Seasonal Photosynthesis, Respiration, and Calcification of a Temperate Maërl Bed in Southern Portugal

Abstract

Rhodolith (maërl) beds are biodiversity hotspots with a worldwide distribution. Maërl is the general term for free-living non-geniculate rhodoliths or coralline red algae. In southern Portugal, maërl 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 maërl 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 • C in winter and 24 • C in summer, irradiance varied from 5 to 75 µmol 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 µmol CaCO 3 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 • C to 26 • C). Phymatolithon lusitanicum photosynthesis increased twofold after a threshold of 18 • C (from 2.2 at 18 • C to 3.87 µmol O 2 m −2 s −1 at 20 • C), whereas respiration increased fourfold with temperature after a threshold of 22 • C (from −0.38 at 18 • C to −1.81 µmol O 2 m −2 s −1 at 24 • 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.

Bottom Trawling Threatens Future Climate Refugia of Rhodoliths Globally

“…Our models suggest that range contractions will intensify under the combination of high temperatures and ocean acidification (e.g., in the Gulf of Mexico and the Caribbean Sea), corroborating previous experimental evidence with temperate (Noisette et al, 2013;Qui-Minet et al, 2019;Sordo et al, 2020) and tropical populations (Steller et al, 2007). Even in colder regions, such as the North Sea, the anticipated increase in acidification and temperature regimes is expected to result in range contraction (Supplementary Figure 5).…”

Reduced physiological performance in a free-living coralline alga induced by salmon faeces deposition

“…Many coralline algae are low-light adapted and can even survive periods of prolonged darkness (Roberts et al 2002, Burdett et al 2012, Sordo et al 2020). In the coralline alga Phymatholithon calcareum, no deleterious effect was observed after being kept 4 wk in the dark, suggesting that this species is able to survive several months in the darkness (Wilson et al 2004).…”

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

“…In a previous study (Sordo et al, in press), the environmental conditions at the studied maërl bed (located in Armação de Pêra, southern Portugal, at 22 m depth) were measured in a continuous basis during more than 2 years. In this period, temperature ranged from 14 to 17°C and irradiance from 6 to 67 μmol m −2 s −1 .…”

“…The information available on this geographical area is restricted to descriptive studies on the morphology and composition of maërl beds (Carro, López, Peña, Bárbara, & Barreiro, ; Peña et al, ) and its associated flora (Peña & Bárbara, ), plus a study where the metabolic rates of the algae were assessed at different temperatures, and the abiotic conditions of the beds were monitored for a 2‐year period (Sordo, Santos, Barrote, Freitas, & Silva, in press). Even if the studied maërl bed is under relatively stable conditions, unusual high temperature and irradiance conditions were recorded during a sporadic heat wave.…”