Investigating natural biofilms on floating marine microplastics and the implications for ocean color remote sensing

New satellites are needed to quantify the upwards of 170 trillion microplastic particles floating on the ocean surface that concentrate in the convergence zones of the ocean gyres. Past research on cleaned microplastic pieces harvested from the ocean gyres indicates a highly consistent endmember across the spectrum with distinct absorption features at 931, 1215, 1417, 1732 and 2313 nm. Here, surface-floating microplastic pieces with natural biofilm were collected at 21 stations across 4700 km of the North Pacific gyre convergence zone, also referred to as the “Great Pacific Garbage Patch,” to assess the impact of biofilm on spectral properties. Reflectance of damp, biofilmed microplastic pieces were measured in bulk from 350 to 2500 nm shortly after collection with ambient sunlight on the ship deck and the biofilm was separated and stored for DNA sequencing. Biofilm was found on all collected samples and the red algal genus Tsunamia alone accounted for ∼53% of the eukaryotic microbiome of the harvested biofilm. Biofilm absorbed light broadly across blue wavelengths (400–500 nm) and in a narrowband at 674 nm consistent with chlorophyll- a and other photosynthetic pigments in red algae. Hence, biofilm could impact ocean color chlorophyll-a and fluorescence line height retrievals if particle concentrations were high enough. No significant differences were found in band-depths estimated at 1215 and 1732 nm and only minor differences at 931 nm between biofilmed and biofilm-removed samples. This new endmember of damp, biofilmed microplastic most closely resembles the spectral properties of microplastic pieces as they naturally occur in the ocean gyres. New sensors for marine debris detection may consider narrow bands between 670 and 680 nm to discriminate red algal biofilmed microplastic from phytoplankton, in addition to the NIR and SWIR bands characteristic of floating marine microplastics. • We collected fresh floating marine debris in the Great Pacific Garbage Patch. • Biofilm presence was found to modify plastic reflectance properties. • The biofilm microbiome was dominated by red algae and gamma proteobacteria.