  Marine plastic debris is widely recognized as a global environmental issue.
Sun-micron plastic particles, with an upper size limit of 20 um, have been
identified as having the highest potential for causing damage to marine
ecosystems. Having accurate methods for quantifying the abundance of such
particles in a natural environment is essential for defining the extent of the
problem they pose. Using an optical micro-Raman tweezers setup, we have
identified the composition of particles trapped in marine aggregates collected
from the coastal surface waters around the subtropical island of Okinawa.
Chemical composition analysis at the single-particle level indicates dominance
by low-density polyethylene, which accounted for 75% of the total sub-micron
plastics analyzed. Our results show the occurrence of plastics at all test
sites, with the highest concentration in areas with high human activities. The
average, smallest sub-micron plastics size is (2.53 +/- 0.85)um for
polystyrene. We also observed additional Raman peaks on the plastics spectrum
with decreasing debris size which could be related to structural modification
due to weathering or embedding in organic matter. By single-particle level
sub-micron plastics identification, we can begin to understand their dispersion
in the ocean and define their toxicity and impacts on marine biodiversity and
food chain.
