THE EFFECT OF NUTRIENT LIMITATION AND ITS INTERACTION WITH LIGHT UPON THE PRODUCTS OF PHOTOSYNTHESIS IN MERISMOPEDIA TENUISSIMA (CYANOPHYCEAE)1

The metabolic fate of photosynthetically-fixed CO₂ was determined by labeling samples of Merismopedia tenuissima Lemmerman for 30 min with NaH¹⁴CO₃ and analyzing its incorporation into low molecular weight compounds, polysaccharide and protein. In N- and P-sufficient cultures, relative incorporation into protein increased as the irradiance used during the labeling period was decreased to 20 μE · m^-2 s^-1. This pattern was found for cells grown at irradiances of either 20 or 180 μE · m^-2· s^-1, although incorporation into protein was greater in cultures grown at the higher irradiance. In N-limited continuous cultures, relative incorporation into protein was low, independent of growth rate, and the same for samples tested at 20 or 180 μE · m^-2· s^-1 irradiance. In contrast, ¹⁴C incorporation into protein by P-limited cultures increased as growth rate increased, and at relative growth rates greater than 0.25, the incorporation was greater at 20 than at 180 μE · m^-2· s^-1. However, the total RNA content and maximum photosynthetic rate of the cultures was the same at all growth rates tested. The interaction between nutrient concentration and light intensity was studied by growing-limited continuous cultures at the same dilution rate, but different irradiances. Relative incorporation into protein was highest in cultures grown at 20 μE · m^-2· s^-1, in which the relative growth rate was 0.4. These results suggest that photosynthetic carbon metabolism may respond to relative growth rate μ/μ_max rather than to growth rate directly.     

Effect of temperature on photosynthesis-light response and growth of four phytoplankton species isolated from a tidal freshwater river

Three cyanobacteria (Microcystis aeruginosa Kütz. emend. Elenkin, Merismopedia tenuissima Lemmermann, and Oscillatoria sp.) and one diatom (Aulacoseira granulata var. angustissima O. Mull. emend. Simonsen) were isolated from the tidal freshwater Potomac River and maintained at 23° C and 40 μmol photons·m^?2·s^?1 on a 16:8 L:D cycle in unialgal culture. Photosynthetic parameters were determined in nutrient‐replete cultures growing exponentially at 15, 20, 25, and 30° C by incubation with ¹⁴C at six light levels. P ^B_max was strongly correlated with temperature over the entire range for the cyanobacteria and from 15 to 25° C for Aulacoseira, with Q₁₀ ranging from 1.79 to 2.67. The α values demonstrated a less consistent temperature pattern. Photosynthetic parameters indicated an advantage for cyanobacteria at warmer temperatures and in light‐limited water columns. P ^B_max and I_k values were generally lower than comparable literature and field values, whereas α was generally higher, consistent with a somewhat shade acclimated status of our cultures. Specific growth rate (μ), as measured by chlorophyll change, was strongly influenced by temperature in all species. Oscillatoria had the highest μ at all temperatures, joined at lower temperatures by Aulacoseira and at higher temperatures by Microcystis. Values of μ for Aulacoseira were near the low end of the literature range for diatoms consistent with the light‐limited status of the cultures. The cyanobacteria exhibited growth rates similar to those reported in other studies. Q₁₀ for growth ranged from 1.71 for Aulacoseira to 4.16 for Microcystis. Growth rate was highly correlated with P ^B_max for each species and the regression slope coefficients were very similar for three of the species.     

Meteorological drivers of the dynamics of autotrophic picoplankton

1. The tiny non‐motile autotrophic picoplankton (APP; size range 0.2–2 μm) occur in all types of aquatic habitats and are comprised of prokaryotic as well as eukaryotic taxa. In the Boreal Zone, the majority of lakes have high concentrations of coloured humic substances that can adversely affect lake light climate and cause steep summertime stratification resulting in epilimnetic nutrient depletion. APP are more effective in nutrient and light acquisition than larger phytoplankton and thus should be competitive in humic lakes. 2. Most lacustrine APP studies have been based on short sampling periods, and thus, interannual variation and its drivers are still unclear. We studied APP in the small, boreal, humic Lake Valkea‐Kotinen during five open‐water periods in 2002–06 to determine interannual variation and the importance of meteorological drivers for APP dynamics. 3. Total APP showed a bimodal annual pattern, but the timing and vertical location of the two maxima varied during the study. In general, APP thrived in warm water and the most important abiotic factor controlling APP was stability of the water column (N_s). On average, 82% of APP were found in the epilimnion or metalimnion during summertime stratification. 4. There was niche separation of APP and larger phytoplankton in the lake because, with only one exception, APP maxima occurred separately from the maxima of larger phytoplankton. 5. Two groups, solitary eukaryotic APP and colonial picocyanobacteria (Merismopedia warmingiana), responded differently to the abiotic factors. Solitary APP preferred high water colour and low pH, both of which occurred after heavy rain, whereas colonial APP did not fare well when water colour was high. Our findings suggest that when future climate change‐related processes increase incoming allocthonous organic matter load from the catchment, solitary eukaryotic APP will be favoured.     

Verteilung und Aufteilung der DNS bei einigen Cyanophyceen,festgestellt durch ihre DAPI-Fluoreszenz

The DNA of Blue-Green Algae may be specifically and most efficiently elucidated by the combined use of the fluorochrome DAPI and DNAse treatment. InChroococcus turgidus andSynechococcus aeruginosus it forms a network which usually is positioned at the periphery of the centroplasm. At the beginning of cell division it seems to be invaginated by the ingrowing cell wall. Later it must be distributed by other means because two equal parts are connected by a number of fine and straight DNA threads. InOscillatoria limosa usually all parts of the centroplasm are interspersed with the DNA network. In surface view, the DNA occasionally seems to consist of a number of independant rods, but in reality these are connected to the general network. Division apparantly occurs by ingrowth of the crosswall and usually results in equal daughter nuclear equivalents although occasionally they are unequal. Four strains ofMicrocoleus vaginatus were found to have nuclear equivalents of the same general appearance as inOscillatoria limosa.

     

Phenotype variability of identical genotypes: the need for a combined approach in cyanobacterial taxonomy demonstrated on Merismopedia-like isolates

Five Merismopedia-like cyanobacterial strains were collected from microbial mats at Norderney Island, subcultured in the laboratory, and finally grown as unicyanobacterial cultures. As a sixth strain, Merismopedia glauca from the ?Sammlung von Algenkulturen“ at Göttingen (SAG) was used for comparisons. According to morphological and physiological characteristics initially observed in the field and during initial subculturing, the five strains were assigned to the species Merismopedia glauca, Merismopedia punctata, or Merismopedia elegans. However, after prolonged maintenance under laboratory conditions, the formation of platelet-like colonies stopped, whereas cell sizes, production of extracellular polymeric substances, and division patterns were stably maintained. These physiological and morphological parameters allowed us to divide the six strains into two clusters. This division was further supported by the profiling of total cell protein and phycobilisomes using SDS-PAGE. The nearly complete 16S rDNA sequence of three of the six isolates was determined. The comparative sequencing analysis revealed an almost 100% identity of these three Merismopedia-like strains. The evolutionary distance dendrogram constructed placed this Merismopedia cluster into a common line of descent with Synechocystis sp. strain PCC6906. Based on the analysis of common stretches of 1,050 nucleotides, the overall similarity between the sequence types of ?Merismopedia“ and ?Synechocystis“ is 96–97%. The values of the different methods for taxonomic classification of unicyanobacterial strains, the relationship of the cyanobacterial genera Merismopedia, Synechococcus, Synechocystis, and Eucapsis sp., and the functional role of different Merismopedia morphologies within microbial mats are discussed. It is suggested that all analyzed Merismopedia strains be combined into one species, namely Merismopedia punctata Meyen (1839).