Distinct Survival Strategies in Oligotrophic and Eutrophic ecotype Synechococcus and Bacteria cocultures under Iron Limitation and Warming Conditions

Phytoplankton and bacteria interactions underpin primary production and nutrient cycling in both oligotrophic and eutrophic aquatic environments, and greatly impact marine biogeochemical cycles under climate change. However, how phytoplankton and bacteria interactions vary under simultaneous environmental fluctuations remains largely unexplored, especially when considering differing ecotypes. Here, we explored the interactions between Synechococcus and its associated heterotrophic bacteria under concurrent warming and iron limitation, and compared the differences between the oligotrophic strain YX04-1 and eutrophic strain XM-24 ecotypes using the 16S rRNA gene amplicon, as well as metagenomic and metatranscriptomic sequencing. Our results revealed that community composition and gene expression in the oceanic Synechococcus sp. YX04-1 coculture were more sensitive to warming, while the coastal Synechococcus sp. XM-24 coculture responded more strongly to Fe limitation. The resilience of oligotrophic Synechococcus sp. YX04-1 and its associated bacteria to iron deficiency might be due to a mutualistic triangular survival strategy, involving the release of dissolved organic matter, the transfer of low-molecular weight DOM, and the feedback of public goods. In contrast, the eutrophic XM-24 coculture experienced increased competition and exploitation of organic matter and public goods driven by the dominant mixotrophic associated bacteria under concurrent warming and Fe limitation. These contrasting scenarios highlight the more positive and tighter interactions in the oligotrophic Synechococcus and bacteria coculture compared to the eutrophic one, which may assist the oligotrophic species in adapting to changing ocean conditions.