Stephen Knobloch

Marine sponges can harbour diverse bacteria that contribute to host metabolism and defence. Identifying these stable members of sponge bacterial communities remains a necessary step in understanding their ecological roles and underlying co-evolutionary processes. In this study, we applied high-throughput sequencing of 16S rRNA gene amplicons, ribosomal nucleotide variant analysis and fluorescence in situ hybridisation to characterise the core members of the bacterial community in the marine sponge Halichondria panicea from Icelandic waters. We show that the core bacterial community across all samples consisted of a single, dominant bacterial taxon, for which we propose a candidate status 'Candidatus Halichondribacter symbioticus'. Comparison against public databases showed that 'Ca. H. symbioticus' is both a highly abundant specialist in H. panicea and a low abundant opportunist in other sponge species. Additionally, H. panicea with and without 'Ca. H. symbioticus' co-exist in similar locations in the North Atlantic. This dichotomy paired with the presence of geographically distinct ribosomal sequence variants of the symbiont make H. panicea an interesting sponge species for studying sponge-symbiont co-evolution and functional interactions.

Atlantic salmon aquaculture is expanding, and with it, the need to find suitable replacements for conventional protein sources used in formulated feeds. Torula yeast (Cyberlindnera jadinii), has been identified as a promising alternative protein for feed and can be sustainably cultivated on lignocellulosic biomasses. The present study investigated the impact of torula yeast on the growth performance and gut microbiome of freshwater Atlantic salmon. A marine protein base diet and a mixed marine and plant protein base diet were tested, where conventional proteins were replaced with increasing inclusion levels of torula yeast, (0%, 10%, 20%). This study demonstrated that 20% torula yeast can replace fish meal without alteration to growth performance while leading to potential benefits for the gut microbiome by increasing the presence of bacteria positively associated with the host. However, when torula yeast replaced plant meal in a mixed protein diet, results suggested that 10% inclusion of yeast produced the best growth performance results but at the 20% inclusion level of yeast, potentially negative changes were observed in the gut microbial community, such as a decrease in lactic acid bacteria. This study supports the continued investigation of torula yeast for Atlantic salmon as a partial replacement for conventional proteins.

Marine sponges host bacterial symbionts with biotechnological potential, yet isolation of true sponge symbionts remains difficult due to their host dependency. Moreover, attempts to grow sponges for their pharmacologically-active compounds outside of their habitat often results in a shift of their microbial community. In this study we evaluate suitable sponge cultivation methods that allow maintenance of both the marine sponge Halichondria panicea and its associated bacteria in an ex situ environment. In addition, we present a method for co-cultivation of sponge explants and microbes separated by a membrane in a multi-chamber device. Tests on ex situ cultivation of H. panicea under different controlled conditions showed that only high water exchange rates in the aquarium enabled maintenance of its dominant symbiont "Candidatus Halichondribacter symbioticus" at a high relative abundance in the sponge body, a prerequisite for co-cultivation. The bacterial enrichment retrieved from co-cultivation contained bacteria from nine different classes in addition to sequences corresponding to "Ca. H. symbioticus". This represents an increase of the cultivable bacterial classes from H. panicea compared to standard isolation techniques on solid media plates. The current study provides insights into sponge-microbe maintenance under ex situ conditions and proposes a new method for the isolation of sponge-associated bacteria.