Precipitação de CaCO3 em algas marinhas calcárias e balanço de CO2 atmosférico: os depósitos calcários marinhos podem atuar como reservas planetárias de carbono?

Abstract

CaCO3 precipitation in calcareous seaweeds and the atmospheric CO2 concentration: can the marine calcareous deposits act as a planetary carbon sink? Human activities over the last two centuries provoked an increase in the concentration of carbon dioxide (CO2) in the atmosphere. One of the effects of this increase is an increment in the primary production of some terrestrial plants which in this way act carbon sinks. In the oceans the CO2 is stored mainly as calcium carbonate (CaCO3). An increase in the concentration of CO2 leads to seawater acidification and to a decrease in carbonate ions availability. Calcareous organisms, in principle, does not act as carbon sinks because the calcification process produces CO2. However, the majority of CO2 measurements in marine communities neglect the calcareous seaweed formations. In calcareous seaweeds the CO2 produced during the calcification process is utilized by photosynthesis. Moreover, increases in the CO2 concentration can lead to an increase in growth rates, although these rates can be negatively affected by a decrease in the availability of carbonate ions. The objective of this work is to test if an increase in the CO2 concentration, and seawater acidification, affects the growth rate of marine calcareous seaweeds. Information on seaweed calcification is scarce, specially on the non-articulated coralline algae, which are the main group of calcareous seaweeds. This group is difficult to work with in laboratory, and therefore the first stage of the project was dedicated to establish seaweed cultures and work methodologies. We tested different calcareous seaweeds including non Corallinaceae, articulated and non articulated coralline algae. Calcification presents a constant relation with growth and is higher on non articulated corallines, what compensates their slow growth. For the experiments of CO2 injection we utilizes vials of 50 mL with 35 mL of MCM artificial medium, irradiance of 42 μmol·m·s, temperature of 25°C and one week incubation time. The experiments of CO2 enrichment were made with nodules of Lithophyllum sp. incubated after bubbling different volumes of CO2. To isolate the acidification effect from the effect of CO2, injections were made in media with, and without, addition of CaCO3 as buffer. Incubations in medium acidified with HCl were also performed to verify the effect of acidification independent of CO2 increment. The growth was estimated by calcification, measured trough the alkalinity anomaly technique. For the buffered medium, injections of gradually higher volumes of CO2 led to a proportional increase in growth rates until the limit of 0.2 L; above this limit the growth rates remained the same, regardless the increase of CO2. This limit is probably due to saturation of the medium and not of the seaweed, once the bubbling was limited only to the beginning of the incubation time. In the groups without buffering, small volumes of CO2 (0.06 L) resulted in an increase in the growth rate. Volumes slightly higher (0.2 L) resulted in a decrease in the growth rate. In higher volumes seaweed began to die. These results are consistent with those obtained by acidifying with HCl. Lithophyllum sp. presented no variation in growth rate between pH 8.0 and 7.5. A decrease in growth rates occurred at pH 7.0, and death occurred at pH below 6.5. The results are not conclusive due to methodological limitations, but indicate that Lithophyllum sp., and probably other coralline algae, can act as carbon sinks under certain pH values. Due to the utilization of a carbonate rich artificial culture medium the observed pH 7.0 limit for growth must be higher in natural environment. More extensive work is necessary to understand the role of these seaweeds on the global atmospheric CO2 increase, for which the techniques and procedures described in this work can be utilized.