Experiment: Elevated CO2 levels affect the activity of nitrate reductase and carbonic anhydrase in the calcifying rhodophyte Corallina officinalis

The concentration of CO2 in global surface ocean waters is increasing due to rising atmospheric CO2 emissions, resulting in lower pH and a lower saturation state of carbonate ions. Such changes in seawater chemistry are expected to impact calcification in calcifying marine organisms. However, other physiological processes related to calcification might also be affected, including enzyme activity. In a mesocosm experiment, macroalgal communities were exposed to three CO2 concentrations (380, 665, and 1486 µatm) to determine how the activity of two enzymes related to inorganic carbon uptake and nutrient assimilation in Corallina officinalis, an abundant calcifying rhodophyte, will be affected by elevated CO2 concentrations. The activity of external carbonic anhydrase, an important enzyme functioning in macroalgal carbon-concentrating mechanisms, was inversely related to CO2 concentration after long-term exposure (12 weeks). Nitrate reductase, the enzyme responsible for reduction of nitrate to nitrite, was stimulated by CO2 and was highest in algae grown at 665 µatm CO2. Nitrate and phosphate uptake rates were inversely related to CO2, while ammonium uptake was unaffected, and the percentage of inorganic carbon in the algal skeleton decreased with increasing CO2. The results indicate that the processes of inorganic carbon and nutrient uptake and assimilation are affected by elevated CO2 due to changes in enzyme activity, which change the energy balance and physiological status of C. officinalis, therefore affecting its competitive interactions with other macroalgae. The ecological implications of the physiological changes in C. officinalis in response to elevated CO2 are discussed.

Experiment: Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels

Future atmospheric CO2 levels will most likely have complex consequences for marine organisms, particulary photosynthetic calcifying organisms. Corallina officinalis L. is an erect calcifying macroalga found in the inter- and subtidal regions of temperate rocky coastlines and provides important substrate and refugia for marine meiofauna. The main goal of the current study was to determine the physiological responses of C. officinalis to increased CO2 concentrations expected to occur within the next century and beyond. Our results show that growth and production of inorganic material decreased under high CO2 levels, while carbonic anhydrase activity was stimulated and negatively correlated to algal inorganic content. Photosynthetic efficiency based on oxygen evolution was also negatively affected by increased CO2. The results of this study indicate that C. officinalis may become less competitive under future CO2 levels, which could result in structural changes in future temperate intertidal communities.

Experiment: Ultraviolet radiation modulates the physiological responses of the calcified rhodophyte Corallina officinalis to elevated CO2

Ocean acidification reduces the concentration of carbonate ions and increases those of bicarbonate ions in seawater compared with the present oceanic conditions. This altered composition of inorganic carbon species may, by interacting with ultraviolet radiation (UVR), affect the physiology of macroalgal species. However, very little is known about how calcareous algae respond to UVR and ocean acidification. Therefore, we conducted an experiment to determine the effects of UVR and ocean acidification on the calcified rhodophyte Corallina officinalis using CO2-enriched cultures with and without UVR exposure. Low pH increased the relative electron transport rates (rETR) but decreased the CaCO3 content and had a miniscule effect on growth. However, UVA (4.25 W m-2) and a moderate level of UVB (0.5 W m-2) increased the rETR and growth rates in C. officinalis, and there was a significant interactive effect of pH and UVR on UVR-absorbing compound concentrations. Thus, at low irradiance, pH and UVR interact in a way that affects the multiple physiological responses of C. officinalis differently. In particular, changes in the skeletal content induced by low pH may affect how C. officinalis absorbs and uses light. Therefore, the light quality used in ocean acidification experiments will affect the predictions of how calcified macroalgae will respond to elevated CO2.

Respostas de curto período das características fotossintéticas da alga vermelha Batrachospermum delicatulum a temperatura e irradiância

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Distribution of stream macroalgae in the northwest region of Sao Paulo State, southeastern Brazil

Forty-four stream segments were sampled from May to October in 1992 and 1993 in the northwest region of São Paulo State, southeastern Brazil (19 degrees 45'-21 degrees 25'S, 49 degrees 05'-51 degrees 30'S). Thirty-six macroalgal subgeneric taxa were found and Chlorophyta was the dominant algal group (47% species), followed by Cyanophyta (33.5%), Rhodophyta (14%) and Chrysophyta (5.5%). The most widespread species were Stigeoclonium helveticum (25% sites), Batrachospermum delicatulum and Compsopogon coeruleus (20.5%). Distribution was patchy, with species number per sampling site ranging from 0 to six (3.1 +/- 1.7) and correlated positively with species abundance. Species cover ranged from 0 to 61% of the stream bottom (19.1 +/- 19.7%). Most sites (57%) were dominated by one or two macroalgae species. No significant difference was found between the frequency distribution of variables measured for streams and for total macroalgae, but the most widespread species differed for most parameters and occurred over wider ranges of environmental conditions. Mean species number and abundance were close to values found in distinct regions or biomes of North America. Higher conductance and lower oxygen values, as well as rocky substrata, generally constituted the most favourable combination of conditions for the development of macroalgae in the region. The pattern of strong dominance of few species was considered to be typical for stream macroalgal communities in general.

Photosynthetically active radiation during the experiment, and lipid content of sub- and intertidal specimens of the endemic Antarctic red alga, Palmaria decipiens

In coastal waters, Antarctic rhodophytes are exposed to harsh environmental conditions throughout the year, like low water temperatures ranging from -1.8°C to 2°C and high light during the summer season. Photosynthetic performance under these conditions may be affected by slowed down enzymatic reactions and the increased generation of reactive oxygen species. The consequence might be a chronic photoinhibition of photosynthetic primary reactions related to increased fragmentation of the D1 reaction centre protein in photosystem II. It is hypothesized that changes in lipid composition of biomembranes may represent an adaptive trait to maintain D1 turnover in response to temperature variation. The interactive effects of high light and low temperature were studied on an endemic Antarctic red alga, Palmaria decipiens, sampled from two shore levels, intertidal and subtidal, and exposed to mesocosm experiments using two levels of natural solar radiation and two different temperature regimes (2-5°C and 5-10°C). During the experimental period of 23 days, maximum quantum yield of photosynthesis decreased in all treatments, with the intertidal specimens exposed at 5-10°C being most affected. On the pigment level, a decreasing ratio of phycobiliproteins to chlorophyll a was found in all treatments. A pronounced decrease in D1 protein concentration occurred in subtidal specimens exposed at 2-5°C. Marked changes in lipid composition, i.e. the ratio of saturated to unsaturated fatty acids, indicated an effective response of specimens to temperature change. Results provide new insights into mechanisms of stress adaptation in this key species of shallow Antarctic benthic communities.

Chlorophyll and carotenoid binding in a simple red algal light-harvesting complex crosses phylogenetic lines

The membrane proteins of peripheral light-harvesting complexes (LHCs) bind chlorophylls and carotenoids and transfer energy to the reaction centers for photosynthesis. LHCs of chlorophytes, chromophytes, dinophytes, and rhodophytes are similar in that they have three transmembrane regions and several highly conserved Chl-binding residues. All LHCs bind Chl a, but in specific taxa certain characteristic pigments accompany Chl a: Chl b and lutein in chlorophytes, Chl c and fucoxanthin in chromophytes, Chl c and peridinin in dinophytes, and zeaxanthin in rhodophytes. The specificity of pigment binding was examined by in vitro reconstitution of various pigments with a simple light-harvesting protein (LHCaR1), from a red alga (Porphyridium cruentum), that normally has eight Chl a and four zeaxanthin molecules. The pigments typical of a chlorophyte (Spinacea oleracea), a chromophyte (Thallasiosira fluviatilis), and a dinophyte (Prorocentrum micans) were found to functionally bind to this protein as evidenced by their participation in energy transfer to Chl a, the terminal pigment. This is a demonstration of a functional relatedness of rhodophyte and higher plant LHCs. The results suggest that eight Chl-binding sites per polypeptide are an ancestral trait, and that the flexibility to bind various Chl and carotenoid pigments may have been retained throughout the evolution of LHCs.