A total of 36 waterbodies of mainland Greece were investigated for the occurrence of microcystins and other bioactive peptides (anabaenopeptins, anabaenopeptilides) during the period 1994 - 2001. Samples were analysed by three different methods (High Performance Liquid Chromatography [HPLC], Enzyme Linked Immunosorbent Assay [ELISA] and Protein Phosphatase 1 Inhibition Assay[PP1IA]) to assess the peptide(s) concentration in water, water blooms and aquatic fauna.

Ten different microcystins and two anabaenopeptins (A and B) were found in water blooms indicating the widespread occurrence of cyanotoxins in Greece. Microcystin occurrence is characterised by the predominance of MC-RR and MC-LR, followed by MC-YR or other microcystins or anabaenopeptins. MC-LA and demethylated variants of MC-LR and MC-RR were also present, but not abundant in the samples. The total microcystin and anabaenopeptin contents ranged from 40 to 2565 µg g^-1 freeze-dried bloom material and from undetectable to 48 µg g^-1 freeze-dried bloom material, respectively.

Quantitative relationships can be recognised between the individual microcystins composing the total microcystin content and concentration, in addition to the qualitative relationships previously known. Qualitative and quantitative variation of the microcystins indicated that there may be geographical trends in the distribution of microcystins. Quantitative data on the anabaenopeptins produced in natural cyanobacteria populations are presented for the first time.
Microcystins were found in water samples collected from most waterbodies. Total intracellular microcystin concentration ranged from undetectable to 13,230 μg L^-1. Total anabaenopeptin concentration ranged from undetectable to 1,725 μg L-1.

Microcystin(s) was detected in the muscle and visceral tissues of eight species of fish, a frog, a water snail and a water mussel. Microcystin concentration ranged from 20 – 1,500 ng g-1 dw and from 25 – 5,400 ng g-1 dw in the muscle and visceral tissue of the fish and frog, respectively. In the mussel and water snail tissue, microcystin concentration ranged from 1,650 – 3,495 ng g-1 dw. Chemical (HPLC) analysis resolved peaks having the same UV absorption spectrum as anabaenopeptin-, or anabaenopeptilide-like compounds, which have not been previously reported in aquatic fauna tissues. Comparison of the PP1IA and ELISA showed that values obtained with PP1IA where higher than those obtained with ELISA. Anabaenopeptins and/or anabaenopeptilides occurring in fauna tissue may account for the higher PP1IA values as we found that PP1 activity was inhibited by the purified anabaenopeptins A (45-60% inhibition) and B (5-75% inhibition). Purified anabaenopeptilides 90A and 90B exhibited weaker PP1 inhibition activity (5-35% and 5-23% inhibition, respectively).

The occurrence of the rare cyanobacterium Aphanizomenon ovalisporum is reported for the first time in Greece. Three strains of the genus Limnothrix isolated from Lake Kastoria, were characterised based on their morphological features and 16S rRNA gene sequences. The Limnothrix isolates were morphologically assigned to Limnothrix redekei and formed a separate cluster in the cyanobacterial 16S rRNA gene tree. Other isolates, three strains of Anabaena and one strain of Chroococcus were also characterised based on their morphological features.