Constitutive and activated toxigenic activity of Halomicronema metazoicum (Cyanoprokaryota, Cyanophyta)

Phormidium-like cyanobacteria produce a variety of biologically active molecules. We isolated a free-living strain of Halomicronema metazoicum, previously known uniquely as a symbiont of marine sponges, and demonstrated that its spent medium was toxic for various protozoans and fish parasites. However, we still ignore if its mats contain constitutive or activated defences influencing the physiology of other organisms, with potential influences on organisms that share the same environment. The goal of this study is to clarify if Phormidium-like cyanobacteria accumulate toxic bioactive molecules in the colony tissues, for example for anti-grazing purposes, or if they constitutively secrete them in the medium, with a potential allelopathic function. To address this hypothesis, the toxigenic power of cyanobacterial spent medium and of disintegrated cells (to simulate lysis) was assayed using standard toxicity-tests on nauplii of the brine shrimp Artemia salina. Toxigenic effects were triggered by the spent culture medium, but the homogenates (disintegrated cyanobacteria cells) did not show any acute toxicity. In addition, chronic toxicity tests of the spent medium demonstrated a reduced but still present toxigenic effect, with an inverse correlation between toxicity and medium dilution. The present research contributes to the understanding of cyanobacteria chemical-ecology and shows that they may play fundamental ecological roles, influencing the survival and the blooms of various co-habiting species by constitutively releasing toxic exudates. Since the bioactive compounds produced by H. metazoicum are stable in water, and considering the bioactivity of exudates on invertebrates' physiology, our results might be exploited for the development of novel biotechnologies in the fields of aquaculture, ecological conservation and medicine, due to the potential antitumoral and anti-parasitic activity demonstrated in strictly related species.