Increasing abundance of nitrogen-fixing Trichodesmium in Australian waters

Context The filamentous cyanobacteria, genus Trichodesmium, is a marine diazotroph that converts atmospheric nitrogen into more bioavailable forms and is therefore important in nutrient cycling in oligotrophic marine ecosystems. Aims This study aimed to confirm findings from historical field observations, remote sensing, laboratory experiments, and models that Trichodesmium may become increasingly important under climate change. Methods We used Tweedie models with a 16-year time series of Trichodesmium observations in Australian waters, based on samples collected with bottles, nets, and the Continuous Plankton Recorder (for microscopic analysis) and environmental DNA from filtered water. Key results The highest Trichodesmium abundances were found in tropical regions, primarily in response to warmer temperatures. However, Trichodesmium is found in lower abundances throughout Australian waters. There are two species common in Australian waters, T. thiebautii dominating on the eastern coast and T. erythraeum on the west, each with slightly different thermal niches. Conclusion Trichodesmium abundance has increased in Australia since 2009. This trend is expected to persist as warming continues, strengthening stratification and reducing the upward mixing of nitrate. Implications This study has deepened our understanding of Trichodesmium in Australian waters, underscoring the importance of ongoing observations as the extent of ocean suitable for Trichodesmium growth increases.