Ulothrix

Abstract: Photosynthesis-irradiance (P-I) characteristics of periphyton (microphytobenthos) have been considered primarily for entire assemblages How P-I responses vary with mat thickness and with community composition has not been considered in detail We used a combined approach of modeling, microscale determinations of photosynthetic rate and light attenuation, and whole-assemblage O 2 flux measurements to explore P-I relationships The modeling approach suggested that the onset of photosynthetic saturation and photoinhibition will occur at higher irradiance and that whole-mat photoinhibition (decreased photosynthesis at very high irradiance), biomass-specific maximum photosynthetic rate, and initial slope of the P-I function (a) should decrease as assemblage thickness increases or light attenuation increases Spherical light microsensor profiles for a variety of stream algae indicated a strongly compressed photic zone with attenuation coefficients of 70‐1791 m21 for scalar photosynthetic photon fluence density The O2 microelectrode measurements showed little if any photoinhibition at 2 and 4 mm depths in one filamentous green algal (Ulothrix) assemblage, with a relatively low attenuation coefficient, and no photoinhibition in a second Ulothrix community An assemblage dominated by a unicellular cyanobacterium exhibited little photoinhibition at 2 and 4 mm, and a dense cyanobacterial (Phormidium)/xanthophyte (Vaucheria) community exhibited no photoinhibition at all The microelectrode data revealed increases in a over several millimeters of depth (photoacclimation) These data supported the model predictions with regard to the effects of mat optical thickness on whole-assemblage values for a and photoinhibition Whole-community O2 flux data from 15 intact assemblages revealed positive relationships between chlorophyll a density and maximum photosynthetic rate or a expressed per unit area; the relationships with chlorophyll a were negative when photosynthetic rates were expressed per unit chlorophyll a None of the whole assemblages exhibited photoinhibition Thus, the data from the whole communities were consistent with model predictions

Abstract: (1) The algal colonization of rocky shores in the Firth of Clyde, Scotland, was investigated by both observations and experiments in nature, and by culture experiments in the laboratory. (2) The species whose propagules were most abundant in the water were also the commonest colonists of glass slides exposed on the shore, and patchiness in the distribution of the propagules in the water was reflected on the slides. Both the species-diversity of the colonists and the abundance of individual species were greater beneath a canopy of Ascophyllum nodosum than in uncovered sites, and at 2.49 m above Lowest Astronomical Tide level as compared with 2.89 m. Irrespective of the inoculum of propagules available, only species that normally inhabit the upper littoral zone successfully colonized slides exposed at that level. (3) Culture experiments in a simulated tidal regime revealed other factors that might influence algal settlement and development. For example, a period of unstressed conditions immediately following settlement was found to increase the degree of aerial exposure tolerated by young plants. High settlement densities enhanced this effect. (4) A thick turf of adult Enteromorpha intestinalis plants acted as a barrier, significantly reducing the number of spores of Ulothrix pseudoflacca reaching the substratum beneath. Conversely, it also enabled spores already established beneath the turf to withstand longer exposures to air than those unprotected by a moistureretaining canopy. (5) The different levels at which plants of Prasiola stipitata, Ulothrix pseudoflacca, Blidingia minima and Fucus spiralis could survive in the simulated tidal regime matched the normal vertical distribution pattern of these species on the shore.

Abstract: The photosynthetic performance of macroalgae isolated in Antarctica was studied in the laboratory. Species investigated were the brown algae Himantothallus grandifolius, Desmarestia anceps, Ascoseira mirabilis, the red algae Palmaria decipiens, Iridaea cordata, Gigartina skottsbergii, and the green algae Enteromorpha bulbosa, Acrosiphonia arcta, Ulothrix subflaccida and U. implexa. Unialgal cultures of the brown and red algae were maintained at 0°C, the green algae were cultivated at 10°C. IK values were between 18 and 53 μmol m−2 s−1 characteristic or low light adapted algae. Only the two Ulothrix species showed higher IK values between 70 and 74 μmol m−2 s−1. Photosynthesis compensated dark respiration at very low photon fluence rates between 1.6 and 10.6 μmol m−2 s−1. Values of α were high: between 0.4 and 1.1 μmol O2 g−1 FW h−1 (μmol m−2 s−1)−1 in the brown and red algae and between 2.1 and 4.9 μmol O2 g−1 FW h−1 (μmol m−2 s−1)−1 in the green algal species. At 0°C Pmax values of the brown and red algae ranged from 6.8 to 19.1 μmol O2 g−1 FW h−1 and were similarly high or higher than those of comparable Arctic-cold temperate species. Optimum temperatures for photosynthesis were 5 to 10°C in A. mirabilis, 10°C in H. grandifolius, 15°C in G. skottsbergii and 20°C or higher in D. anceps and I. cordata. P: R ratios strongly decreased in most brown and red algae with increasing temperatures due to different Q10 values for photosynthesis (1.4 to 2.5) and dark respiration (2.5 to 4.1). These features indicate considerable physiological adaptation to the prevailing low light conditions and temperatures of Antarctic waters. In this respect the lower depth distribution limits and the northern distribution boundaries of these species partly depend on the physiological properties described here.