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Temperature amplifies the effect of high CO2 on the photosynthesis, respiration, and calcification of the coralline algae Phymatolithon lusitanicum

Publication . Sordo, Laura; Santos, Rui; Barrote, Isabel; Silva, João

The combination of ocean acidification (OA) and global warming is expected to have a significant effect on the diversity and functioning of marine ecosystems, particularly on calcifying algae such as rhodoliths (maërl) that form extensive beds worldwide, from polar to tropical regions. In addition, the increasing frequency of extreme events, such as heat waves, threatens coastal ecosystems and may affect their capacity to fix blue carbon. The few studies where the simultaneous effects of both temperature and CO2 were investigated have revealed contradictory results. To assess the effect that high temperature spells can have on the maërl beds under OA, we tested the short-time effects of temperature and CO2 on the net photosynthesis, respiration, and calcification of the recently described species Phymatolithon lusitanicum, the most common maërl species of southern Portugal. Photosynthesis, calcification, and respiration increased with temperature, and the differences among treatments were enhanced under high CO2. We found that in the short term, the metabolic rates of Phymatolithon lusitanicum will increase with CO2 and temperature as will the coupling between calcification and photosynthesis. However, under high CO2, this coupling will favor photosynthesis over calcification, which, in the long term, can have a negative effect on the blue carbon fixing capacity of the maërl beds from southern Portugal.

2019Journal article

Open Access

Mäerl calcification, photosynthesis and respiration in an acidified ocean

Publication . Nieves, Laura Sordo de las; Silva, João Miguel Sousa da; Coelho, Rui; Barrote, Isabel

With the increase of atmospheric CO2 and the associated acidification of the oceans, rhodolith (mäerl) beds are under severe threat. The general lack of consensus regarding the foreseeable effects of Ocean Acidification (OA) on coralline algae is largely due to the divergences of results obtained in different scientific experiments. These divergences may be related to differences in temperature, irradiance, CO2 levels, time of exposure and also on technical difficulties concerning the experimental methodologies used. This thesis aimed to determine the photosynthesis, calcification and respiration rates of the mäerl species Phymatolithon lusitanicum under natural conditions in Southern Portugal and to assess the effect that ocean acidification will have on these processes under different irradiances, temperatures, CO2 concentrations and times of exposure. Dark respiration and photosynthesis increased with temperature in summer and spring and decreased in winter and autumn while calcification rates did not change seasonally. A direct CO2 control system was developed and found to be reliable to assess the short and long term effect of OA on coralline algae. In the short term, photosynthesis and calcification increased with CO2 and temperature, but after prolonged exposure this pattern was reversed and algae exposed to high CO2 showed lower photosynthetic and calcification rates and accumulated growth with respect to control algae, effects that were enhanced with increasing irradiances. Dark respiration was unaffected by CO2 but increased with temperature. The results suggest that temperature, irradiance, CO2 level and time of exposure are determinant factors in ocean acidification experiments with coralline algae. Both temperature and high light intensified the effect of high CO2 on Phymatolithon lusitanicum and these will be determinant factors on the long-term resilience of Lusitanian rhodolith beds to OA.

2017-10-19Doctoral thesis

Open Access

High CO2 decreases the long-term resilience of the free-living coralline algae Phymatolithon lusitanicum

Publication . SORDO, LAURA; Santos, Rui; Barrote, Isabel; Silva, João

Maerl/rhodolith beds are protected habitats that may be affected by ocean acidification (OA), but it is still unclear how the availability of CO2 will affect the metabolism of these organisms. Some of the inconsistencies found among OA experimental studies may be related to experimental exposure time and synergetic effects with other stressors. Here, we investigated the long-term (up to 20months) effects of OA on the production and calcification of the most common maerl species of southern Portugal, Phymatolithon lusitanicum. Both the photosynthetic and calcification rates increased with CO2 after the first 11months of the experiment, whereas respiration slightly decreased with CO2. After 20months, the pattern was reversed. Acidified algae showed lower photosynthetic and calcification rates, as well as lower accumulated growth than control algae, suggesting that a metabolic threshold was exceeded. Our results indicate that long-term exposure to high CO2 will decrease the resilience of Phymatolithon lusitanicum. Our results also show that shallow communities of these rhodoliths may be particularly at risk, while deeper rhodolith beds may become ocean acidification refuges for this biological community.

2018-05Journal article

Open Access

Seasonal photosynthesis, respiration, and calcification of a temperate Maërl bed in southern Portugal

Publication . SORDO, LAURA; Santos, Rui; Barrote, Isabel; Freitas, Cátia; Silva, João

Rhodolith (maerl) beds are biodiversity hotspots with a worldwide distribution. Maerl is the general term for free-living non-geniculate rhodoliths or coralline red algae. In southern Portugal, maerl beds are mainly composed of Phymatolithon lusitanicum, recently identified as a new species and commonly misidentified as Phymatolithon calcareum. Photosynthesis, respiration, and growth rates of the algae were measured seasonally, as well as the photosynthetic pigment composition. To characterize the seasonal and interannual patterns of key abiotic conditions in the largest described maerl bed of the Portuguese coast, temperature, irradiance, and dissolved oxygen were continuously monitored over a 2-year period. At the bed depth (22 m), temperature ranged between 14 degrees C in winter and 24 degrees C in summer, irradiance varied from 5 to 75 mu.mol m(-2) s(-1) , and dissolved oxygen from 5.8 to 7.25 mg O-2 L-1. We found a strong linear relationship (r(2) = 0.95) between gross primary production (GPP) and relative electron transport rates (rETRs). Both methods led to similar results and an average molar ratio of 0.24. Photosynthesis and respiration increased in summer and decreased in autumn and winter. In the summer of 2013, the growth rates were twofold higher (1.34 mu.mol CaCO3 g(-1) day(-1)) than in the other seasons. In winter and spring, to compensate for light deprivation and low temperature, algae increased their chlorophyll a and carotenoid concentrations while also decreasing their phycobilin concentration, in this case probably due to nutrient limitation. To isolate the role of temperature on the algae's metabolism, the photosynthetic and respiration rates of individual thalli were measured at eight different temperatures in the laboratory (from 12 degrees C to 26 degrees C). Phymatolithon lusitanicum photosynthesis increased twofold after a threshold of 18 degrees C (from 2.2 at 18 degrees C to 3.87 mu mol O-2 m(-2) s(-1) at 20 degrees C), whereas respiration increased fourfold with temperature after a threshold of 22 degrees C (from -0.38 at 18 degrees C to -1.81 (mu mol O-2 m(-2) s(-1) at 24 degrees C). The significant increases on respiration, photosynthetic rates, and maximum growth with temperature reveal that the metabolic rates of P. lusitanicum are highly sensitive to ocean warming.

2020Journal article

Open Access

A direct CO2 control system for ocean acidification experiments: testing effects on the coralline red algae Phymatolithon lusitanicum

Publication . Sordo, L.; Santos, Rui; Reis, João; Shulika, Alona; Silva, João

Most ocean acidification (OA) experimental systems rely on pH as an indirect way to control CO2. However, accurate pH measurements are difficult to obtain and shifts in temperature and/or salinity alter the relationship between pH and pCO(2). Here we describe a system in which the target pCO(2) is controlled via direct analysis of pCO(2) in seawater. This direct type of control accommodates potential temperature and salinity shifts, as the target variable is directly measured instead of being estimated. Water in a header tank is permanently re-circulated through an air-water equilibrator. The equilibrated air is then routed to an infrared gas analyzer (IRGA) that measures pCO(2) and conveys this value to a Proportional-Integral-Derivative (PID) controller. The controller commands a solenoid valve that opens and closes the CO2 flush that is bubbled into the header tank. This low-cost control system allows the maintenance of stabilized levels of pCO(2) for extended periods of time ensuring accurate experimental conditions. This system was used to study the long term effect of OA on the coralline red algae Phymatolithon lusitanicum. We found that after 11 months of high CO2 exposure, photosynthesis increased with CO2 as opposed to respiration, which was positively affected by temperature. Results showed that this system is adequate to run long-term OA experiments and can be easily adapted to test other relevant variables simultaneously with CO2, such as temperature, irradiance and nutrients.

2016-09Journal article

Open Access