Summary: Marine sponges constitute major parts of coral reefs and deep‐water communities. They often harbour high amounts of phylogenetically and physiologically diverse microbes, which are so far poorly characterized. Many of these sponges regulate their internal oxygen concentration by modulating their ventilation behaviour providing a suitable habitat for both aerobic and anaerobic microbes. In the present study, both aerobic (nitrification) and anaerobic (denitrification, anammox) microbial processes of the nitrogen cycle were quantified in the sponge Geodia barretti and possible involved microbes were identified by molecular techniques. Nitrification rates of 566 nmol N cm ⁻³ sponge day ⁻¹ were obtained when monitoring the production of nitrite and nitrate. In support of this finding, ammonia‐oxidizing Archaea (crenarchaeotes) were found by amplification of the amo A gene, and nitrite‐oxidizing bacteria of the genus Nitrospira were detected based on rRNA gene analyses. Incubation experiments with stable isotopes ( ¹⁵ NO ₃ ^‐ and ¹⁵ NH ₄ ⁺ ) revealed denitrification and anaerobic ammonium oxidation (anammox) rates of 92 nmol N cm ⁻³ sponge day ⁻¹ and 3 nmol N cm ⁻³ sponge day ⁻¹ respectively. Accordingly, sequences closely related to ‘ Candidatus Scalindua sorokinii’ and ‘ Candidatus Scalindua brodae’ were detected in 16S rRNA gene libraries. The amplification of the nir S gene revealed the presence of denitrifiers, likely belonging to the Betaproteobacteria . This is the first proof of anammox and denitrification in the same animal host, and the first proof of anammox and denitrification in sponges. The close and complex interactions of aerobic, anaerobic, autotrophic and heterotrophic microbial processes are fuelled by metabolic waste products of the sponge host, and enable efficient utilization and recirculation of nutrients within the sponge–microbe system. Since denitrification and anammox remove inorganic nitrogen from the environment, sponges may function as so far unrecognized nitrogen sinks in the ocean. In certain marine environments with high sponge cover, sponge‐mediated nitrogen mineralization processes might even be more important than sediment processes.

Aerobic and anaerobic microbial key processes were quantified and compared to microbial numbers and morphological structure in Mediterranean sponges. Direct counts on histological sections stained with DAPI showed that sponges with high microbial abundances (HMA sponges) have a denser morphological structure with a reduced aquiferous system compared to low microbial abundance (LMA) sponges. In Dysidea avara, the LMA sponge, rates of nitrification and denitrification were higher than in the HMA sponge Chondrosia reniformis, while anaerobic ammonium oxidation and sulfate reduction were below detection in both species. This study shows that LMA sponges may host physiologically similar microbes with comparable or even higher metabolic rates than HMA sponges, and that anaerobic processes such as denitrification can be found both in HMA and LMA sponges. A higher concentration of microorganisms in the mesohyl of HMA compared to LMA sponges may indicate a stronger retention of and, hence, a possible benefit from associated microbes.

Offshore platforms, subsea pipelines, wells and related fixed structures supporting the oil and gas (O&G) industry are prevalent in oceans across the globe, with many approaching the end of their operational life and requiring decommissioning. Although structures can possess high ecological diversity and productivity, information on how they interact with broader ecological processes remains unclear. Here, we review the current state of knowledge on the role of O&G infrastructure in maintaining, altering or enhancing ecological connectivity with natural marine habitats. There is a paucity of studies on the subject with only 33 papers specifically targeting connectivity and O&G structures, although other studies provide important related information. Evidence for O&G structures facilitating vertical and horizontal seascape connectivity exists for larvae and mobile adult invertebrates, fish and megafauna; including threatened and commercially important species. The degree to which these structures represent a beneficial or detrimental net impact remains unclear, is complex and ultimately needs more research to determine the extent to which natural connectivity networks are conserved, enhanced or disrupted. We discuss the potential impacts of different decommissioning approaches on seascape connectivity and identify, through expert elicitation, critical knowledge gaps that, if addressed, may further inform decision making for the life cycle of O&G infrastructure, with relevance for other industries (e.g. renewables). The most highly ranked critical knowledge gap was a need to understand how O&G structures modify and influence the movement patterns of mobile species and dispersal stages of sessile marine species. Understanding how different decommissioning options affect species survival and movement was also highly ranked, as was understanding the extent to which O&G structures contribute to extending species distributions by providing rest stops, foraging habitat, and stepping stones. These questions could be addressed with further dedicated studies of animal movement in relation to structures using telemetry, molecular techniques and movement models. Our review and these priority questions provide a roadmap for advancing research needed to support evidence‐based decision making for decommissioning O&G infrastructure.

The benefit of engaging volunteers in marine citizen science projects goes beyond generation of data and has intrinsic value with regards to community capacity-building and education. Yet, despite the documented benefits of citizen science, there can be barriers to the process of developing strategic citizen science projects and translating data into valued results with natural resource management applications. This paper presents four case-studies from fifteen years of Reef Check Australia (RCA) marine citizen science research and education projects. These case studies convey approaches and lessons-learned from the process of designing, implementing and sharing citizen science programs with the goal to create valuable social and environmental outcomes: 1) Demonstrating citizen science data quality through a precision study on data and analysis of 15 years of standardized Reef Check (RC) reef health data in Queensland, Australia. 2) Identifying and responding to data gaps through volunteer monitoring of sub-tropical rocky reefs in South East Queensland, Australia. 3) Adapting citizen science protocols to enhance capacity building, partnerships and strategic natural resource management applications through reef habitat mapping. 4) Tailoring new pathways for sharing citizen science findings and engaging volunteers with the community via a Reef Check Australia Ambassadors community outreach program. These case studies offer insights into considerations for developing targeted and flexible citizen science projects, showcasing the work of volunteers and project stakeholders, and collaborating with partners for applications beneficial to research, management and education.