Did you say marine snow? Zooming into different types of organic matter particles and their importance 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, having an important role in controlling global climate. Suspended particles represent a major substrate of organic carbon for heterotrophic microorganisms, being more likely to get remineralized. Ascending particles, depending on their content, 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 discuss characteristics of known particle-types and associated sampling methods. We further identify gaps in knowledge and highlight the need to better understand the single particles ecosystem to improve upscaling rates to the global scale.