Processes of calcification and sedimentation of the tropical marine green macro-alga genus Halimeda and effects of ocean acidification on its calcareous microstructure

Abstract

Calcifying green macro-algae of the genus Halimeda are common organisms in tropical shallow marine environments. These ramified benthic algae grow by forming successional segments that exhibit an internal skeletal microstructure of the calcium carbonate polymorph aragonite. The calcareous segments become part of the sediment after death. As macro-algae of the genus Halimeda often occur in large quantities and are able to build extensive bioherms, dropped segments from these algae are considered as an important source for carbonate sediments in many shallow water and coral reef environments. Thus detailed knowledge on the calcification of the alga is crucial for estimations on the carbonate budget and sediment dynamics of tropical settings, as this process directly determines the sediment contribution of Halimeda. Furthermore, it is a prerequisite when effects on the formation of its calcium carbonate microstructure under ocean acidification have to be assessed. In this study, internal microstructural features of segments from the species Halimeda opuntia, a cosmopolitan species of the genus Halimeda, are investigated using scanning electron microscopy. The first aim is to study the alga´s calcified microstructure in detail in order to be able to explain the formation of skeletal features in relation to known physiological processes of the alga. Thereby, lifetime primary cementation is identified to be an important process for calcium carbonate deposition in the algal segment. The second aim is to determine potential alterations in the formation of these microstructural features due to elevated seawater pCO2 and the corresponding shift in seawater carbon chemistry. Laboratory experiments show that especially the process of lifetime primary cementation is affected by elevated seawater pCO2. Based on the microstructural investigations, a theoretical model is developed on how physiological daytime and nighttime processes influence the formation of skeletal features in the genus Halimeda. The model also illustrates the basic relationships between changes in the seawater carbon chemistry and changes observed in the skeletal microstructure of the segment under elevated seawater pCO2. As a third objective, segments from living Halimeda and segments recovered from surface sediments are studied and compared using scanning electron microscopy to investigate the occurrence of post-sedimentary processes that alter the original skeletal microstructure. By the investigation of thin-sections of numerous sedimentary segments, species-specificity of Halimeda sediments is observed. Segments found in sediments predominantly originate from heavily calcified lithophytic species of the genus Halimeda, such as from the lineage Opuntia. Microstructural investigations also reveal that the process of lifetime primary cementation strongly determines the preservation potential of Halimeda segments in the sediment. Thus ocean acidification is assumed to impair both the alga´s environmental competitiveness (e.g., grazing protection, pathogen defense, structural integrity) and its carbonate sediment contribution to tropical coastlines and reef islands.