Growth and toxicity of Halomicronema metazoicum (Cyanoprokaryota, Cyanophyta) at different conditions of light, salinity and temperature

Abstract

Cyanobacteria may live in the water column and in the benthos of aquatic environments, or be symbionts of other organisms, as in the case of Phormidium-like cyanobacteria, known to influence the ecology of freshwater and marine ecosystems. A strain of Phormidium-like cyanobacteria has been recently isolated as a free-living epiphyte of leaves of Posidonia oceanica (L.) Delile in the Mediterranean sea and its biology and ecology are herein investigated. It was identified as Halomicronema metazoicum, previously known uniquely as a symbiont of marine sponges. We cultivated it in a range of light irradiances, temperatures and salinities, to establish the most suitable conditions for the production of allelopathic and toxic compounds. The bioactivity of its spent culture medium was measured by means of standard toxicity tests performed on two model organisms. Our results indicate that at least two bioactive compounds are produced, at low and high irradiance levels and at two temperatures. The main compounds influencing the survival of model organisms are produced at the highest temperature and high or intermediate irradiance levels. The present research contributes to the understanding of critical toxigenic relationships among cyanobacteria and invertebrates, possibly influencing the ecology of such a complex environment as P. oceanica. Future isolation, identification and production of bioactive compounds will permit their exploitation for biotechnologies in the field of ecological conservation and medical applications.

Constitutive and activated toxigenic activity ofHalomicronema metazoicum(Cyanoprokaryota, Cyanophyta)

“…Strains were kept in continuous culture at the Stazione Zoologica Anton Dohrn. Its purity was previously ascertained by means of SEM and molecular investigations (Mutalipassi et al, 2019) demonstrating the absence of contaminant organisms. The axenic conditions of the culture were reconfirmed during this study also by also means of SEM analyses (Figure 1).…”

“…metazoicum were isolated from mother cultures and individually cultivated in sterile 2 L Erlenmeyer flasks containing 1.5 L of f / 2 medium (Figure 2). Culture conditions were set to provide the highest production of toxic compounds (Mutalipassi et al, 2019). In particular, photosynthetic active radiation, temperature and salinity were set at 140 µE, 22°C, and 44 PSU.…”

“…These compounds are produced to face the complexity of interactions among communities associated with complex ecosystems as those of seagrass leaves (Mazzella et al, 1992). H. metazoicum (Phormidium-Leptolyngbya group) has been isolated by previous authors from the surface of P. oceanica leaves (Ruocco et al, 2018) and its spent medium showed a clear toxigenic activity (Mutalipassi et al, 2019).…”

“…Under favourable conditions, cyanobacteria rapidly multiply, doubling in 1 day or less, and mature strains of cyanobacteria may move from plankton to benthos and vice‐versa (Huisman et al, 2018). Consequently, they may influence the composition of various marine communities, by producing allelopathic compounds active in benthic or planktonic environments (Mutalipassi et al, 2019). Recent observations demonstrated that Halomicronema metazoicum , a filamentous non‐heterocystous cyanobacterium symbiont of P. ficiformis , can also be free‐living, associated with the leaves of the seagrass Posidonia oceanica (Ruocco et al, 2018).…”

“…metazoicum ( Phormidium ‐ Leptolyngbya group) has been isolated by previous authors from the surface of P . oceanica leaves (Ruocco et al, 2018) and its spent medium showed a clear toxigenic activity (Mutalipassi et al, 2019). Although this species was previously considered an exclusive endobiont of marine sponges (Ledermann et al, 2017), its discovery as a free‐living organism is not surprising, because congeneric cyanobacteria were firstly isolated from benthic microbial mats and also found in heavily polluted coastal streams (Abed et al, 2002).…”