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The community structure and physiological characteristics of three microbial mat communities in Byers Peninsula (Livingston Island, South Shetland Islands, Antarctica) were compared. One of the mats was located at the edge of a stream and was dominated by diatoms (with a thin basal layer of oscillatorian cyanobacteria), whereas the other two mats, located over moist soil and the bottom of a pond, respectively, were dominated by cyanobacteria throughout their vertical profiles. The predominant xanthophyll was fucoxanthin in the stream mat and myxoxanthophyll in the cyanobacteria-dominated mats. The sheath pigment scytonemin was absent in the stream mat but present in the soil and pond mats. The stream mat showed significantly lower delta13C and higher delta15N values than the other two mats. Consistent with the delta15N values, N2 fixation was negligible in the stream mat. The soil mat was the physiologically most active community. It showed rates of photosynthesis three times higher than in the other mats, and had the highest rates of ammonium uptake, nitrate uptake and N2 fixation. These observations underscore the taxonomic and physiological diversity of microbial mat communities in the maritime Antarctic region.

The three-dimensional structures of two types of cyanobacterium-dominated microbial mats from meltwater ponds on the McMurdo Ice Shelf were as determined by using a broad suite of complementary techniques, including optical and fluorescence microscopy, confocal scanning laser microscopy, scanning electron microscopy with back-scattered electron-imaging mode, low-temperature scanning electron microscopy, and microanalyitical X-ray energy dispersive spectroscopy. By using a combination of the different in situ microscopic techniques, the Antarctic microbial mats were found to be structures with vertical stratification of groups of cyanobacteria and mineral sediments, high contents of extracellular polymeric substances, and large void spaces occupied by water. In cyanobacterium-rich layers, heterocystous nostocalean and nonheterocystous oscillatorialean taxa were the most abundant taxa and appeared to be intermixed with fine-size deposits of epicellular silica and calcium carbonate. Most of the cyanobacterial filaments had similar orientations in zones without sediment particles, but thin filaments were tangled among thicker filaments. The combination of the microscopic techniques used showed the relative positions of biological and mineral entities within the microbial mats and enabled some speculation about their interactions.

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We have investigated the ecological importance of N2-fixation in cyanobacterial mats, dominated by oscillatorean species, in ponds of the Bratina Island area of the McMurdo Ice Shelf, Antarctica (78 degrees S, 166 degrees E). Nitrogenase activity, estimated as acetylene reducing activity (ARA), was found in all the mats investigated (n = 16). The average ARA was 75.9 mmol ethylene m-2 h-1, ranging from 6 to 201 mmol ethylene m-2 h-1. Nitrogenase activity was positively correlated with dissolved reactive phosphorus concentration in pondwater and the C/N ratio of the mat, and was negatively correlated with pondwater NH4+-N concentrations and natural abundance of 15N in the mats. ARA was restricted to the upper, oxic layer of the mats. Experiments conducted to ascribe ARA to different groups of prokaryotes suggested that ARA was mainly conducted by heterocystous cyanobacteria, since no activity was found in the dark and the activity was inhibited by the photosystem II inhibitor DCMU (3-[3,4-dichlorophenyl]-1,1-dimethyl urea). In spite of 24 h of daylight, nitrogenase activity showed a diel cycle with maximum activity at midday (10-18 h) and minimal activity at early morning (6-10 h) when pond temperatures were at their minima. Light dependency of nitrogenase activity for three cyanobacterial communities showed that the irradiance required for saturating ARA was low, in every case lower than 100 mmol photon m-2s-1. Irradiance rarely fell below 100 mmol photon m-2s-1 during Antarctic summer days and ARA was likely to be light saturated for much of the time. We estimate that N2 fixation represented on average a N input into the ponds of over 1 g m-2y-1. This value appears to be the highest N input to this Antarctic ecosystem.

The relationship between the requirement for boron and the form of N supplied in nutrient media to cyanobacterium Anabaena sp. PCC 7119 was investigated. When cells were grown in a medium which contained nitrate or ammonium-N, boron deficiency in the nutrient media did not inhibit growth or change cell composition. However, when cells were dependent on N2 fixation, the lack of boron inhibited growth (i.e. growth ceased after 96 hours under these conditions). Additionally, boron-deficient cells showed a significant decrease in their content of phycobiliproteins and chlorophyll and accumulated carbohydrates within 24 hours of removing boron from the nutrient media. Inhibition of photosynthetic 02 evolution accompanied the decrease in photosynthetic pigments. Boron deficiency symptoms were relieved when either boron or combined N was added to boron-deficient cultures. The degree of recovery depended upon the age of the cultures. Assays of nitrogenase activity showed that, after 2 hours of growth, nitrogenase activity of boron-deficient cells was inhibited by 40%. After 24 hours a total inactivation of nitrogenase activity was observed in boron-deficient cells. These results strongly suggest an involvement of boron in N2 fixation in cyanobacteria.Boron is an essential element for all higher plants but it is not required by most phototrophic microorganisms. A requirement for boron in marine and freshwater diatoms was clearly shown (13). In contrast, others (3,7,9,17) have failed to demonstrate a requirement for boron in Chlorella and other green algae.Stimulation of growth rates with added boron in the absence ofcombined N has been reported for the N2-fixing cyanobacteria Nostoc muscorum, Calothrix parietina, and Anabaena cylindrica (6, 9). Boron, however, had little effect on the growth of these cyanobacteria when nitrates were added to the medium. Growth of non-N2-fixing Microcystis aeruginosa also was not enhanced by boron (9). Gerloff (9) suggested that the lack of response to boron in cells grown in nutrient media containing nitrates was due to boron contamination of the nitrate salt used to prepare the growth media rather than to the involvement of boron in the N2-fixation process. By using salts containing no detectable amounts of boron, we reported (16) that boron deficiency during 3 months of culture did not affect the growth of the non-N2-fixing cyanobacterium Anacystis nidulans. We concluded that the requirement for boron is not a general feature of all cyanobacteria and suggested that the essentiality of boron could be restricted to N2-fixing strains (16).

Transposon mutagenesis of Anabaena sp. PCC7120 led to the isolation of a mutant strain, PHB11, which grew poorly at pH values above 10. The mutant strain exhibited pronounced Na+ sensitivity; this sensitivity was higher under basic conditions. Mutant PHB11 also showed an inhibition of photosynthesis that was much more pronounced at alkaline pH. Reconstruction of the transposon mutation of PHB11 in the wild-type strain reproduced the phenotype of the original mutant. The wild-type version of the mutated gene was cloned and the mutation complemented. In mutant strain PHB11, the transposon had inserted within an ORF that is part of a seven-ORF operon with significant sequence similarity to a family of bacterial operons that are believed to code for a novel multiprotein cation/proton antiporter primarily involved in resistance to salt stress and adaptation to alkaline pH. The Anabaena operon was denoted mrp (multiple resistance and pH adaptation) following the nomenclature of the Bacillus subtilis operon; the ORF mutated in PHB11 corresponded to mrpA. Computer analysis suggested that all seven predicted Anabaena Mrp proteins were highly hydrophobic with several transmembrane domains; in fact, the predicted protein sequences encoded by mrpA, mrpB and mrpC showed significant similarity to hydrophobic subunits of the proton pumping NADH : ubiquinone oxidoreductase. In vivo expression studies indicated that mrpA is induced with increasing external Na+ concentrations and alkaline pH; mrpA is also upregulated under inorganic carbon (Ci) limitation. The biological significance of a putative cyanobacterial Mrp complex is discussed.