBIAL MATS FROM LAKE HOARE, A PERMANENTLY ICE-COVERED ANTARCTIC LAKE

We investigated the composition of benthic microbial mats in permanently ice-covered Lake Hoare, Antarctica, and their irradiance vs. photosynthetic oxygen exchange relationships. Mats could be subdivided into three distinct depth zones: a seasonally ice-free “moat” zone and two under-ice zones. The upper under-ice zone extended from below the 3.5 m thick ice to approximately 13 m and the lower from below 13 m to 22 m. Moat mats were acclimated to the high irradiance they experienced during summer. They contained photoprotective pigments, predominantly those characteristic of cyanobacteria, and had high compensation and saturating irradiances (E_c and E_k) of 75 and 130 μmol photons·m⁻²·s⁻¹, respectively. The moat mats used light inefficiently. The upper under-ice community contained both cyanobacteria and diatoms. Within this zone, biomass (as pigments) increased with increasing depth, reaching a maximum at 10 m. Phycoerythrin was abundant in this zone, with shade acclimation and efficiency of utilization of incident light increasing with depth to a maximum of 0.06 mol C fixed·mol⁻¹ incident photons under light-limiting conditions. Precipitation of inorganic carbon as calcite was associated with this community, representing up to 50% of the carbon sequestered into the sediment. The lower under-ice zone was characterized by a decline in pigment concentrations with depth and an increasing prevalence of diatoms. Photosynthesis in this community was highly shade acclimated and efficient, with E_c and E_k below 0.5 μmol·m⁻²·s⁻¹ and 2 μmol·m⁻²·s⁻¹, respectively, and maximum yields of 0.04 mol C fixed·mol⁻¹ incident quanta. Carbon uptake in situ by both under-ice and moat mats was estimated at up to 100 and 140 mg·m⁻²·day⁻¹, based on the photosynthesis–irradiance curves, incident irradiance, and light attenuation by ice and the water column.     

Bacteriohopanepolyols across environmental gradients in Lake Vanda,Antarctica

Bacteriohopanepolyols (BHPs) are bacterial membrane lipids that may be used as biological or environmental biomarkers. Previous studies have described the diversity, distribution, and abundance of BHPs in a variety of modern environments. However, the regulation of BHP production in polar settings is not well understood. Benthic microbial mats from ice‐covered lakes of the McMurdo Dry Valleys, Antarctica provide an opportunity to investigate the sources, physiological roles, and preservation of BHPs in high‐latitude environments. Lake Vanda is one of the most stable lakes on Earth, with microbial communities occupying specific niches along environmental gradients. We describe the influence of mat morphology and local environmental conditions on the diversity and distribution of BHPs and their biological sources in benthic microbial mats from Lake Vanda. The abundance and diversity of C‐2 methylated hopanoids (2‐MeBHP) are of particular interest, given that their stable degradation products, 2‐methylhopanes, are among the oldest and most prevalent taxonomically informative biomarkers preserved in sedimentary rocks. Furthermore, the interpretation of sedimentary 2‐methylhopanes is of great interest to the geobiology community. We identify cyanobacteria as the sole source of 2‐MeBHP in benthic microbial mats from Lake Vanda and assess the hypothesis that 2‐MeBHP are regulated in response to a particular environmental variable, namely solar irradiance.     

Modern conophyton‐like microbial mats discovered in Lake Vanda,Antarctica

Lake Vanda is a cold nonturbulent, perennially ice‐covered lake in the valleys of southern Victoria Land, Antarctica. Observations made and samples collected under the 3.5 m ice in 1980 by SCUBA divers reveal that an extensive benthic microbial mat dominated by the filamentous blue‐green algae (cyanobacteria) Phormidium frigidum and Lyngbya martensiana is growing there. As is the case in other Antarctic lakes investigated by us thus far, the mat in Lake Vanda traps and binds sediment and precipitates calcite and is undisturbed by grazers and burrowers. Therefore, stromatolitic laminae are being generated. Unlike the other Antarctic lakes investigated in this region, Lake Vanda has (a) an ice cover and water that transmits significantly more light; (b) an ice cover that is permeable to gases and aeolian sediment; (c) no zone of lift‐off mat where photosynthetically generated oxygen would render the mat buoyant and cause it to separate from the substrate and float away; and (d) mat that has a distinctive pinnacle macrostructure. Although the laminae being laid down by the Lake Vanda mat do not retain the cone and ridge morphology of the living mat, the pinnacle macrostructure of the mat is similar to the Precambrian Conophyton stromatolites as well as microbial structures forming in Yellowstone hot springs, freshwater marshes in the Bahamas, and hypersaline intertidal mats in Baja California, Mexico, and Shark Bay, Australia. This suggests (a) Conophyton‐like structures similar to those abundant during the Precambrian can form under widely varying environmental conditions and (b) high latitudes should not be overlooked as sites of formation of ancient stromatolites.     

Growth dynamics of a laminated microbial mat in response to variable irradiance in an Antarctic lake

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Annual growth layers as proxies of past growth conditions for benthic microbial mats in a perennially ice-covered Antarctic lake

Perennial microbial mats can be the dominant autotrophic community in Antarctic lakes. Their seasonal growth results in clearly discernible annual growth layering. We examined features of live microbial mats from a range of depths in Lake Hoare, Antarctica, that are likely to be preserved in these layers to determine their potential as proxies of past growth performance. Cyanobacteria dominated the mat for all but the deepest depth sampled. Changes in areal concentrations of phycobilin pigments, organic matter and extracellular polysaccharide and in species composition did not correspond to changes in various water column properties, but showed a linear relationship with irradiance. Carbonate accumulation in the mats correlated with biomass markers and may be inferred as an index of mat performance. We examined the carbonate content of annual layers laid down from 1958–1959 to 1994–1995 in sediment cores from 12 m depth. The carbonate content in the layer showed a significant correlation with the mean summer air temperature. These data suggest a link between air temperature and microbial mat growth performance, and suggest that it is mediated via irradiance. Laminated microbial mats in Antarctic lakes have the potential to act as fine-resolution records of environmental conditions in the recent past, although interpretation is complex.     

Phytoplankton nutrient deficiency in lakes of the McMurdo dry valleys, Antarctica

1. The influence of inorganic nitrogen and phosphorus enrichment on phytoplankton photosynthesis was investigated in Lakes Bonney (east and west lobes), Hoare, Fryxell and Vanda, which lie in the ablation valleys adjacent to McMurdo Sound, Antarctica. Bioassay experiments were conducted during the austral summer on phytoplankton populations just beneath the permanent ice cover in all lakes and on populations forming deep-chlorophyll maxima in the east and west lobes of Lake Bonney. 2. Phytoplankton photosynthesis in surface and mid-depth (13 m) samples from both lobes of Lake Bonney were stimulated significantly (P < 0.01) by phosphorus enrichment (2 μM) with further stimulation by simultaneous phosphorus plus NH₄⁺ (20 μM) enrichment. Similar trends were observed in deeper waters (18 m) from the east lobe of Lake Bonney, although they were not statistically significant at P < 0.05. Photosynthesis in this lake was never enhanced by the addition of 20 μM NH₄⁺ alone. Simultaneous addition of phosphorus plus nitrogen stimulated photosynthesis significantly (P < 0.01) in both Lake Hoare and Lake Fryxell. No nutrient response occurred in Lake Vanda, where activity in nutrient-enriched samples was below unamended controls; results from Lake Vanda are suspect owing to excessively long sample storage in the field resulting from logistic constraints. 3. Ambient dissolved inorganic nitrogen (DIN) (NH4⁺+ NO₂^?+ NO₃^?): soluble reactive phosphorus (SRP) ratios partially support results from bioassay experiments indicating strong phosphorus deficiency in Lake Bonney and nitrogen deficiency in Lakes Hoare and Fryxell. DIN : SRP ratios also imply phosphorus deficiency in Lake Vanda, although not as strong as in Lake Bonney. Particulate carbon (PC): particulate nitrogen (PN) ratios all exceed published ratios for balanced phytoplankton growth, indicative of nitrogen deficiency. 4. Vertical nutrient profiles in concert with low advective flux, indicate that new (sensu Dugdale & Goering, 1967) phytoplankton production in these lakes is supported by upward diffusion of nutrients from deep nutrient pools. This contention was tested by computing upward DIN : SRP flux ratios across horizontal planes located immediately beneath each chlorophyll maximum and about 2 m beneath the ice (to examine flux to the phytoplankton immediately below the ice cover). These flux ratios further corroborated nutrient bioassay results and bulk DIN : SRP ratios indicating phosphorus deficiency in Lakes Bonney and Vanda and potential nitrogen deficiency in Lakes Hoare and Fryxell. 5. Neither biochemical reactions nor physical processes appear to be responsible for differences in nutrient deficiency among the study lakes. The differences may instead be related to conditions which existed before or during the evolution of the lakes.     

The microfauna of algal mats and artificial substrates in Southern Victoria Land lakes of Antarctica

The first microfaunal colonization study of continental lakes in Antarctica is reported. In addition to the general taxonomic composition of six Antarctic lakes, detailed community composition analyses of two adjacent lakes, Lake Fryxell and Lake Hoare, using polyurethane foam artificial substrates suspended in the open water and placed directly over benthic algal mats are also presented. Protozoans, rotifers, tardigrades and nematodes constituted the microfaunal community in all lakes. Higher metazoans do not occur in lakes that we studied. Protozoans were the most diverse of the four groups, and the first dinoflagellates reported from southern Victoria Land, Gymnodinium and Glenodinium, were collected from Lake Fryxell. The greatest number of protozoan taxa (55) were associated with the algal mats in the more productive Lake Fryxell. Both the physical and chemical properties and taxonomic diversity showed marked variability between all lakes that were studied, especially the adjacent lakes, Fryxell and Hoare. Ecological variables which regulate taxonomic diversity in other lakes of the world also appear to exert similar influences in remote Antarctic lakes.     

UV radiation and potential biological effects beneath the perennial ice cover of an antarctic lake

High-resolution spectral scans of solar ultraviolet radiation (UVR) were obtained directly beneath the 4.0–5.0 m thick, perennial ice cover of Lake Hoare, South Victoria Land, Antarctica. Both UVA (320–400 nm) and UVB (280–320 nm) radiation were detectable beneath the ice using a diver-deployed, underwater scanning spectroradiometer which permitted accurate measurement in the 280–340 nm range, while avoiding effects of surface shading and/or hole effects. UVR at wavelengths <310 nm was not detectable below the ice. This lower wavelength UVB appears to penetrate the Lake Hoare ice to depths of no more than 1.5 m during relatively cloud-free austral summer days. Based upon estimated biologically effective UVR dosages and DNA dosimeter data, exposure of benthic and planktonic microbes to the UVR encountered immediately beneath the ice is unlikely to inhibit microbial metabolism. Although waters of oligotrophic antarctic lakes are highly transparent to UVR, the thick, high scattering and optically dense ice covers on many of these lakes offers organisms a degree of protection largely unavailable in temperate and tropical systems. Thinning or complete loss of these overlying ice covers is likely to have major consequences for the structure of antarctic lake microbial communities.     

The balance between photosynthesis and grazing in Antarctic mixotrophic cryptophytes during summer

SUMMARY 1. Grazing and photosynthetic contributions to the carbon balance of planktonic, mixotrophic cryptophytes in Lakes Fryxell and Hoare in the Taylor Valley, Antarctica were measured during November and December 2000.
2. The cryptophytes never became entirely photosynthetic, although carbon derived from grazing decreased in December. Individual grazing rates ranged between 5.28 and 10.08 bacteria cell⁻¹ day⁻¹ in Lake Fryxell and 0.36–11.76 bacteria cell⁻¹ day⁻¹ in Lake Hoare. Grazing rates varied temporally and with depth in the water column. In Lake Fryxell, which is a meromictic lake, highest grazing occurred just above the chemocline. Individual photosynthetic rates ranged from 0.23 to 1.35 pg C cell⁻¹ h⁻¹ in Lake Fryxell and 0.074 to 1.08 pg C cell⁻¹ h⁻¹ in Lake Hoare.
3. Carbon acquisition by the cryptophyte community gained through grazing ranged between 8 and 31% during November in Lake Fryxell, dropping to between 2 and 24% in December. In Lake Hoare grazing contributed 12–21% of the community carbon budget in November and 1–28% in December. Around 4% of the carbon acquired from grazing and photosynthesis was remineralised through respiration.
4. Mixotrophy is probably a major survival strategy for cryptophytes in the extreme lakes of the Dry Valleys, because perennial ice-cover severely limits light penetration to the water column, whereas these phytoflagellates are not normally mixotrophic in lower latitude lakes. The evidence suggests that mixotrophy may be a mechanism for supplementing the carbon budget, as well as a means of acquiring nutrients for growth.     

Seasonal changes in the underwater light climate of two Canadian shield lakes

This study examines the seasonal variation in the underwater spectral distribution of light in a mesotrophic (Lake Cromwell) and an oligo-mesotrophic (Lake Croche) temperate lake. Gilvin is primarily responsible for the strong selective attenuation of blue light in both lakes. As a result of differing gilvin concentrations light transmission maxima of downwelling and upwelling spectra are near 615 nm in Lake Cromwell and 599 nm in Lake Croche. With increases in depth both upwelling and downwelling radiance fluxes decrease, are shifted to longer wavelengths and become more monochromatic. The greatest penetration of light occurs in the summer and spring after which a gradual decrease occurs through fall to a minimum value in winter. Under the winter cover the P ₅₀ of downwelling light shifts 10 to 20 nm towards shorter wavelengths. Seasonal changes in downwelling irradiance are related to solar altitude, concentration of suspended particles, phytoplankton populations, amount of gilvin, mixing and winter cover. The brownish colouration of these lakes is explained by reflectance of spectrally impure orangish-red light.     

Influence of light intensity and wavelength on phototactic behaviour of larval silver perch Bidyanus bidyanus and golden perch Macquana ambigua and the effectiveness of light traps

Larval golden perch, Macquaria ambigua , and silver perch, Bidyanus bidyanus , were exposed to light gradients in wavebands centred on 400, 496, 601 and 695 nm at nominal quantum irradiance values of 0–1, 1–0 and 10 μmol m⁻² s^−l. Silver perch larvae displayed stronger phototactic behaviour than golden perch, and both species were most responsive to light in the 601 nm waveband. The intensity of phototactic responses in both species was greater at higher irradiance levels. Enhanced responsiveness to longer wavelengths reflects possible adaptations to life in turbid habitats where the underwater light field is dominated by yellow/orange wavebands.
At night, traps fitted with 12 h yellow lightsticks attracted more golden perch larvae than traps with blue, green, orange, red or no lightstick. The efficacy of yellow lightsticks may be due to yellow/orange wavebands not being attenuated under water as rapidly as blue or red wavebands. Yellow lightsticks also emit a greater intensity of light over a longer time than other colours tested, which may have increased the effectiveness of yellow traps. Light traps were ineffective during the day.     

MICROALGAL LIGHT-HARVESTING IN EXTREME LOW-LIGHT ENVIRONMENTS IN MCMURDO SOUND, ANTARCTICA1

Microalgal pigment composition, photosynthetic characteristics, single-cell absorption efficiency (Qa_(λ)) spectra, and fluorescence-excitation (FE) spectra were determined for platelet ice and benthic communities underlying fast ice in Mc Murdo Sound, Antarctica, during austral spring 1988. Measurements of spectral irradiance (E_(λ)) and photosynthetically active radiation (PAR) as well as samples for particulate absorption measurements were taken directly under the congelation ice, within the platelet layer, as profiles vertically through the water column, and at the benihic surface. Light attenuation by.sea ice, algal pigments, and particulates reduced PAR reaching the platelet ice layer to 3%(9–33 fimol photons m-²-^?s-¹) of surface values and narrowed its spectral distribution to a band between 400 and 580 nm. Attenuation by the water column further reduced PAR reaching the sea floor (28–m depth) to 0.05% of surface levels (< 1 μmol photons m-² s-¹), with a spectral distribution dominated by 470–580–nm wavelengths. The photoadaptive index (I) for platelet ice algae (5.9–12.6 μmol photons m-^2.s-¹) was similar to ambient PAR, indicating that algae had acclimated to their light environment (i.e. the algae were light-replete). Maximum Qa_(λ) at the blue absorption peak (440 nm) was 0.63, and enhanced absorption was observed from 460–500 nm and was consistent with observed high cellular chlorophyll (chi) c:chl a and fucoxanthin: chl a molar ratios (0.4 and 1.2, respectively). Benthic algae were light-limited despite the maintenance of very low I_k values (4–11 μmol photons.m-^2.s-¹). Extremely high fucoxanthin: chi a ratios (1.6) in benthic algae produced enhanced green light absorption, resulting in a high degree of complementation between algal absorption and ambient spectral irradiance. Qa_(λ) values for benthic algae were maximal (0.9) between 400 and 510 nm but remained >0.35 even at absorption minima. Strong spectral flattening, a characteristic of intense pigment packaging, was also apparent in the Qa_(λ) spectra for benthic algae. FE and Qa_(λ) spectra were similar in shape for platelet ice algae, indicating that the efficiency at which absorbed energy was transferred to photosystem II (PSII) was independent of wavelength. Fluorescence emission by benthic algae was greatest for the 500–560–nm excitation wavelengths, suggesting that most energy absorbed by accessory pigments was transferred to PSII. These results suggest that under ice algae employ complementary pigmentation and maximize absorption efficiency as adaptive strategies to low-light stress. Regulating the distribution of absorbed energy between PSI and PSII may be an adaptive response to the restricted spectral distribution of irradiance.     

Diel patterns of photosynthate biosynthesis by phytoplankton in permanently ice-covered Antarctic lakes under continuous sunlight

Diel patterns of photosynthate biosynthesis by Antarctic freshwaterphytoplankton growing under the variable but continuous sunlightof summer were found to be similar in many respects to thosereported from other aquatic environments where light/dark periodsalternate. Lipid synthesis by freshwater phytoplankton in LakesVanda and Fryxell predominated during periods when solar radiationand photosynthesis were most intense; the inverse was generallytrue of the protein and polysaccharide fractions. The majorphotosynthetic end-products in both lakes were protein and polysaccharide,which together accounted for 60–81% of the total cellularcarbon incorporation. Less than 4% of the carbon was incorporatedinto lipid in Lake Vanda; >12% appeared in the lipid fractionin Lake Fryxell. The Lake Fryxell populations showed evidenceof photoinhibition of complete photosynthesis during ‘midday’when irradiance was most intense. I_k values, computed from thephotosynthesis irradiance relationships in Lake Fryxell, corroborateother studies suggesting that the phytoplankton populationsin permanently ice-capped Antarctic lakes are among the mostshade-adapted yet reported.     

Fluorometric Characterization of Two Picocyanobacteria Strains from Lakes of Different Underwater Light Quality

Unicellular autofluorescent picoplankton ranging from 0.6 to 0.9 ym in diameter were isolated from Lake Maggiore and from Lake Balaton. The cyanobacterial isolates contain two accessory pigments: phycoerythrin (PE) and phycocyanin (PC) respectively. The in vivo spectral properties of the two clones were compared to identify characteristics of the pigments. In vivo fluorescence excitation and emission spectra revealed that clones with PE are composed only of phycoerythrobilin chromophores and lack phycourobilin. Glycerol treatment enhances the fluorescence yield up to 3 times and improves the detection sensitivity of PE particularly at 436 and 520 nm and of PC at 600 nm. The vertical profiles of the underwater irradiance at different wavelengths were measured in both lakes to study the light quality of the natural environment of the two strains. Growth rates of both clones growing at different light intensities and wavelengths, selected by the same filters used for vertical profiles, were estimated. The results showed a difference in growth rate of phycoerythrin and phycocyanin containing cells exposed to an equal quantum flux of preferential illumination. In particular the maximum growth rate was reached by PE cells exposed to green light and by PC cells exposed to red light.     

Diel activity of Gammarus pulex (Crustacea) in a South Swedish stream: comparison of drift catches vs baited traps

The benthos of a perennially ice-covered Antarctic lake, Lake Hoare, contained three distinct signatures of lipophilic pigments. Cyanobacterial mats found in the moat at the periphery of the lake were dominated by the carotenoid myxoxanthophyll; carotenoids: chlorophyll a ratios in this high light environment ranged from 3 to 6.8. Chlorophyll c and fucoxanthin, pigments typical of golden-brown algae, were found at 10 to 20 m depths where the benthos is aerobic. Anaerobic benthic sediments at 20 to 30 m depths were characterized by a third pigment signature dominated by a carotenoid, tentatively identified as alloxanthin from planktonic cryptomonads, and by phaeophytin b from senescent green algae. Pigments were not found associated with alternating organic and sediment layers. As microzooplankton grazers are absent from this closed system and transformation rates are reduced at low temperatures, the benthos beneath the lake ice appears to contain a record of past phytoplankton blooms undergoing decay.     

Salinity, depth and the structure and composition of microbial mats in continental Antarctic lakes

1. Lakes and ponds in the Larsemann Hills and Bølingen Islands (East‐Antarctica) were characterised by cyanobacteria‐dominated, benthic microbial mats. A 56‐lake dataset representing the limnological diversity among the more than 150 lakes and ponds in the region was developed to identify and quantify the abiotic conditions associated with cyanobacterial and diatom communities. 2. Limnological diversity in the lakes of the Larsemann Hills and Bølingen Islands was associated primarily with conductivity and conductivity‐related variables (concentrations of major ions and alkalinity), and variation in lake morphometry (depth, catchment and lake area). Low concentrations of pigments, phosphate, nitrogen, DOC and TOC in the water column of most lakes suggest extremely low water column productivity and hence high water clarity, and may thus contribute to the ecological success of benthic microbial mats in this region. 3. Benthic communities consisted of prostrate and sometimes finely laminated mats, flake mats, epilithic and interstitial microbial mats. Mat physiognomy and carotenoid/chlorophyll ratios were strongly related to lake depth, but not to conductivity. 4. Morphological‐taxonomic analyses revealed the presence of 26 diatom morphospecies and 33 cyanobacterial morphotypes. Mats of shallow lakes (interstitial and flake mats) and those of deeper lakes (prostrate mats) were characterised by different dominant cyanobacterial morphotypes. No relationship was found between the distribution of these morphotypes and conductivity. In contrast, variation in diatom species composition was strongly related to both lake depth and conductivity. Shallow ponds were mainly characterised by aerial diatoms (e.g. Diadesmis cf. perpusilla and Hantzschia spp.). In deep lakes, communities were dominated by Psammothidium abundans and Stauroforma inermis. Lakes with conductivities higher than ±1.5 mS cm^?1 became susceptible to freezing out of salts and hence pronounced conductivity fluctuations. In these lakes P. abundans and S. inermis were replaced by Amphora veneta. Stomatocysts were important only in shallow freshwater lakes. 5. Ice cover influenced microbial mat structure and composition both directly by physical disturbance in shallow lakes and by influencing light availability in deeper lakes, as well as indirectly by generating conductivity increases and promoting the development of seasonal anoxia. 6. The relationships between diatom species composition and conductivity, and diatom species composition and depth, were statistically significant. Transfer functions based on these data can therefore be used in paleolimnological reconstruction to infer changes in the precipitation–evaporation balance in continental Antarctic lakes.     

Action spectra of microalgal photosynthesis and depth distribution of spectral scalar irradiance in a coastal marine sediment of Limfjorden, Denmark

Abstract The role of complementary spectral utilization of light for the zonation of different groups of oxygenic phototrophic organisms in sediments was studied. The marine sediment was covered by a dense population of diatoms with an underlying population of cyanobacteria. Action spectra for photosynthesis and spectral scalar irradiance, E ₀, were measured directly in the sediment at a spatial resolution of 0.1 mm by the use of oxygen and light microsensors. The action spectrum for the diatoms was similar to the attenuation spectrum of the scalar irradiance, K ₀, in the diatom layer with Chl. a and carotenoids being the major photosynthetic pigments. The action spectrum of the cyanobacteria showed photosynthesis maxima at the absorption regions of Chl. a and phycocyanin. The measured depth distribution of spectral scalar irradiance and the action spectra of diatoms and cyanobacteria were used to calculate the spectral quality for photosynthesis of the 400–700 nm light to which the two populations were exposed. This spectral quality was compared to that of the light incident on the sediment surface. Due to preferential extinction of wavelengths, at which their photosynthetically active pigments had maximal absorption, the relative light quality for diatoms was reduced to 85% of the quality of incident light at a similar total quantum flux. This effect was partly due to spectral alterations of light backscattered from the underlying sediment with cyanobacteria. The cyanobacteria at the bottom of the euphotic zone, in contrast, experienced a light spectrum which was favorably altered, to 107% in quality, due to absorption by the overlying diatoms. It was concluded that these changes in spectral light quality can be considered as only one of more factors explaining the zonation of the two phototrophic populations, and that total light intensity and the chemical microenvironment are probably more important factors.     

The role of zoobenthos in energy flow in two shallow lakes

Net production of zoobenthos in two shallow and eutrophic lakes, i.e. the S-basin of Mývatn, Iceland (maximum depth 4.2 m, mean depth 2.3 m) and Hjarbæk Fjord, Denmark (maximum depth 6.5 m, mean depth 1.9 m) were calculated as 878 and 1093 kJ m^-2 yr^-1, respectively. The zoobenthos in both lakes was dominated by Chironomidae (Diptera) living partly as filtrators feeding on suspended particles (phytoplankton) and partly as surface feeders foraging on benthic algae and/or seston. Respiration and consumption were estimated from the literature. Net production efficiency averaged 0.41 and 0.48 in Hjarbæk Fjord and Mývatn, respectively. Ingestion was dominated by herbivorous chironomids, while detritivorous tubificids were insignificant. Zoobenthic production made up 86% of total secondary production (zooplankton plus zoobenthos) in both lakes. The trophic efficiency between net primary production and benthic net secondary production was 8% and 11% in Hjarbæk Fjord and Lake Mývatn S-basin, respectively.     

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.     

A mechanistic analysis of light and carbon use efficiencies

We explore the extent to which a simple mechanistic model of short-term plant carbon (C) dynamics can account for a number of generally observed plant phenomena. For an individual, fully expanded leaf, the model predicts that the fast-turnover labile C, starch and protein pools are driven into an approximate or moving steady state that is proportional to the average leaf absorbed irradiance on a time-scale of days to weeks, even under realistic variable light conditions, in qualitative agreement with general patterns of leaf acclimation to light observed both temporally within the growing season and spatially within plant canopies. When the fast-turnover pools throughout the whole plant (including stems and roots) also follow this moving steady state, the model predicts that the time-averaged whole-plant net primary productivity is proportional to the time-averaged canopy absorbed irradiance and to gross canopy photosynthesis, and thus suggests a mechanistic explanation of the observed approximate constancy of plant light-use efficiency (LUE) and carbon-use efficiency. Under variable light conditions, the fast-turnover pool sizes and the LUE are predicted to depend negatively on the coefficient of variation of irradiance. We also show that the LUE has a maximum with respect to the fraction of leaf labile C allocated to leaf protein synthesis ( a_lp ), reflecting a trade-off between leaf photosynthesis and leaf respiration. The optimal value of a_lp is predicted to decrease at elevated [CO₂] _a , suggesting an adaptive interpretation of leaf acclimation to CO₂. The model therefore brings together a number of empirical observations within a common mechanistic framework.