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Autochthonous calcite precipitation is an important process for C-fixation in hard-water lakes, which is mainly induced by the photosynthesis of planktonic microorganisms. Among these, the widespread calcifying green alga Phacotus lenticularis (Ehrenberg) Diesing contributes to biogenic calcite precipitation in temperate regions. Its role in carbonate precipitation needs to be investigated, because there are no studies dedicated to the quantitative contribution of Phacotus shells to long term carbonate sequestration in hard-water lake sediments. In order to fill this gap, the Phacotus shell content in the sediments of four German hard-water lakes was determined and compared to the fraction of Phacotus shells in the total suspended autochthonous calcite of the euphotic zone.It was found that the Phacotus shells contributed at least 10% to the autochthonous carbonate precipitation in the upper water column in three investigated lakes. During a Phacotus mass occurrence with a cell density of 1.8 × 106 Ind L-1 in Lake Hopfensee, even 59% of the 3.6 mg L-1 total carbonate concentration consisted of Phacotus shells. In contrast to this high amount, the topmost basinal sediment contained a Phacotus shell content between 80 and 36,252 individuals per mg dry sediment, representing only 0.02% to 2.28% of the total carbonate sediment content. In a gravity core from Lake Grosser Ostersee, dating back ~150 years, the Phacotus shell content was continuously below 0.24% whereas the shell diameters remained equal to those of living individuals found in the water column proving that Phacotus shells are capable to persist in the sediment after deposition.A main reason for the large discrepancy between Phacotus shell abundance in the euphotic zone and in sediment was found to be the gross authigenic carbonate precipitation, which dilutes the sedimenting Phacotus shells that accumulate exclusively during short and intensive population peaks in summer. Additionally, dissolution of the carbonate shells during sedimentation was proven to be a relevant factor in Lake Igelsbachsee by means of reducing the number of Phacotus shells reaching the lake bottom. These facts explain that short-term high Phacotus carbonate contents of the total suspended carbonate in the water column do not mirror the contribution of Phacotus shells in the sedimentary record.

(2000). Phacotus lenticularis (Chlorophyta) population dynamics in both nature and culture. SIL Proceedings, 1922-2010: Vol. 27, No. 2, pp. 700-703.

In north-eastern German hardwater lakes, the unicellular flagellate Phacotus lenticularis (Chlorophyta, Chlorophyceae, Chlamydomonadales) occurs regularly in mass developments during the summer months. The most striking characteristic of this chlamydophycean alga is the presence of a calcified two-shelled lens-like lorica. The morphology of P. lenticularis loricae in 10 lakes was studied by scanning electron microscopy. Four lorica morphotypes could be discriminated; several were observed together even within a single water sample. We developed an artificial culture medium that imitated the chemical conditions of hardwater lakes to such an extent that calcified loricae were formed that were highly similar to those found under natural conditions. Phacotus strains from three of the lakes were studied with respect to their calcification patterns under defined culture conditions. Each strain produced a lorica morphology that corresponded to one of those occurring in the lake from which it had been is...

Summary A green flagellate of the genus Phacotus with a spherical calcite-lorica was found in field collections from limerich waters in the Baltic Lake District of the G.D.R. LM observations showed wide coincidence with Phacotus lenticularis (EHRENBERG) STEIN var. sphaerica WISLOUCH, described from Poland. SEM examinations demonstrated the autonomous character of this taxon for which the new status Phacotus sphaericus (WISLOUCH) GIERING is suggested.

Summary Envelopes, loriea and cell walls of a number of freshwater organisms from the vicinity of Hamburg (F.R.G.) were assayed for Fe and Mn mineralization by laser microprobe mass analysis(LAMMA) and energy dispersive X-ray spectroscopy. The organisms investigated were: (1) Leptothrix ochracea (Chlamydobacteriales), (2) Anthophysa steinii and (3) Chrysococcus rufescens (Chrysophyceae), (4) Trachelomonas (T.) volvocina, (5) T. volvocinopsis, (6) T. hispida, (7) T. lefevrei, (8) T. spec. and (9) Strombomonas subcurvata (Euglenophyceae), (10) Phacotus lenticularis (Chlorophyceae), (11) Helicodendron spec. (Hyphomylcetes). Three types of mineralization could be distinguished: T. volvocina, T. volvocinopsis and T. hispida showed only Fe incrustation, whereas most organisms investigated accumulated both Fe and Mn into their mineralized parts. ThP quotient Fe: Mn differed considerably within a given species, even if its individuals were derived from the same water sample. Strombomonas subcurvata so far showed only Mn incrustation of its lorica. All three types of mineralization may be found within thr same water sample. It is therefore concluded that Fe and Mn mineralization does not solely depend upont the chemistry of the water.

The biogenic carbonate precipitation by the freshwater alga Phacotus lenticularis may play a role in long-term carbon (C) fixation that has not yet been quantified. This is partly due to the absence of a standardised methodology to representatively sample and assess the cell density and sedimentation of P. lenticularis in lakes. The objective of the present study was to define an adequate sampling methodology taking into account the spatiotemporal variation of P. lenticularis as well as the sedimentation and dissolution of calcite shells. Simultaneous measurements in three different lake sub-basins of Lake Grosser Ostersee, Germany, showed that the spatial cell density of P. lenticularis was similar in each sub-basin. At all sites, the vertical P. lenticularis cell density maxima corresponded with the slowly downshifting thermocline from depths of 2 to 6 m. During the entire growth period, composite samples from 0 to 7 m included 89% of the total P. lenticularis population. Lake bathymetry, as well as external factors like wind exposure, did not appear to affect the abundance of these calcifying algae. Sediment traps at a depth below the thermocline (at 7 m) and 1 m above the lake bottom recorded sedimentation rates of P. lenticularis shell halves (sh) between 1.1×106 and 1.1×108 sh m−2 d−1, while mean cell concentrations in the water column were between 1.1×108 and 1.7×109 shells per m². Sinking velocity ranged between 3 and 4 m d−1. Sediment from traps installed at a depth of 7 m did not reflect mean shell concentrations in the water column above. Dissolution of carbonates reduced the number of shells in sediment traps at the lake bottom and during the storage of samples. A laboratory experiment showed that even distilled water used for dilution during microscopic analysis led to dissolution of P. lenticularis shells. In conclusion, combined sampling of P. lenticularis from open water and sediment traps close to the lake bottom delivers a most representative assessment of biogenic carbonate precipitation. Due to dissolution effects, high temporal resolution along with appropriate sample preservation are crucial, whereas spatial representativeness was already achieved with low number of sampling sites per lake.