The functional roles that marine mycoplankton fulfil are poorly understood, resulting in a lack of knowledge of their ecology. Here we show, using DNA Stable Isotope Probing with 13 C-labelled diatom polysaccharide microgels, that mycoplankton assimilate algal-derived particulate organic carbon (POC), identifying two genera, Malassezia and Cladosporium, which are active saprotrophs in coastal waters. We subsequently isolated polysaccharide-utilising Cladosporium strains from the same ecosystem and that are well-represented in marine mycoplankton assemblages. At the study site, Cladosporium occurs across multiple years and is associated with diatoms. During growth with the polysaccharide laminarin, Cladosporium spp. secrete the extracellular carbohydrate-active enzyme glucan 1,3-b-glucosidase. These results show that some marine mycoplankton have a saprotrophic functional role in processing algal polysaccharides. Mycoplankton may, therefore, be involved in the trophic transfer of phytoplankton produced POC in marine food webs, and because bacterioplankton occupy the same niche, potential interactions maybe taking place that are yet to be characterised.

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Lichens are a well-known symbiosis between a host mycobiont and eukaryote algal or cyanobacterial photobiont partner(s). Recent studies have indicated that terrestrial lichens can also contain other cryptic photobionts that increase the lichens’ ecological fitness in response to varying environmental conditions. Marine lichens live in distinct ecosystems compared with their terrestrial counterparts because of regular submersion in seawater and are much less studied. We performed bacteria 16S and eukaryote 18S rRNA gene metabarcoding surveys to assess total photobiont diversity within the marine lichen Lichina pygmaea (Lightf.) C. Agardh, which is widespread throughout the intertidal zone of Atlantic coastlines. We found that in addition to the established cyanobacterial photobiont Rivularia, L. pygmaea is also apparently host to a range of other marine and freshwater cyanobacteria, as well as marine eukaryote algae in the family Ulvophyceae (Chlorophyta). We propose that symbiosis with multiple freshwater and marine cyanobacteria and eukaryote photobionts may contribute to the ability of L. pygmaea to survive the harsh fluctuating environmental conditions of the intertidal zone.