Community Patterns Of Labyrinthulomycetes And Their Roles In Carbon Cycles In Typical Coastal And Oceanic Waters

Heterotrophic microbes drive the marine carbon cycles and sinks,and play a key role in regulation of the global climate.However,our understanding to the heterotrophic microbial communities and functions remains largely from studies on prokaryotes and microzooplankton.Recent evidence has shown that the fungus-like heterotrophic microeukaryotes,typically known as Labyrinthulomycetes protists,are ubiquitous in the global ocean and possess a high fraction of biomass;but their roles in the food webs and carbon cycles have long been overlooked,with their diversity,community patterns,and functions in carbon cycles still unclear.In this study,we first employed high-resolution time-series observations at the nearshore and repeated spatial observations across the nearshore to offshore waters of North Carolina,analyzed the abundance,diversity,and community composition of the Labyrinthulomycetes,and revealed partitioning preferences of this group to different seasons（summer/winter）and habitats（nearshore/shelf/offshore）.Notably,a few dominant populations were identified to be persistent and prevalent as observed for the bacteria;these populations exhibited extensive associations with cyanobacteria and heterotrophic bacteria,including significant concurrent occurrences or time-lagged responses.The remaining majority of populations,however,exhibited short-blooming patterns and patchy distributions;and these populations often co-bloomed with specific fungi or eukaryotic algae.It suggested potential effects of specific substrates,hosts,or micro-habitats（bottom-up control）,or strong density-dependent selection（top-down control）.These results supported multifarious and key roles of Labyrinthulomycetes in the marine carbon cycles which were mediated by the microbial food webs.In the pelagic waters of eastern Indian Ocean,the average Labyrinthulomycetes biomass(7.4μg C L^-1)was found higher than that of prokaryotes(5.7μg C L^-1).Unlike the sharp decline of the prokaryotic abundance with depth,patchy Labyrinthulomycetes blooms were observed in some stations,where their cell abundance kept high across different water layers.Analysis in community structure revealed persistent distributions of many populations from the epipelagic to the bathypelagic,suggesting potential sinking and contribution to the biological pump.Aplanochytrium and Aurantiochytrium were dominant genera of the epipelagic zone and the dark ocean,respectively;some unclassified populations were enriched around the deep chlorophyll maximum layer;and more populations were patchy and scattered.The partitioning patterns suggested potential ecotypes with differential functions in the pelagic carbon cycle.By exploring the influence of upwelling to the microbial eukaryotic communities,we further revealed a significant response of the Labyrinthulomycetes to the pulsed resources（e.g.,diatom-derived organic matter）supplies,which partially explained the potential ecological driving mechanisms of the patchy Labyrinthulomycetes blooms.Overall,this study revealed distinct community patterns and potential drivers of the Labyrinthulomycetes via multi-scale spatiotemporal observations and comparative analysis for different microbial groups.With carbon metabolic information of cultured strains considered,the potential roles of different ecotypes were also evaluated in the carbon cycle and biological pump.This study provides novel perspectives,hypotheses,and in-situ observational evidence for the ecological mechanisms how heterotrophic microbes participate in the marine carbon cycle and sink processes.