From thallus to communities: scale effects and photosynthetic performance in macroalgae communities

Abstract

Experiments on aquatic plant photosynthesis are often performed on spatial scales much smaller than natural systems. The photosynthesis-irradiance relationship (P-I curve) depends, however, on whether photosynthesis is measured on leaves, shoots, plants or entire communities, so that results cannot be reliably extrapolated from the limited spatial scale of experiments to the larger scale of natural systems. To evaluate how the photosynthetic response changes with spatial scale for aquatic plants, we measured the photosynthetic production as a function of irradiance for thallus pieces, single-species and multi-species communities of 4 different marine algae (Fucus serratus, Chordaria flagelliformis, Enteromorpha sp. and Ahnfeltia plicata). Thallus pieces followed the wellknown hyperbolic shape of P-I curves; that is, the community P-I curves approached linearity. Thallus pieces, thus, saturated at irradiances below 160 µmol photons m -2 s -1, while the communities started to saturate at values 4 times higher and never became fully saturated even at 1750 µmol photons m -2 s -1. This increasing linearisation of the P-I relationship for communities influenced the photosynthetic parameters, as maximum gross photosynthesis and compensation irradiances increased 3 to 8 times. The increasing linearisation of the P-I curves is attributed to the 3-dimensional structure of the communities, since communities with a low extinction per unit leaf area, and thus an even distribution of irradiance among thallus parts, used their potential photosynthesis capacity more efficiently. Although not significantly different, multi-species communities had a slightly more linear response and a higher gross photosynthesis than single-species communities, suggesting a more efficient distribution and photosynthetic use of light among the photosynthetic tissue. An increase in spatial scale from thallus pieces to communities decreased variation in photosynthetic rates between species, implying that differences between species are much smaller on natural scales than experiments on single thallus pieces suggest. The findings imply that experiments on the thallus scale should be avoided or treated with caution when predicting the ecological performance of plants in their natural habitat.KEY WORDS: Community · Photosynthesis · Production · Macroalgae · IrradianceResale or republication not permitted without written consent of the publisher

Realistic changes in seaweed biodiversity affect multiple ecosystem functions on a rocky shore

“…which is an index of light use efficiency and represents the initial slope of the photosynthesis-irradiance curve (Jassby andPlatt 1976, Binzer andMiddelboe 2005). We focused our analyses on maximum nitrate uptake and photosynthetic rates (i.e., V max and P max ), and nitrate uptake efficiency and light use efficiency values (i.e., a N and a P ).…”

High pH in shallow-water macroalgal habitats

“…We do not, however, know how general the ability to acclimate to high pH is or the temporal aspects of acclimatization. (2) Recent studies have shown that measurements on thallus pieces cannot always be extrapolated to whole macroalgal communities (Binzer & Middelboe 2005) because large parts of the biomass in a community are continuously light-limited. Measurements of photosynthesis on a more relevant ecological scale (e.g.…”

Primary productivity of intertidal macroalgal assemblages: comparison of laboratory and in situ photorespirometry

“…Bracken et al 2008). Levels of primary production seen in diverse communities indicate that photosynthesis does not show a typical saturation curve, as is usually seen in productivity-irradiance curves for single species (Lobban et al 1985), but increases in a more linear fashion (Binzer & Sand-Jensen 2002a, Binzer & Sand-Jensen 2002b, Middleboe & Binzer 2004Binzer & Middleboe 2005). In fact, primary production within diverse assemblages has been shown to increase in a relatively linear fashion with increasing irradiance up to maximum levels of natural irradiance (Middleboe & Binzer 2004).…”

“…Although both techniques are useful under certain circumstances, they have numerous shortcomings. Laboratory incubations often fail to scale up primary production to that which occurs in nature (Binzer & Middleboe 2005). Furthermore, laboratory incubations are performed under artificial conditions and often use excised tissue samples (Flores-Moya et al 1995, Gómez et al 1997), which do not necessarily give an accurate estimate of production by whole plants.…”