Rivularia

Abstract: The composition of carbon and oxygen stable isotopes was determined for calcite associated with the freshwater cyanobacteria Homoeothrix Crustacea, Phormidium incrustatum, and Rivularia haematites and the green alga Gongrosira incrustans in a UK travertine‐depositing stream. The δ13C values of Rivularia calcites deposited in summer were significantly higher than those deposited during winter. This was interpreted as the result of photosynthetic activity within colonies. The δ13C values were similar for the three cyanobacterium species studied, and colonies of Homoeothrix sampled over a 531‐m length of stream showed progressive downstream enrichment of 13C (2.0‰). This resulted from CO2 degassing of the downstream water, augmented during summer by bryophyte photosynthesis. Subsequent deposition of calcite takes place irrespective of photosynthetic activity of the algae and may blur the isotopic characteristics in older colonies and fossil assemblages.

Abstract: The recognition that many ancient stromatolites are organo-sedimentary structures of microbial origin established the validity of interpreting them by comparison with modern counterparts. The study of modern stromatolites from different environmental settings such as freshwater lakes and creeks (Irion and Muller 1968; Golubic and Fischer 1975), thermal springs (Walter et al. 1972; Doemel and Brock 1974), hypersaline lagoons (Davies 1970; Horodyski and Vonder Haar 1975; Horodyski 1977), and other freshwater and marine coastal environments (Monty 1965, 1967; Gebelein 1969; Golubic 1973, 1976; Golubic and Focke 1978) shows that several different models for ancient stromatolites exist today. Among other things, the characteristic lamination of stromatolites has been shown to result from the periodical binding of detrital particles, the periodical deposition of calcium carbonate, the periodical differentiation of microbial communities, periodical changes in growth pattern of the constitutive algae, etc. Scores of processes have been described to date, but very few have been studied in great detail. Our purpose is to document and discuss a most interesting case, that of the microstromatolites formed by monospecific populations of a filamentous photosynthetic (oxygenic) blue-green alga: Rivularia (Rivulariaceae, Nostocales, Cyanophyta).

Abstract: A weight-of-evidence approach was used to examine how nutrient availability influences stream benthic algal community structure and to validate nutrient-response thresholds in assessing nutrient limitation. Data from 104 southern California streams spanning broad nutrient gradients revealed that relative abundance of N2-fixing heterocystous cyanobacteria (Nostoc, Calothrix), and diatoms (Epithemia, Rhopalodia)-containing cyanobacterial endosymbionts, decreased with increasing ambient inorganic N concentrations within the low end of the N gradient. Response thresholds for these N2 fixers were 0.075 mg l−1 NO3-N, 0.04 mg l−1 NH4-N, and an N:P ratio (by weight) of 15:1. The NO3-N threshold was independently validated by observing nitrogenase gene expression using real-time reverse transcriptase PCR. Morphometric analysis of cyanobacterial endosymbionts in Epithemia and Rhopalodia indicated that endosymbiont biovolume per diatom cell decreased with increasing NO3-N. Our findings indicate that abundance of heterocyst-containing cyanobacteria and endosymbiont-containing diatom cells are good indicators for rapid nutrient biomonitoring. Because heterocystous cyanobacteria and Epithemia/Rhopalodia were not always recorded together at N-limited sites, examining both assemblages jointly may provide a more comprehensive assessment of stream nutrient limitation than using either assemblage alone.