Radiocystis geminata – Kinneret Algae Atlas

Phylum: Cyanobacteriophyta
Class: Cyanophyceae
Order: Chroococcales
Habitat: planktonic
Distinctive features: colonies of spherical cells organized in radiating uniseriate rows extending from the center. No cyanotoxins are known for this species.
Organization: colonies
Color: blue-green
Cell shape: sphere
Colony shape: spherical (more or less)
Cell diameter (D): 1-3.7 µm; median: 2 μm; N=609
Cell biovolume: 0.5 – 25 μm³, median: 4 μm³; N=609
Biovolume equation: Sphere V, μm³ = 4/3 π (D/2)³
Colony diameter: up to 70 µm
Cells per colony: 40-100 (600)

Morphological features

Colonies are more or less spherical or slightly elongate, made of small (2 µm) sperical cells organized in radiating uniseriate rows extending from the center to all directions, embedded in colorless mucilage (Plate 1). Cells with gas vesicles. Can be distinquished from Microcystis spp. by the smaller cell size and organization in radiating rows. Cell size varies only slightly, OK not to measure cell size routinely, use a median value instead.

Plate 1. with 1 or more photos of the species, and descriptive text below each photo.Photo by Alla Alster.

Ecology

Radiocystis germinata has always been a member of the Kinneret plankton, although there were periods during which it was nearly absent (e,g, 1993-1995; 2005-2006; 2011-2014) but then it returned (Fig. 1). When present, it is often very abundant, with hundreds of colonies made of thousands of cells per mL (Fig. 1). Peak abundance is in summer, from july to September, while in winter the species is practically absent (Fig. 2). The number of cells in a colony shows a weak seasonal cycle, with larger colonies in winter and smaller in summer (Fig. 2). The variation in cell size is minor (Fig. 2).

Environmental conditions

Abundance increases with water temperature. Indifferent to salinity at the range occurring in Kinneret. Most abundant when pH ranges 8.5-9, 100-120 mg CaCO₃ mL^-1 (not shown).

Additional figures

Figure 1. Time series of Radiocystis germinata mean epilimnion cell abundance (cells mL^-1), Lake Kinneret, 1970-2020

Figure 2. The annual pattern of water column cell abundance (left), colony size (middle) and cell size (right) of Radiocystis germinata, based on data for 2006-2014. Statistics shown are: median – middle line; 25th to 75th percentiles – box content; 90th and 10th percentiles - top and bottom bars, respectively.

Cite this record as: Dr. Tamar Zohary, Dr. Alla Alster. 21 July 2026. Electronic publication. Israel Oceanographic & Limnological Research. https://kinneret-algae-atlas.org/ Searched on —.