Fungal symbionts impact cyanobacterial biofilm durability and photosynthetic efficiency

Marine Mycobiomes Colonize Mediterranean Sponge Hosts in a Random Fashion

Marine sponges are widespread, sessile, filter-feeding animals, known for living in association with complex prokaryotic communities structured by host species. Though marine fungi are ubiquitous across marine environments, little is known about sponge-associated fungal communities (mycobiome). Indeed, aside from a few studies based on the isolation of fungal strains for biotechnological purposes, little information is available to understand the diversity and structure of sponge mycobiome. Here, a metabarcoding approach based on the ITS1 marker was applied to examine the structure and composition of fungal communities associated with four Mediterranean sponges. The species: Petrosia ficiformis, Chondrosia reniformis, Crambe crambe, and Chondrilla nucula were analyzed along with the surrounding seawater, revealing Aspergillus (1-56%), Cladosporium (1-75%), Malassezia (1-38.5%), and Pennicillium (1.5-36%) as the most represented fungal genera. Our data showed high intra-specific variability and no clear core mycobiome within each of the sponge species host, suggesting stochastic and perhaps transient community membership. This study sheds light on one of the most abundant yet least understood components of the marine ecosystem. Unraveling the dynamics of fungal interactions within sponge holobionts is essential to advance our understanding of their ecological roles and functions. By addressing the enigmatic nature of sponge-associated fungi, this research opens new avenues for exploring their contributions to marine ecosystems and resolving the many unanswered questions in this field.

Screening fungal partners for enhancing the vitality of a xenic algal culture via photosynthetic efficiency

The xenic strain Chlorella sorokiniana was grown together with selected fungal strains to investigate the effect of fungi on the algal photosynthetic performance during cultivation. The introduction of well-selected fungal strains can potentially increase algal cultivation efficiency. The bacteria that inhabited the algae were identified and the coexistence of the fungi with the algae and bacteria in liquid and solid media was examined. Chlorophyll a fluorescence measurement, a commonly used method for determining the efficiency of plant photosynthesis under stressful conditions, was used to assess the condition of the algae. The algae were cultivated for eight weeks without supplementing the nutrient solution. The experiments showed that the fungal strains Clonostachys rosea, Rhodotorula mucilaginosa and Mortierella alpina formed stable interactions with the microalga C. sorokiniana and the bacteria in the microalgal culture. The time of the measurement and treatments caused changes in the fluorescence curve patterns. Differences in the profiles of the curves in different phases revealed modifications in the operation of the light-dependent photochemical reactions. Generally, the most positive changes in the chlorophyll a fluorescence induction curves (OJIP) were recorded in the double inoculation of C. sorokiniana with R. mucilaginosa + M. alpina and R. mucilaginosa + C. rosea. The results show that selected combinations of fungal strains can be a tool to improve the photosynthetic efficiency of C. sorokiniana.

A thallus-forming N-fixing fungus-cyanobacterium symbiosis from subtropical forests

Fungi engage in diverse symbiotic relationships with phototrophs. Lichens, symbiotic complexes involving fungi and either cyanobacteria, green algae, or both, have fungi forming the external layer and much of the interior. We found an erect thallus resembling a lichen yet with an unexpected thallus structure composed of interwoven cyanobacterial filaments with numerous fungal hyphae inserted within individual cyanobacterial sheaths, contrasting with typical lichen structure. Phylogenetics identified the fungus as a previously undescribed species, Serendipita cyanobacteriicola, closely related to endophytes, and the cyanobacterium belongs to the family Coleofasciculaceae, representing a genus and species not yet classified, Symbiothallus taiwanensis. These thalli exhibit nitrogen-fixing activity similar to mosses but lower than cyanolichens. Both symbiotic partners are distinct from known lichen-forming symbionts, uncovering a phylogenetically and morphologically unprecedented thallus-forming fungus-cyanobacterium symbiosis. We propose the name "phyllosymbia" for these thalli to underscore their unique symbiotic nature and leaf-like appearance. This finding marks a previously unknown instance of fungi solely residing within structures generated by cyanobacteria.

Engineered Cyanobacteria-Based Living Materials for Bioremediation of Heavy Metals Both In Vitro and In Vivo

The pollution caused by heavy metals (HMs) represents a global concern due to their serious environmental threat. Photosynthetic cyanobacteria have a natural niche and the ability to remediate HMs such as cadmium. However, their practical application is hindered by a low tolerance to HMs and issues related to recycling. In response to these challenges, this study focuses on the development and evaluation of engineered cyanobacteria-based living materials for HMs bioremediation. Genes encoding phytochelatins (PCSs) and metallothioneins (MTs) were introduced into the model cyanobacterium Synechocystis sp. PCC 6803, creating PM/6803. The strain exhibited improved tolerance to multiple HMs and effectively removed a combination of Cd2+, Zn2+, and Cu2+. Using Cd2+ as a representative, PM/6803 achieved a bioremediation rate of approximately 21 μg of Cd2+/OD750 under the given test conditions. To facilitate its controllable application, PM/6803 was encapsulated using sodium alginate-based hydrogels (PM/6803@SA) to create "living materials" with different shapes. This system was feasible, biocompatible, and effective for removing Cd2+ under simulated conditions of zebrafish and mice models. Briefly, in vitro application of PM/6803@SA efficiently rescued zebrafish from polluted water containing Cd2+, while in vivo use of PM/6803@SA significantly decreased the Cd2+ content in mice bodies and restored their active behavior. The study offers feasible strategies for HMs bioremediation using the interesting biomaterials of engineered cyanobacteria both in vitro and in vivo.

Biological control of predatory fungi inhabiting activated sludge in wastewater treatment

The unfavorable phenomenon of activated sludge bulking that occurs in sewage treatment plants (WWTPs) is caused by the over-proliferation of filamentous bacteria that should be limited by the Lecane rotifers that feed on them; however, predatory, rotiferovorous fungi that often inhabit WWTPs pose a real threat to these organisms. To solve this problem, we investigated the interaction of the fungus Clonostachys rosea, which is a known Biological Control Agent (BCA) and the predacious Zoophagus sp. in simplified laboratory culture conditions. The presence of C. rosea in the cultures reduced the number of active traps, thus translating into a much smaller number of rotifers being caught. The mycelium of C. rosea was labeled with a red fluorescent protein (RFP). The life cycle of C. rosea that were attacking Zoophagus sp. (hunting for rotifers) is described. C. rosea spores germinate into single-celled forms and penetrate the interior of the Zoophagus mycelium where they feed on the cytoplasm. Then is the mycelium produced abundantly and forms conidiophores. This type of life strategy has not been known before. The obtained results demonstrated the potential of C. rosea as a BCA that can be used to protect rotifers in the event of an infection of activated sludge by the predatory fungi that threaten the rotifer population.