Gas vesicle genes in Planktothrix spp. from Nordic lakes: strains with weak gas vesicles possess a longer variant of gvpC

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Abstract
In cyanobacteria of the genus Planktothrix, there are three length variants of gvpC, the gene that encodes the outer protein of the gas vesicle. Sequence analyses indicated that the three allelic variants of gvpC differ principally in the presence or absence of a 99 nt and a 213 nt section. Strains with the new variant, gvpC 28, which encodes a 28 kDa form of GvpC, produce gas vesicles that collapse at the relatively low critical pressure (p c) of 0·61–0·75 MPa. The authors have identified 12 classes of gvp genotypes that differ in the number and arrangement of alternating gvpA–gvpC genes and in the presence of ΩC, a fragment of gvpC. The gvpC 28 gene was found to be the most common variant of gvpC amongst 71 strains of Planktothrix isolated from Nordic lakes: 34 strains contained only gvpC 28; 22 strains, which possessed only the shorter gvpC 20 gene, produced gas vesicles with a higher p c of 0·76–0·91 MPa; and 15 strains, which possessed both gvpC 20 and gvpC 28, also produced the stronger gas vesicles. Genotypes with only the gvpC 28 genes were more common amongst green Planktothrix strains (33 out of 38) than red strains (one out of 33). It is suggested that there is competition between the strains producing the two types of gas vesicles, with the stronger forms favoured in lakes deeper than 60 m, in which the combination of cell turgor pressure and hydrostatic pressure can collapse the weaker gas vesicles. The fact that none of the Nordic lakes are deeper than 67 m would explain the absence of the gvpC 16-containing strains that produce even narrower gas vesicles of p c 1·0–1·2 MPa, which are common in the much deeper Lake Zürich.