Malcolm T. Downes

Community structure and pigment organisation of cyanobacteria-dominated microbial mats in Antarctica

Benthic microbial mat communities were sampled from 20 lakes, ponds and streams of the McMurdo Sound region, Antarctica. At least five distinct assemblages could be differentiated by their cyanobacterial species composition, pigment content and vertical structure. The most widely occurring freshwater communities were dominated by thin-trichome (0·5–3 µm) oscillatoriacean species that formed benthic films up to several millimetres thick. ‘Lift-off mats’ produced mucilaginous mats 1–5 cm thick at the surface and edge of certain ponds. Another group of oscillatoriacean communities was characteristic of hypersaline pond environments; these communities were dominated by species with thicker trichomes such as Oscillatoria priestleyi. Black mucilaginous layers of Nostoc commune were widely distributed in aquatic and semi-aquatic habitats. Dark brown sheath pigmentation was also characteristic of less cohesive mats and crusts dominated by Pleurocapsa, Gloeocapsa and Calothrix. High performance liquid chromatograp...

ANTARCTIC CYANOBACTERIA: LIGHT, NUTRIENTS, AND PHOTOSYNTHESIS IN THE MICROBIAL MAT ENVIRONMENT1

The microenvironmental and photosynthetic characteristics of Antarctic microbial mats were measured in a series of ponds near McMurdo Sound. As elsewhere in Antarctica, these cold‐water benthic communities were dominated by oscillatoriacean cyanobacteria. Despite large variations in mat thickness, surface morphology, and color, all of the communities had a similar pigment organization, with a surface carotenoid‐rich layer that overlaid a deep chlorophyll maximum (DCM) enriched in phycocyanin as well as chlorophyll a. Spectroradiometric analyses showed that the DCM population inhabited an orange‐red shade environment. In several of the mats, the deep‐living trichomes migrated up to the surface of the mat within 2 h in response to a 10‐fold decrease in surface irradiance. The euphotic layer of the mats was supersaturated in oxygen and contained ammonium and dissolved reactive phosphorus concentrations in excess of 100 mg N·m−3 or P·m−3. Integral photosynthesis by core samples was saturated at low irradiances and varied two‐ to threefold throughout the continuous 24‐h radiation cycle. Oxygen microelectrode analyses showed that the photosynthetic rates were slow to negligible near the surface and maximal in the DCM. These compressed, nutrient‐rich euphotic zones have some properties analogous to planktonic systems, but the integrated photosynthetic responses of the community reflect the strong self‐shading within the mat and physiological dominance by the motile, DCM populations.

Microbial biomass, photosynthesis and chlorophyll a related pigments in the ponds of the McMurdo Ice Shelf, Antarctica

In this study we examine the magnitude and variability of benthic microbial biomass in a contrasting series of ponds on the McMurdo Ice Shelf. We also assess the photoautotrophically active portion of this biomass by the relationship between photosynthetic rates and chlorophyll a

Variation in nutrient removal from a stream by watercress (Nasturtium officinale R. Br.)

Large seasonal variation was observed in the removal of nitrate, phosphate and ammonium from a small stream (flow rate: 0.071–0.108 m3s−1) entering Lake Taupo in North Island, New Zealand. Loss of all three nutrients was greatest between mid-November and mid-February, the time of maximum growth of watercress (Nasturtium officinale R. Br.). Nitrate loss varied markedly throughout the day and night, with maximum removal occurring at midday and minimum removal at midnight. The diel variation in nitrate loss was closely correlated with changes in dissolved oxygen concentration in the stream, but with a lag of 4h. There was no pronounced diel change in rates of ammonium or phosphate removal. In-situ root incubations with watercress demonstrated that the plant is capable of removing nitrate and phosphate at rates that can account for most of the loss of stream nutrients. The diel pattern of nitrate uptake by the adventitious roots paralleled that of nitrate loss from the stream. There was no diel change in the rate of phosphate uptake. As a measure of the assimilation capacity of watercress for nitrate-nitrogen, the levels of nitrate reductase in root, stem and leaf tissue were monitored. Enzyme activity was greatest in the leaves and varied throughout the day and night. The implications of the large-scale nutrient removal by watercress are discussed in relation to nutrient loading budgets.

Evidence for regional climate change in the recent evolution of a high latitude pro-glacial lake

Lake Wilson, a perennially ice-capped, deep (>100 m) lake at 80°S in southern Victoria Land was investigated in January 1993. Water chemistry and physical structure showed three distinct layers; an upper c. 35 m mixed layer of low salinity, moderately turbid water; a less turbid mid layer, 20 m thick of slightly higher salinity and supersaturated with oxygen; and a deep 20 m brackish layer (conductivity c. 4000 μS cm −1 ) with anoxic conditions in the lower 5 m. Extreme supersaturation of N 2 O (up to 400 times air saturation) together with high nitrate concentration (4000 mg m −3 ) was recorded in the deep layer. Phytoplankton biomass and photosynthetic activity was confined to the upper mixed layer and the band of supersaturated dissolved oxygen located at 40–55 m appears to represent a relict layer from when the lake level was lower. The evidence from a comparison of profiles between 1975 and 1993 suggests that Lake Wilson has risen 25 m since 1975, synchronous with a period of lake level rise in the McMurdo Dry Valleys lakes to the north at 77°S. Geochemical diffusion models indicate that Lake Wilson had evaporated to a smaller brine lake about 1000 yrs BP, which also fits the pattern shown by the McMurdo Dry Valleys lakes. Climate changes influencing lake levels have thus covered a wide area of southern Victoria Land.

A sensitive fluorometric technique for the measurement of phycobilin pigments and its application to the study of marine and freshwater picophytoplankton in oligotrophic environments

A sensitive and specific technique is described for the estimation of phycobiliprotein in freshwater and marine picophytoplankton. The method uses fluorescent properties to detect phycoerythrin concentrations as low as 40 ng L-1 from a 1 L water sample and is capable of distinguishing between R-phycoerythrin, C-phycocyanin and C-phycoerythrin. The application of the method to the study of natural picophytoplankton populations in marine and freshwater environments is described. Nitrate concentrations appear to influence picophytoplankton cellular C-phycoerythrin concentrations in surface waters and increasing cellular C-phycoerythrin fluorescence with water depth suggests that this pigment plays a role as a photosynthetic accessory pigment.

Extraction of chlorophyll and carotenoid pigments from Antarctic benthic mats for analysis by HPLC

The efficiency of 9:1 acetone-water, DMSO and boiling 9:1 ethanol-water in extracting chlorophyll and carotenoid pigments from benthic cyanobacterial mats from Antarctica for HPLC (high performance liquid chromatography) analysis was examined. Considerable breakdown of chlorophylla was observed after 5 min extraction in boiling ethanol and 2 h extraction in DMSO. Over 50% of the chlorophylla was degraded to chlorophyllidea and there was substantial loss of carotenoids after a 15 h exposure of ground cells to cold 9:1 acetone-water.Mild sonication of ground mat material in 9:1 acetone-water followed by a 4 h extraction at 4 ° C was found to minimise chlorophylla breakdown and dramatically improved the extraction efficiency of chlorophylla, myxoxanthophyll and -carotene.

Underwater light and photosynthesis at three sites in Pelorus Sound, New Zealand

Abstract Optical and photosynthctic measurements were made at 3 sites in Pelorus Sound, New Zealand, in late summer 1985. The attenuation of underwater light increased towards the landward end of the Sound, with an abrupt rise in scattering parameters (the scattering coefficient b, asymptotic reflectance Ra, and the ratio of scalar to downwelling irradiance Eo/E d) in the inner sound region (Mahau Sound). The effects of scattering were offset by changing water column depth and a high average irradiance was experienced by the phytoplankton community (22–34% of surface irradiance) at the 3 sites. Spectral scans of the water from Pelorus Sound showed an increasing absorbance by dissolved organic material (DOM) as well as paniculate materials towards the inner sound. Maximum photo‐synthetic rates for the water column and per unit chlorophyll a were similarly high at all 3 sites, but chlorophyll a concentrations were consistently low (0.5–2.5 mg nr‐3). Population losses to zooplankton or other herbivores seem ...