Mucospheres produced by a mixotrophic protist impact ocean carbon cycling

Abstract

Mixotrophic protists (unicellular eukaryotes) that engage in both phototrophy (photosynthesis) and phago-heterotrophy (engulfment of particles)—are predicted to contribute substantially to energy fluxes and marine biogeochemical cycles. However, their impact remains largely unquantified. Here we describe the sophisticated foraging strategy of a widespread mixotrophic dinoflagellate, involving the production of carbon-rich ‘mucospheres’ that attract, capture, and immobilise microbial prey facilitating their consumption. We provide a detailed characterisation of this previously undescribed behaviour and reveal that it represents an overlooked, yet quantitatively significant mechanism for oceanic carbon fluxes. Following feeding, the mucospheres laden with surplus prey are discarded and sink, contributing an estimated 0.17–1.24 mg m−2 d−1 of particulate organic carbon, or 0.02–0.15 Gt to the biological pump annually, which represents 0.1–0.7% of the estimated total export from the euphotic zone. These findings demonstrate how the complex foraging behaviour of a single species of mixotrophic protist can disproportionally contribute to the vertical flux of carbon in the ocean.

Diverse RNA viruses associated with diatom, eustigmatophyte, dinoflagellate and rhodophyte microalgae cultures

“…While most of the Partitiviridae do not induce symptoms in their hosts, hypovirulence has been reported in the alpha-, beta- and gammapartitiviruses 55 . Further analysis is required to assess the effects of Orion durna-like virus 1 and Diktys durna-like virus 1 on their potentially harmful dinoflagellate hosts G. carpenteri and P. lima 48,56 .…”

“…Culture media and temperature conditions were specific to each species and were as follows: Nannochloropsis oceanica 24°C, f/2 medium; Nannochloropsis oculata 24°C, f/2 medium; Thalassiosira weissflogii 20°C, f/2 medium; Rhodella maculata 24°C, L1 medium (minus Si); Euglena gracilis 20°C, Euglena medium; Prorocentrum cf. balticum (UTSPH2D4) 48 ; 20°C K medium-Si; Prorocentrum lima 25°C modified K medium 58 ; Gambierdiscus carpenteri (UTSHI2C4) 25°C modified K medium 48,59 . To harvest each microalgal culture, the cells from 100-250 mL were pelleted by centrifugation at 200g for 4 mins and the supernatant discarded.…”

“…Further analysis is required to assess the effects of Orion durna-like virus 1 and Diktys durna-like virus 1 on their potentially harmful dinoflagellate hosts G. carpenteri and P. lima 48,56 .…”

“…Culture media and temperature conditions were specific to each species and were as follows: Gambierdiscus carpenteri (UTSHI2C4) 25°C modified K medium 48,59 . To harvest each microalgal culture, the cells from 100-250 mL were pelleted by centrifugation at 200g for 4 mins and the supernatant discarded.…”

“…Additional read mapping using our metagenomic or SRA-based data did not allow the retrieval of the full-length sequence (Figure A4). Whether Althepos endorna-like virus 1 and Almopos endorna-like virus 1 impact the fitness of their G. carpenteri host remains to be investigated but might have significant implications for the management of this potentially harmful species 48 . Considering the persistent lifestyle reported for endornaviruses 49 and the high similarities in terms of RdRp homologies and genomic organisation, it is likely that Althepos endorna-like virus 1 and Almopos endorna-like virus 1 share the same infectious properties as other members of the genus Alphaendornavirus and might therefore constitute another example of capsid-less persistent viruses associated with protist hosts.…”