Macroalgal photosynthetic responses to light in relation to thallus morphology and depth zonation

Abstract

We show how photosynthesis and UV sensitivity of algae are related to thallus morphology and depth distributions. This was studied for typical depth zonations of red and brown macroalgae in the Skagerrak (ca. 25 psu) and the Baltic Sea (6.5 psu). The algae were collected from the water surface down to 20.5 m of depth, whereby each species was sampled at its maximum abundance depth. Altogether, we measured photosynthetic and respiratory rates of 19 red and 13 brown algal species as O 2 evolution at different light intensities. Photosynthesis versus irradiance curves (PI curves) showed that light-saturated net photosynthetic rates (P max ), respiratory rates in darkness (R d ) and the initial slope (α) were strongly related to algal morphology with higher values for thinner species. The compensation irradiance (I c ) and saturating irradiance (I k ) were strongly related to water depth with lower values at greater depth. A novel approach to analyse PI data with principal component analysis (PCA) is presented. The method makes it possible to assign a quantitative morphological gradient to algal species based on photosynthetic properties. Such a gradient can be used in ecological studies as an alternative to more subjective discrete subdivisions into functional-form groups. Another type of PCA analysis, with the relative shapes of the PI curves as input data, summarises α and convexity but discards all interference of morphology. This results in a gradient of genuine physiological responses, which in our study was strongly correlated to maximum abundance depth. The UV sensitivity of the same 32 algal species was determined as the change in net O 2 evolution after exposure to UV light and the recovery after this treatment. Deeper-growing algae were more sensitive to UV and species with thinner thalli recovered better after UV treatment in the Skagerrak. No such trends were observed for the algae in the northern Baltic Sea, which suggests that no real deepwater species occur here. This is further supported by the lack of a clear pattern in I c and I k values with depth for the algae in the Baltic Sea. Our results advocate that the reduced species diversity of the Baltic Sea is also coupled to a loss of functional groups in the sense of general photosynthetic performance and not only in the sense of pure morphology (loss of canopy-forming species).

Understanding the intertidal zonation of two estuarine red macroalgae fromex situphotoacclimative responses

“…Thallus morphology is related to the photophysiological responses of species living in the intertidal zone (Markager & Sand-Jensen, 1996;Taylor et al, 1998;Johansson & Snoeijs, 2002;Runcie et al, 2003). In our study, the larger surface area of C. caespitosa might be advantageous at low light in terms of increased light absorption per unit biomass.…”

“…These parameters usually follow the distribution of macroalgae at different tidal levels, which occur at scales from microscale (Williams & Carpenter, 1990;Beach & Smith, 1996) to hundreds of metres (Lüning, 1981;Chapman & Johnson, 1990). Differences in photokinetic parameters have been identified between species (Lüning, 1981;Mathieson & Dawes, 1986;Henley & Ramus, 1989b;Johansson & Snoeijs, 2002) and within species (Coutinho & Yoneshigue, 1988;Silva et al, 1998;Ekelund et al, 2007). The light compensation point for growth (E c ) represents the minimum light requirements and sets the lowest depth limits for subtidal macroalgae (Mathieson & Norall, 1975).…”

Growth kinetics related to physiological parameters in young Saccorhiza dermatodea and Alaria esculenta sporophytes exposed to UV radiation

“…Macroalgae growing in shallower depth are often more tolerant to UV and recover well after periods of high UV radiation. Morphologically, species with tougher and thicker thalli may also be less sensitive to UVR as a result of more protective tissue (Johansson and Snoeijs 2002).…”

Photosynthesis and daily metabolic carbon balance of the invasive <i>Caulerpa racemosa</i> var. <i>cylindracea</i> (Chlorophyta: Caulerpales) along a depth gradient

“…Regarding depth (Raniello et al, 2006), changes in pigment composition were thought to represent algal photoacclimation responses in order to optimize light capture (increase in α) and photosynthetic performance (decrease in E k ) as light becomes limiting. These are common responses seen in some macroalgae species able to develop over broad depth gradients (Ramus et al, 1977;Markager and Sand-Jensen, 1992;Gómez et al, 1997;Johansson and Snoeijs, 2002). Nonetheless, the extent to which the ability of Mediterranean populations of C. racemosa to photoacclimate effectively is responsible for productivity and potential colonization success remains unknown.…”