Effect of temperature on photosynthesis, growth and calcification rates of the free-living coralline alga Lithophyllum margaritae

Abstract

Rhodolith beds are the dominant submerged calcifying aquatic vegetation in some coastal marine environments worldwide but few quantitative data are available regarding their physiology. In the Gulf of California (Mexico), Lithophyllum margaritae (Rhodophyta, Corallinaceae) is the most abundant nongeniculate, rhodolith-forming coralline species. Over their gulf-wide distribution, rhodoliths are exposed to a wide range of seasonal temperatures (~8–32ºC). The effect of changes in temperature on the photosynthetic and calcification rates of this species is unknown. We therefore evaluated the effect of temperature (10–30ºC) on the photosynthetic and calcification rates of L. margaritae rhodoliths in the lab and examined the effect of seasonal changes in temperature on growth rates in the field. Photosynthetic rates were evaluated polarographically and calcification rates were evaluated in the lab using both the buoyant weight technique and total alkalinity method, and in the field through alizarin staining. To the best of our knowledge, this is the first time that these three methods are used simultaneously to evaluate growth rates in coralline algae. Photosynthetic, calcification and growth rates showed wide fluctuations as a result of laboratory or field temperature. Photosynthetic (P_max) and respiratory rates both increased five-fold as incubation temperature increased to 25–30ºC. Similarly, calcification rates in the lab and growth rates in the field increased with higher temperatures. The lab data suggest that rhodolith growth is seasonally regulated by seawater temperature. The buoyant weight and total alkalinity techniques for determining calcification rate were comparable at low temperatures, but variability increased with temperature and this will be examined in further studies. Field growth rates, presented as apical tip extension, were significantly higher in summer (5.02 ± 1.16 mm yr^–1) than in winter (0.83 ± 0.16 mm yr^–1), supporting the lab results. The strong effects of temperature on photosynthetic, calcification and growth rates of Lithophyllum margaritae in the Gulf of California suggest that changes in sea surface temperature directly regulate bed production.