Tiny photosynthetic organisms (picocyanobacteria, mainly Synechococcus strains smaller than 2 micrometers) in the Baltic Sea can substantially help transport carbon from surface waters to deeper layers by forming colonies and aggregates that sink more readily.

A study from the Department of Ecology, Environment and Plant Sciences at Stockholm University (led by Martin Ekman, with co- author Rachel Foster) shows that the tendency of different strains to aggregate directly controls how much of their biomass is exported downward.

Individual cells are too small to sink efficiently on their own, but colony- forming strains appear more often in larger size fractions and in sediment-trap material. Their contribution to exported biomass was broadly proportional to their abundance in surface waters (often a large share of the phytoplankton community).

Researchers sampled surface water plus depths of 25 m and 75 m at Landsort Deep (a long- term monitoring site) during spring and summer, used sediment traps below the sunlit zone, and combined microscopy, flow cytometry, size fractionation (>10 µm vs. free/small cells), and genetic sequencing (16S rRNA).

Seasonal differences appeared between colony- forming and mostly single- cell strains, suggesting aggregation is an important ecological trait.

This matters for the biological carbon pump: phytoplankton photosynthesis takes up CO₂, and sinking biomass moves that carbon deeper, helping regulate atmospheric CO₂. Remineralization of the sinking material also contributes to oxygen- poor (anoxic) bottom waters in places like the Baltic Sea.

Because picophytoplankton are expected to become more abundant in warmer, more stratified, nutrient-poorer future oceans, understanding how their biomass can still be exported is relevant for predicting carbon cycling and atmospheric CO₂ levels. The authors note that similar processes should be checked in other picophytoplankton-dominated regions, such as open- ocean gyres.

The peer- reviewed paper is “Differential aggregation properties among Baltic Sea picocyanobacterial strains impact their export,” published in The ISME Journal (2026).

The biological carbon pump (BCP) is a suite of biologically mediated processes that transfer carbon fixed by photosynthesis in the sunlit surface ocean (euphotic zone) into the ocean interior, sequestering it away from exchange with the atmosphere on timescales of months to millennia.

It is a major component of the global carbon cycle. Phytoplankton perform roughly half of Earth’s photosynthesis, drawing down dissolved inorganic carbon (DIC, including CO₂) and helping the ocean absorb atmospheric CO₂. Without the BCP, atmospheric CO₂ would be substantially higher (estimates often cite ~200 ppm higher in a world without export).

The BCP is often discussed alongside the solubility (or physical) pump and the carbonate (or alkalinity and hard- tissue) pump. A related concept is the microbial carbon pump, which transforms labile dissolved organic carbon (DOC) into more refractory forms that can persist for centuries to millennia.

Core Stages of the Biological Carbon Pump

1. Primary production: Phytoplankton (including diatoms, coccolithophores, and picocyanobacteria such as Synechococcus) fix CO₂ into organic matter (particulate organic carbon (POC) and dissolved organic carbon (DOC) via photosynthesis in the euphotic zone.
2. Food- web processing and transformation: Much of the fixed carbon is rapidly recycled in the surface layer by grazing, respiration, and bacterial degradation (the microbial loop). A fraction escapes recycling and is available for export.
3. Export from the surface: Organic carbon (and some inorganic carbon) moves downward via multiple pathways.
4. Attenuation and remineralization: As material sinks or is transported, microbes and zooplankton consume and respire it, releasing CO₂ (DIC) at various depths. The depth of remineralization largely determines how long the carbon is sequestered.
5. Sequestration: Carbon that reaches greater depths (or is transported into poorly ventilated water masses) remains isolated from the atmosphere longer.

Main Export Mechanisms (Pathways)

Modern frameworks often group processes into three broad categories (gravitational, migrant, and mixing/physical injection pumps), sometimes expanded into six more specific “particle injection pumps”.⁠

1. Gravitational (sinking) pump: The classic pathway, often the largest contributor to net export.

• Sinking particles: individual cells (especially larger phytoplankton), phytodetritus, marine snow (aggregates of cells, debris, transparent exopolymer particles- TEP), and zooplankton fecal pellets.
• Aggregation is critical: small cells (e.g., picocyanobacteria <2 µm) rarely sink alone but can form colonies or stick together into faster- sinking aggregates, as shown in the Baltic Sea study. Ballasting by dense minerals (opal from diatoms, calcium carbonate from coccolithophores) increases sinking speeds.
• Marine snow and fecal pellets sink at rates from meters to hundreds of meters per day. Recent work highlights that the internal viscosity structure of aggregates influences bacterial colonization and degradation rates, more viscous particles degrade more slowly and can sequester carbon more effectively.⁠

2. Migrant (animal- driven) pumps

• Diel vertical migration: Zooplankton (and some fish) feed near the surface at night and migrate to deeper waters by day, where they respire, excrete, and defecate, actively transporting carbon downward.
• Seasonal and ontogenetic (lipid) pump: High- latitude zooplankton (especially copepods) accumulate lipid stores in surface waters during productive seasons, then migrate to depth for overwintering (diapause), releasing carbon via respiration or mortality.

3. Mixing and physical injection (particle injection) pumps:

These transport both particulate and dissolved organic carbon via physical processes acting across different scales:

• Mixed- layer pump: Seasonal deepening and subsequent shoaling of the mixed layer detains organic matter below the new mixed- layer depth.
• Eddy and submesoscale subduction pump: Intense vertical velocities at fronts and eddies inject surface water (and its organic carbon) downward.
• Large- scale subduction pump: Associated with large- scale circulation and Ekman pumping.

Additional contributions come from DOC export (some DOC is labile and quickly remineralized; a fraction becomes refractory via the microbial carbon pump) and from the carbonate pump (formation and sinking of CaCO₃ shells, which has different chemical effects on seawater alkalinity and CO₂).

Global estimates of organic carbon export from the euphotic zone are typically in the range of ~10- 15 Pg C yr⁻¹ (with substantial uncertainty and rapid attenuation so that much less reaches the deep ocean). A significant fraction of century- scale sequestration can occur above 1,000 m in some regions (“continuous vertical sequestration”).

Efficiency, Variability, and Climate Relevance

Export efficiency (the fraction of primary production that is exported) and transfer efficiency (how much of the export reaches greater depths) vary strongly with region, season, ecosystem structure (e.g., size of phytoplankton, zooplankton communities), temperature, nutrient availability, and oxygen levels. Warmer, more stratified, nutrient- poor oceans are expected to favor smaller phytoplankton (including picophytoplankton), which may alter aggregation and export dynamics.

The BCP also influences oxygen distributions: remineralization of sinking organic matter is a major cause of oxygen minimum zones and anoxic conditions in places such as the Baltic Sea.

Climate change is projected to affect the BCP through shifts in primary production, community composition, sinking rates, remineralization rates (often temperature- dependent), and ocean circulation. Earth- system models currently disagree on the net future change in global export flux.

In short, the biological carbon pump is not a single process but an interconnected set of biological, chemical, and physical mechanisms whose combined effect helps regulate atmospheric CO₂ and sustains deep- ocean ecosystems. Aggregation of even the smallest photosynthetic cells (as in the Baltic picocyanobacteria example) is one concrete illustration of how microscopic traits scale up to influence global carbon cycling.

Differential aggregation properties among Baltic Sea picocyanobacterial strains impact their export

Key Findings (from the abstract and available excerpts)

Picocyanobacteria (especially Synechococcus) contributes substantially to carbon export in the ocean, but the mechanism allowing their tiny cells (<2 µm) to reach depth was unclear. This study provides in situ evidence from the Baltic Sea (Landsort Deep area) linking cell aggregation and colony formation directly to sedimentation.

• Picocyanobacterial contribution to phytoplankton biomass export was largely proportional (70- 90%) to their abundance in surface waters.
• Both cell aggregation (enrichment in the >10 µm size fraction) and sedimentation (enrichment in sediment- trap material) varied among different Synechococcus amplicon sequence variants (ASVs/strains).
• A robust linear relationship exists between an ASV’s enrichment in the large-size fraction and its enrichment in sediment traps. This indicates that the degree of aggregation is a direct determinant of how much of that strain sinks.
• Dominant aggregate- forming ASVs showed similar seasonal abundance patterns that differed from single- cell ASVs, suggesting that colony/aggregate formation is an important ecological trait influencing strain distribution and responses to environmental conditions (possibly nutrient dynamics).
• By correlating microscopy observations with sequence data, the authors propose genotypes for the dominant colonial picocyanobacterial morphotypes in the Baltic Sea.

Methods Snapshot

Researchers combined:

• Size- fractionated water sampling (surface, 25 m, 75 m)
• Sediment traps
• Microscopy and flow cytometry
• 16S rRNA gene sequencing of free- living vs. aggregated fractions and trap material

Broader Implications

These results help explain how pico- sized phytoplankton biomass is exported from the photic zone. Because picophytoplankton are expected to become more abundant in warmer, more stratified, nutrient-poorer future oceans, understanding aggregation mechanisms is important for predicting biological carbon pump strength and atmospheric CO₂ levels. The authors recommend investigating similar processes in other picophytoplankton- dominated regions (e.g., open- ocean gyres).

This is the peer- reviewed source behind the Stockholm University press release discussed earlier.

The full text is available via the Oxford Academic link (may require institutional access or open- access options).

Journal: The ISME Journal (2026), volume 20, article wrag214

DOI: 10.1093/ismejo/wrag214

Link: academic.oup.com/ismej/article/20/1/wrag214/8772006

Provided: Stockholm University 

Published: 27 August 2026

Authors: Martin Ekman ,

Elina Viinamäki,

Anders Verstraaten-Svensson,

Ryan W Paerl,

Anders F Andersson,

Helena Höglander,

Jakob Walve,

Rachel A Foster

Abstract

Picocyanobacteria contribute significantly to carbon export but the mechanism by which their biomass is transported to depth is unclear. By analyzing 16S rRNA gene sequences from sediment trap material and size-fractionated water samples from different depths in the Baltic Sea, we show that picocyanobacterial contribution to phytoplankton biomass export was largely proportional (70%–90%) to their surface water abundances. However, we also found that both cell aggregation (enrichment in >10 μm fraction) and sedimentation (trap enrichment) varied among Synechococcus ASVs (amplicon sequence variants). A robust linear relationship was identified between ASV enrichment in traps and ASV enrichment in the >10 μm fraction, suggesting that cell aggregation is a direct determinant of Synechococcus sedimentation. Seasonal abundance patterns were similar among the dominant aggregate-forming ASVs but distinct from those of single-cell ASVs, suggesting aggregate/colony formation being an important trait in determining distribution of Synechococcus strains and their response to environmental conditions. Correlating microscopy- and sequence-based abundances further allowed us to propose genotypes for the dominant Baltic Sea colonial picocyanobacterial morphotypes. Our results provide in situ data linking phenotypic characteristics of picocyanobacteria with sedimentation properties and several lines of evidence for a quantitative dependency between surface water picocyanobacterial aggregate/colony formation and sinking. These findings are important for our understanding of mechanisms involved in export of pico-sized phytoplankton biomass from the photic zone to depth and emphasize the need to investigate occurrence and environmental regulation of cell aggregation in other picophytoplankton-dominated waters (e.g. open ocean gyres).