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

Abstract

Mäerl/rhodolith beds are protected habitats that may be affected by ocean acidification (OA), but it is still unclear how the availability of CO 2 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 20 months) effects of OA on the production and calcification of the most common mäerl species of southern Portugal, Phymatolithon lusitanicum. Both the photosynthetic and calcification rates increased with CO 2 after the first 11 months of the experiment, whereas respiration slightly decreased with CO 2. After 20 months, 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 CO 2 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.

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

“…In a previous long‐term experiment with Phymatolithon lusitanicum (Sordo et al, ), the authors also observed no effect of high CO 2 under low irradiance (8 μmol photons m −2 s −1 ). However, when irradiance was increased during the short‐time incubations (to a maximum of 200 μmol photons m −2 s −1 ), the differences among treatments increased proportionally to CO 2 concentration.…”

“…Even if values close to 50 μmol photons m −2 s −1 were occasionally recorded on the field, most of the time these algae are exposed to low light and temperature conditions. In a previous long‐term experiment with P. lusitanicum , Sordo et al () observed a loss of pigmentation, especially accentuated at the acidified algae, but only after 20 months of high CO 2 conditions. During these experiments, algae were cultivated at low light and temperature conditions, and these conditions increased their resilience to a long‐term exposure to high CO 2 .…”

“…Previous research found that under high CO 2 , algae survived by increasing their calcification rates to compensate the dissolution that occurs during the night (Kamenos et al, ). Even if acidified algae calcify more to compensate the drop of pH, they are still under a constant dissolution/calcification process and in the long term are expected to grow less (Dupont & Pörtner, ; see Sordo et al, ). Kamenos et al () also found that when this pH change occurs abruptly there is an additional weakening of the calcite skeleton.…”

“…The experimental setup used in this experiment is largely based on the one described in Sordo Santos Reis Shulika and Silva () and Sordo et al (), where CO 2 is controlled via direct analysis of p CO 2 in seawater. Following the recommendations of Cornwall and Hurd (), the system was upgraded to eliminate any concerns of pseudoreplication.…”

“…Currently, there is no information on the effect that a short‐term increase of temperature could have on the metabolism of P. lusitanicum under high CO 2 conditions. So far, only two studies (Sordo, Santos, Barrote, & Silva, ; Sordo, Santos, Reis, Shulika, & Silva, ) have addressed the effect of high CO 2 on the recently identified new species Phymatolithon lusitanicum , the most common species found in the maërl beds from southern Portugal (Peña et al, ). The objective of this work is to investigate the short‐term effect of high temperature on the photosynthesis, respiration, and calcification of the free‐living coralline alga Phymatolithon lusitanicum under a future high CO 2 scenario.…”