Calcite production by the calcifying green alga Phacotus lenticularis

Abstract

The importance of carbonate precipitation by phytoplankton in fresh water lakes has not been sufficiently considered in global carbon cycles and climate change scenarios. The objective of this study was to determine the influence of the calcifying bivalved phytoflagellate Phacotus lenticularis (Ehrenberg) Deising 1866 on the total calcite precipitation in five European hard-water lakes. For this purpose, an accurate mass determination of single Phacotus lenticularis shells was required. We developed a novel methodological approach to precisely determine the volume and mass of the calcified shells. Focused ion beam (FIB) techniques were employed to investigate internal structural features. Thin layer cross-sections of the shell profiles were reproduced and perforation as well as the crystalline structure of the calcite plates were monitored. 3D-shell models were computed by 360° rotation of the shell cross-sections using a CAD 3D imaging software to calculate precise volumes and estimate realistic masses. In contrast to previous estimates, we determined a 2.8-fold higher shell mass of 0.86 ng CaCO 3 (SD=0.18) for the highly massive shells at a mean volume per individual of 334.1 µm³ (SD=70). An initial shell porosity of less than 5% was derived from thin layer crosssection images, resulting in a presumed mean shell density of 0.0026 ng µm . The shell diameter was significantly influenced by the lake's origin. The shells from each lake displayed substantial variations in diameter and shape. The pores in the shells showed two variations. Wider pore canals penetrated the whole shell wall, whereas small, elongated pores were located along the interspaces between calcite crystals with tabular habit. The approximate average dimensions of these calcite plates were 1.0×1.6×0.2 µm. The mean lateral wall thickness at the rim and centre of the shell were 1.98 µm (SD=0.42) and 0.79 µm (SD=0.17), respectively. The average carbonate precipitation by Phacotus lenticularis in relation to the total epilimnetic suspended calcite precipitation was 6% in the oligotrophic lake Großer Ostersee (Bavaria, Germany). During the growing season, Phacotus lenticularis contributed up to 21% of the particulate calcium carbonate in the epilimnion. These findings suggest that Phacotus lenticularis should be considered in the assessment of hard-water lake carbon cycling.

Seasonal deposition of authigenic calcite out of isotopic equilibrium with DIC and water, and implications for paleolimnological studies

“…Some of the identified phytoplankton taxa are known to be associated with calcite precipitation, e.g. picocyanobacteria, filamentous cyanobacteria, the diatom Stephanodiscus hantzschii Grunow and chlorophytes (Phacotus lendneri Chodat and representatives of the genus Chlamydomonas) (Stabel 1986;Kosamu and Obst 2009;Lenz et al 2018;Karami et al 2019), however the "vital effects" of particular species are not known and are beyond the scope of this study.…”

Representative monitoring of the calcifying alga <em>Phacotus lenticularis</em> (Chlamydophyceae) in lentic ecosystems

“…6b). This was considered as indicator for carbonate dissolution of Phacotus shells already in the water column (5), which was surprising since the conditions in the epilimnion of a hard-water lake, with a Ca 2+ ionic concentration of 37.6 mg L −1 , pH >8.6 (SD = 0.5) and temperature of 20.9°C (SD = 2.7°C) as described by Lenz et al (2018), did not indicate carbonate undersaturated conditions. These parameters are within acceptable ranges indicated for the occurrence of Phacotus in lakes (Gruenert et al, 2016;Schlegel et al, 2000;Schlegel, 2001).…”

“…Brown-glass flasks were previously prepared with neutral pH Lugol's solution so that a 2% (v/v) concentration would ensure preservation during sample storage, transport and post-sampling. Procedures for plankton analysis of the epilimnion followed the method of Lenz et al (2018). Phacotus shell concentration was determined according to Utermöhl (1958), using an inverted microscope (Leitz Labovert, Stuttgart, Germany) at 200-400x magnification.…”

“…Selected hard-water lakes in Bavaria, Germany, and geographic sampling positions at each lake's deepest point. (Lenz et al, 2018).…”

“…The abundance of Phacotus and its contribution to suspended autochthonous calcite content in hard-water lakes of northern Germany have been studied (Krienitz et al, 1993). Also well understood are the shell growth responses of Phacotus to meteorological changes (Gruenert and Raeder, 2014) and the systematic assessment of natural variations in the general size and particular calcite mass of Phacotus shells, in order to quantitatively determine its role in carbonate sedimentation in hard-water ecosystems (Lenz et al, 2018).…”