Recurrent association between Trichodesmium colonies and calcifying amoebae.

Colonies of the N₂-fixing cyanobacterium Trichodesmium spp. constitute a consortium with multiple microorganisms that collectively exert ecosystem-level influence on marine carbon and nitrogen cycling, shunting newly fixed nitrogen to low nitrogen systems, and exporting both carbon and nitrogen to the deep sea. Here we identify a seasonally recurrent association between puff colonies and amoebae through a two-year survey involving over 10 000 Trichodesmium colonies in the Red Sea. This association was most commonly found in near-shore populations during spring. Microscopic observations revealed consistent amoebae morphology throughout the study, and both morphological characteristics and 18S rRNA gene sequencing suggested that these amoebae are likely to belong to the species Trichosphaerium micrum, an amoeba that forms a CaCO₃ shell. Co-cultures of Trichosphaerium micrum and Trichodesmium grown in the laboratory suggest that the amoebae feed on heterotrophic bacteria and not Trichodesmium, which adds a consumer dynamic to the complex microbial interactions within these colonies. Sinking experiments with fresh colonies indicated that the presence of the CaCO₃-shelled amoebae decreased colony buoyancy. As such, this novel association may accelerate Trichodesmium sinking rates and facilitate carbon and nitrogen export to the deep ocean. Amoebae have previously been identified in Trichodesmium colonies in the western North Atlantic (Bermuda and Barbados), suggesting that this type of association may be widespread. This association may add a new critical facet to the microbial interactions underpinning carbon and nitrogen fixation and fate in the present and future ocean.