Cyanobacterial endobionts within a major marine planktonic calcifier (<i>Globigerina bulloides</i>, Foraminifera) revealed by 16S rRNA metabarcoding

Abstract

We investigated the possibility of bacterial symbiosis in <i>Globigerina bulloides</i>, a palaeoceanographically important, planktonic foraminifer. This marine protist is commonly used in micropalaeontological investigations of climatically sensitive subpolar and temperate water masses as well as wind-driven upwelling regions of the world's oceans. <i>G. bulloides</i> is unusual because it lacks the protist algal symbionts that are often found in other spinose species. In addition, it has a large offset in its stable carbon and oxygen isotopic compositions compared to other planktonic foraminifer species, and also that predicted from seawater equilibrium. This is suggestive of novel differences in ecology and life history of <i>G. bulloides</i>, making it a good candidate for investigating the potential for bacterial symbiosis as a contributory factor influencing shell calcification. Such information is essential to evaluate fully the potential response of <i>G. bulloides</i> to ocean acidification and climate change. To investigate possible ecological interactions between <i>G. bulloides</i> and marine bacteria, 18S rRNA gene sequencing, fluorescence microscopy, 16S rRNA gene metabarcoding and transmission electron microscopy (TEM) were performed on individual specimens of <i>G. bulloides</i> (type IId) collected from two locations in the California Current. Intracellular DNA extracted from five <i>G. bulloides</i> specimens was subjected to 16S rRNA gene metabarcoding and, remarkably, 37–87 % of all 16S rRNA gene sequences recovered were assigned to operational taxonomic units (OTUs) from the picocyanobacterium <i>Synechococcus</i>. This finding was supported by TEM observations of intact <i>Synechococcus</i> cells in both the cytoplasm and vacuoles of <i>G. bulloides</i>. Their concentrations were up to 4 orders of magnitude greater inside the foraminifera than those reported for the California Current water column and approximately 5 % of the intracellular <i>Synechococcus</i> cells observed were undergoing cell division. This suggests that <i>Synechococcus</i> is an endobiont of <i>G. bulloides</i> type IId, which is the first report of a bacterial endobiont in the planktonic foraminifera. We consider the potential roles of <i>Synechococcus</i> and <i>G. bulloides</i> within the relationship and the need to determine how widespread the association is within the widely distributed <i>G. bulloides</i> morphospecies. The possible influence of <i>Synechococcus</i> respiration on <i>G. bulloides</i> shell geochemistry is also explored.