Susana Enríquez

Patterns in decomposition rates among photosynthetic organisms : the importance of detritus C:N:P content

The strength and generality of the relationship between decomposition rates and detritus carbon, nitrogen, and phosphorus concentrations was assessed by comparing published reports of decomposition rates of detritus of photosynthetic organisms, from unicellular algae to trees. The results obtained demonstrated the existence of a general positive, linear relationship between plant decomposition rates and nitrogen and phosphorus concentrations. Differences in the carbon, nitrogen, and phosphorus concentrations of plant detritus accounted for 89% of the variance in plant decomposition rates of detritus orginating from photosynthetic organisms ranging from unicellular microalgae to trees. The results also demonstrate that moist plant material decomposes substantially faster than dry material with similar nutrient concentrations. Consideration of lignin, instead of carbon, concentrations did not improve the relationships obtained. These results reflect the coupling of phosphorus and nitrogen in the basic biochemical processes of both plants and their microbial decomposers, and stress the importance of this coupling for carbon and nutrient flow in ecosystems.

Scaling maximum growth rates across photosynthetic organisms

1. This study provides evidence for the existence of general intrinsic patterns controlling the maximum growth rate of photosynthetic organisms, based on a compilation of data from more than 120 reports, the species ranging from unicellular algae to trees and thick-leaved desert plants. 2. We used thickness and nutrient concentration of the photosynthetic tissue to scale differences in maximum growth rate among plants, based on the demonstrated importance of these plant traits in regulating the maximum growth rates of particular plant groups. 3. The growth rate declined with increasing thickness of the photosynthetic structures and increased with increasing nitrogen and phosphorus concentration. The strong dependence of growth rate and nutrient concentrations on tissue thickness reflect broad-scale patterns and not the adaptive response of individual or closely related species of similar tissue thickness to varying environmental conditions. 4. The scaling of maximum growth rate is similar to the scaling of metabolic rate to animal size. Thickness of the photosynthetic structures therefore plays an important role in the environmental control of plant performance and evolution, setting thresholds for the growth and productivity of photosynthetic organisms.

Reserve design for uncertain responses of coral reefs to climate change

Rising sea temperatures cause mass coral bleaching and threaten reefs worldwide. We show how maps of variations in thermal stress can be used to help manage reefs for climate change. We map proxies of chronic and acute thermal stress and develop evidence-based hypotheses for the future response of corals to each stress regime. We then incorporate spatially realistic predictions of larval connectivity among reefs of the Bahamas and apply novel reserve design algorithms to create reserve networks for a changing climate. We show that scales of larval dispersal are large enough to connect reefs from desirable thermal stress regimes into a reserve network. Critically, we find that reserve designs differ according to the anticipated scope for phenotypic and genetic adaptation in corals, which remains uncertain. Attempts to provide a complete reserve design that hedged against different evolutionary outcomes achieved limited success, which emphasises the importance of considering the scope for adaptation explicitly. Nonetheless, 15% of reserve locations were selected under all evolutionary scenarios, making them a high priority for early designation. Our approach allows new insights into coral holobiont adaptation to be integrated directly into an adaptive approach to management.

Light harvesting among photosynthetic organisms

Examination of light absorption by photosynthetic tissues of 156 specimens from 128 species of chlorophyll a containing photosynthetic organisms, ranging from single-celled cyanobacteria to trees, confirmed the existence of a universal law involving the asymptotic increase in light absorption with increasing chlorophyll a density. The chlorophyll a concentration of photosynthetic tissues decreased as the tissues become thicker, thereby avoiding high areal chlorophyll a density and inefficient light absorption. Light absorption per unit photosynthetic tissue weight was strongly, linearly related to the chlorophyll a concentration, explaining the lower growth rates and higher light requirements of thick, compared to thin photosynthetic organisms and the dominance of thin photosynthetic organisms in shaded environments (...)

Broad-scale comparison of photosynthetic rates across phototrophic organisms

We tested the existence of general patterns in the photosynthetic metabolism of oxygen-evolving organisms, based on a compilation of data for 315 species ranging from cyanobacteria to tree leaves. We used thickness and chlorophyll a concentration of the photosynthetic structure (cell, thallus, leaf) to scale differences in photosynthetic metabolism among plants, because of the demonstrated importance of these plant traits in regulating light absorption properties and photosynthetic rates of particular plant groups. We examined only the properties of the photosynthetic structure because this is the plant unit responsible for the photosynthetic process and thus is closely related to plant productivity, whereas there is a lack of general quantitative descriptors of the whole organism useful for such broad-scale comparisons, and few studies report net photosynthetic rates of whole organisms, including respiration rates of all non-photosynthetic structures. The results demonstrated that descriptors of plant metabolism such as maximum net photosynthesis, initial slope of the photosynthesis-irradiance (PI) curve and dark respiration display strong positive interrelationships. The metabolic rates declined with increasing thickness of the photosynthetic structures and more steeply for photosynthesis than respiration. Photosynthetic rates also changed with increment of volume of the photosynthetic structure resembling patterns that have been previously described for animal metabolism related to body weight. The strong relationship of metabolic rate and chlorophyll a concentration to the thickness of photosynthetic tissue reflects broad-scale patterns and not the adaptive response of individual or closely-related species of similar tissue thickness to varying environmental conditions. Thickness of the photosynthetic structures, therefore, plays an important role in the environmental control of plant performance and, consequently, it might have been an important driver of plant evolution, setting thresholds to the metabolism and productivity of phototrophic organisms.

Form-function analysis of the effect of canopy morphology on leaf self-shading in the seagrass Thalassia testudinum

The variation in seagrass morphology and the magnitude of leaf self-shading within the canopy of Thalassia testudinum, were compared among nine sites in a fringing reef lagoon. We found a significant variation in the growth-form of T. testudinum reflected in a 5.4-fold variation in the attenuation coefficient (Kd) within the canopy. The largest morphological variation was observed in shoot density. Leaf biomass, leaf area index (LAI), and shoot density were positively associated with canopy-Kd and with the percentage of surface irradiance received by the top of the seagrass canopy (% Es). These results provide an explanation for the consistent pattern of depth reduction in seagrass leaf biomass and shoot density reported in the literature. Shoot density and shoot size are two descriptors of the growth-form of T. testudinum related to its clonal life-form. Shoot size was not significantly correlated with canopy-Kd, nevertheless, it showed a significant effect on the slope of the relationship between shoot density and canopy-Kd. According to this model, shoot size also contributes to light attenuation within the seagrass canopy by increasing the effect of shoot density. This form-function analysis suggests that light may have a relevant role in the regulation of the optimal plant balance between horizontal (variation in shoot density) and vertical (variation in shoot size) growth of seagrasses. Other environmental factors and interactions also need to be examined to fully understand the mechanistic bases of the morphological responses of seagrasses to the environment.

Epiphyte Accrual on Posidonia oceanica (L.) Delile Leaves: Implications for Light Absorption

Abstract We examined the pattern of epiphyte accrual along the life-span of Posidonia oceanica leaves, both for the total epiphyte community and the main epiphyte groups (i. e. red encrusting algae and brown erect algae). Moreover, we document the importance of this epiphyte accrual pattern for evaluating P. oceanica-epiphyte interactions by assessing the dependence of the quantity and quality of light absorbed by epiphytes on their accrual pattern. Epiphyte biomass increased with leaf age following a sigmoidal curve (r2 = 0.90, P < 0.001), both for the epiphyte community and for the two main groups. Total epiphyte biomass increased with leaf age at a rate of about 0.03 day−1 to reach a constant maximum value of 2.60 mg DW cm−2 on leaves older than 200 days. Brown erect algae grew about an order of magnitude faster (0.120 day−1) than red encrusting ones (0.017 day−1). However, the former group of algae reached their maximum biomass (1 mg DW cm−2) on 150 days-old leaves, whereas red encrusting algae continued to grow along the whole leaf life-span to reach a maximum biomass of 1.70 mg DW cm−2. The non-linear increase in epiphyte biomass with leaf age involved a non-linear increase in epiphyte light absorption with leaf age, which reached a maximum constant value of 30% of incident light on 250 days-old leaves. Moreover, because red encrusting algae contribute a higher fraction to total epiphyte biomass on older leaves, we observed a shift in absorbed light quality with increasing leaf-age. Our results indicate the importance of accounting for the pattern of epiphyte accrual with leaf age when assessing seagrass-epiphytes interactions, especially for long-lived seagrass species where epiphytes may differ much in growth and biomass between young and old leaves.

Light absorption by marine macrophytes

Tissues of 338 marine macrophytes comprising 103 species, collected from the Atlantic, Mediterranean, South China, and Caribbean Seas, and encompassing a broad range in thallus form and pigmentation, were examined to quantify the importance of phylogenetic differences, spectral variability, and plant form and pigment content to account for differences in the absorption of light by marine macrophytes. Phylogenetic differences accounted for 2.5% of the variance in absorption observed, non-phylogenetic spectral differences being much larger (26%). Differences among individual specimens were much larger (72%), absorption at 675 nm increasing non-linearly as chlorophyll a density1/2, indicating that light absorption increases with increasing chlorophyll a density following a law of diminishing returns, as predicted by theory. The energy return per unit tissue produced (i.e. light absorption per unit plant weight) increased linearly with increasing chlorophyll a concentration. However, the light absorbed per unit weight decreased, for a given chlorophyll a concentration, as plant thickness increased. This indicates that while increasing thickness may increase chlorphyll a density and, hence, the light absorbed by marine macrophyte thalli, this strategy represents a burden limiting potential carbon turnover and plant growth. These results indicate that the diverse repertoire of light absorption by marine macrophytes can be adequately modeled as a continuum, dependent on plant thickness and pigment content, independent of phylogenetic differences.

The Use of the Fluorescence Signal in Studies of Seagrasses and Macroalgae

Seagrasses and macroalgae (seaweeds) are important primary producers and habitats in estuarine and marine intertidal and shallow benthic areas. Unlike terrestrial plants, aquatic macrophytes also show a wide diversity in photosynthetic and accessory pigment systems and chloroplast structure (Larkum and Vesk 2003; Larkum 2003). Furthermore, aquatic plants are exposed to a more variable light field than terrestrial plants with significant spectral changes with depth and water quality and, in shallow waters, extremely high light flashes due to the lensing effects of waves (Enriquez et al. 2002; Hanelt et al. 2003). Considering that the main structural and most likely functional diversity of photosynthetic mechanisms is present in the wide taxonomic diversity of the aquatic environment, the study of the fluorescence signal on marine macrophytes presents unique challenges and opportunities for the better understanding of how photosynthetic organisms utilize light.

Multiple light scattering and absorption in reef-building corals

We present an experimental and numerical study of the effects of multiple scattering on the optical properties of reef-building corals. For this, we propose a simplified optical model of the coral and describe in some detail methods for characterizing the coral skeleton and the layer containing the symbiotic algae. The model is used to study the absorption of light by the layer of tissue containing the microalgae by means of Monte Carlo simulations. The results show that, through scattering, the skeleton homogenizes and enhances the light environment in which the symbionts live. We also present results that illustrate the modification of the internal light environment when the corals loose symbionts or pigmentation.