Planktonic Aggregates as Hotspots for Heterotrophic Diazotrophy: The Plot Thickens

Biological dinitrogen fixation is typically attributed to cyanobacteria in aquatic systems. Recent studies, however, suggest that non-cyanobacterial diazotrophs (NCDs) may also play a role in this process. The ecological distribution and activity of NCDs remain poorly understood. Prior research has shown that NCDs can exist in various aquatic environments, including coastal and deep-sea regions. Their presence in these areas raises questions about their contribution to nitrogen cycling. The role of planktonic aggregates as potential habitats for NCDs has not been fully explored. Aggregates may provide unique microenvironments that support NCD activity. Understanding these dynamics is essential for refining models of nitrogen cycling in aquatic ecosystems.

This review aims to synthesize recent molecular and biogeochemical findings on non-cyanobacterial diazotrophy in pelagic systems. The focus is on identifying the ecological niches where NCDs are active. The study seeks to clarify whether planktonic aggregates serve as hotspots for NCD activity. Aggregates may provide favorable conditions for N₂ fixation due to their chemical properties. The goal is to assess the significance of these microhabitats in nitrogen cycling. The review also highlights gaps in current knowledge about NCDs. The authors propose a conceptual model to guide future research on aggregate-associated diazotrophy. This work aims to stimulate new experimental approaches to study NCDs in situ.

The authors conducted a literature review of molecular and biogeochemical studies on pelagic NCDs. They analyzed data from diverse aquatic ecosystems, including coastal and deep-sea regions. The review focused on planktonic aggregates as potential habitats for NCDs. Molecular techniques such as nifH gene sequencing were used to identify NCDs. Biogeochemical data included measurements of oxygen levels and carbon:nitrogen ratios. The authors compared aggregate-associated and ambient water conditions. They evaluated the energetic feasibility of N₂ fixation in these microzones. The review also proposed a conceptual framework to explain aggregate-associated diazotrophy.

Pelagic NCDs are frequently found in planktonic aggregates across aquatic systems. Aggregates contain reduced-oxygen microzones, which may support N₂ fixation. The C:N ratio in aggregates exceeds the Redfield ratio, indicating excess carbon. Labile carbon availability is higher in aggregates than in surrounding waters. These conditions may provide energy for NCDs to fix nitrogen. Molecular evidence shows diverse NCDs inhabit aggregate surfaces. N₂ fixation rates are elevated in aggregates compared to ambient waters. The findings suggest that aggregates serve as hotspots for heterotrophic diazotrophy.

The authors propose that planktonic aggregates are key sites for NCD activity in aquatic systems. The unique chemical conditions of aggregates may enable N₂ fixation by heterotrophic diazotrophs. These findings suggest that NCDs contribute to nitrogen cycling in ways not fully captured by current models. The conceptual framework presented may guide future experimental studies. The authors emphasize the need for novel methods to study aggregate-associated diazotrophy. They suggest that aggregates may be more important than previously assumed in nitrogen cycling. The review highlights the need for further investigation into NCD ecology and activity. Future work should focus on quantifying the contribution of NCDs to global nitrogen fluxes.