Exposure to aged polypropylene nurdle leachates disrupts photosymbiosis in a kleptoplastic unicellular eukaryote

Kleptoplasty, i.e. the sequestration of functional algal chloroplasts by a host organism, represents a natural case of photosymbiosis from which the host derives crucial energetic benefits. We explored here how this host-symbiont relationship is affected by polypropylene nurdle leachates in a kleptoplastidic foraminifera. When exposed to virgin nurdles, a mild proteome regulation was observed in the host, whereas photosynthetic proteins were more abundant in kleptoplasts, supplying energy to the host. These results show that, de novo protein synthesis in stolen chloroplasts and delivery of host proteins and algal proteins encoded by the host following horizontal gene transfer are necessary to maintain efficient photosymbiosis in a virgin nurdle leachate polluted environment. Conversely, aged nurdles strongly reduced the content of photosynthesis-related proteins in kleptoplasts, disrupting the host-symbiont association. Remodeling of the proteome nevertheless suggested the possibly for an increased energy production in foraminifera, through a switch from mixotrophy to heterotrophy. Benthic foraminifera are therefore truly efficient unicellular eukaryotes, with diverse and sophisticated metabolic adaptive strategies that we are just beginning to discover.