A focus on different types of organic matter particles and their significance in the open ocean carbon cycle

Marine particles are key to the cycling of major elements on Earth and play an important role in the balance of nutrients in the ocean. Three main categories of marine particles link the different parts of the open ocean by shaping carbon distribution: (i) sinking; (ii) suspended, and (iii) ascending. Atmospheric carbon captured by phytoplankton in the surface water, is partly sequestered by sinking particles to the bottom of the ocean and plays an important role in controlling global climate. Suspended particles represent a significant source of organic carbon for heterotrophic microorganisms and are more likely to undergo remineralization compared to sinking particles. Ascending particles, depending on their composition, point of origin, and ascending velocity, may lead to carbon remineralization in the upper layers of the ocean in closer proximity to the atmosphere. Marine particles are hotspots of microbial activity and thus heavily colonized by microorganisms whose dynamics play an important role in organic matter degradation, aggregation and sinking, thus directly influencing the biological carbon pump efficiency. Microbiomes of marine particles differ depending on particle size, source, and age. Nevertheless, these factors are generally overlooked, and particles are mostly studied as “bulk” without considering the high heterogeneity between individual particles. This hinders our understanding of the carbon budget in the ocean and thus future predictions of climate change. In this review we examine known particle-types and associated sampling methods and identify knowledge gaps and emphasize the need for a better understanding of the single-particle ecosystem to enhance global upscaling rates. Furthermore, we introduce a novel concept: the ‘lipid carbon shunt’.