Rainer-B. Volk

Screening of microalgal culture media for the presence of algicidal compounds and isolation and identification of two bioactive metabolites, excreted by the cyanobacteria Nostoc insulare and Nodularia harveyana

Culture medium extracts obtained from 115 culture media of 35 different microalgae species were screened for the presence of algicidal compounds, in particular for compounds which are cytotoxic to Arthrospira (Spirulina) laxissima. In agar plate diffusion tests and in a test system combining thin layer chromatography (TLC) with the use of an aqueous suspension of living A. laxissima cells as spray reagent, 14 microalgae species were found with cytotoxic activity of different intensity to A. laxissima. In a so-called TLC plate diffusion test, using A. laxissima and other microalgae as test organisms, the culture medium extracts of Nodularia harveyana and Nostoc insulare possessed the highest strength and range of algicidal activity. The algicidal compound in the culture medium extracts of Nodularia harveyana was shown to be norharmane (9H-pyrido(3,4-b)indole), a known indole alkaloid. The main algicidal compound in culture medium extracts of Nostoc insulare was identified as 4,4′-dihydroxybiphenyl. The possible applicability of both compounds as therapeutics or as useful agents for removing cyanobacterial water blooms or for developing new antifouling systems is discussed.

Antialgal Activity of Several Cyanobacterial Exometabolites

The isolation of norharmalane (3,4-dihydro-9H-pyrido(3,4-b)indole) from culture media of the cyanobacterium Nodularia harveyana is described. The minimum toxic quantities against selected cyanobacteria of this compound, of the two known cyanobacterial exometabolites 4,4′-dihydroxybiphenyl and norharmane (9H-pyrido(3,4-b)indole) and in addition of harmane (1-methyl-9H-pyrido(3,4-b)indole) were determined using a special TLC plate assay. The three β-carbolines harmane, norharmane and norharmalane were tested both as bases and as hydrochlorides. All four test compounds were found to be cytotoxic against the cyanobacterial test organisms in low quantities (0.5 to 18.0 μg). For the β-carbolines the following structure-response relationships were revealed: the double bond in position 3–4 and possibly the 1-methyl-group increased the cytotoxic effect of these indole alkaloids. Minimum toxic quantities detected for β-carboline bases were in general lower than those of equimolar amounts of the corresponding hydrochlorides. The possible applicability of both β-carbolines and biphenyls as agents in antifouling systems is discussed.

Studies on culture age versus exometabolite production in batch cultures of the cyanobacterium Nostoc insulare

Changes in the content of exometabolites excreted by the cyanobacterium Nostoc insulare during batch cultivation were determined. During linear growth, only the non-toxic compound N,N′-(4,5-dimethyl-1,2-phenylene)-bis-acetamide was detectable in appreciable quantities in the medium, whereas during stationary growth the antimicrobial and cytotoxic exometabolites 4,4′-dihydroxybiphenyl and 9H-pyrido(3,4-b)indole (norharmane) were also present to an increasing degree. Hence it is proposed that biosynthesis of N,N′-(4,5-dimethyl-1,2-phenylene)bis-acetamide in N. insulare is associated with cell proliferation and primary metabolism of this organism. 4,4′-Dihydroxybiphenyl and norharmane, however, are proposed to be products of secondary metabolism that are excreted by N. insulare primarily under nutrient-restricted conditions and under increased pressure of competition with other organisms.