Microplastics in the ocean function as an artificial microbial reef, with diverse communities of eukaryotic and bacterial microbiota colonizing its surface. It is not well understood if these communities are specific for the type of microplastic on which they develop. Here, we carried out a 6‐week long incubation experiment of six common plastic polymers in Bocas del Toro, Panama. The community composition of prokaryotes based on 16S rRNA gene sequencing data, when judged under a null model analysis, shows that neither plastic polymer type nor time exposed to the environment plays a significant role in shaping biofilm communities. However, the null model analyses of eukaryotic communities based on 18S rRNA gene sequences reveal that they can be significantly influenced by plastic polymer type and time incubated. This was confirmed by scanning electron microscopy, which allowed us to distinguish plastic‐specific diatom communities by the end of the incubation period.

Enhancement of primary and export production following dust deposition pulses is well established for High Nutrient Low Chlorophyll regions, but the effect of atmospheric dust on the biogeochemistry of oligotrophic gyre regions remains unclear. Here we report atmospheric dust concentrations measured on Gran Canaria, Canary Islands, concomitantly with upper water biogeochemistry at the oligotrophic time series station ESTOC (European Station for Time series in the Ocean, Canary Islands) during a 2‐year period from winter 1997 to 1999. ESTOC is located in the eastern subtropical Atlantic gyre about 100 km north of Gran Canaria and 500 km west of the Sahara, and receives aeolian dust episodically mainly in winter and summer. We found a close correlation of the magnitude of atmospheric dust concentration in winter with the magnitude of downward particle flux at ESTOC, mainly due to a relative increase in lithogenic matter and carbonate sedimentation. Higher aerosol concentration was not accompanied by higher primary or export production, however, indicating that phytoplankton production remained unaffected by atmospheric nitrogen supply on a seasonal or yearly timescale. However, by estimating bioavailable iron input and the need of the phytoplankton population, we found that the highly episodic dust pulses might exert a feast and famine effect on the phytoplankton. Despite the high lithogenic matter input, carbonate, mainly stemming from coccolithophorids, exceeded in importance the role of lithogenic matter as ballasting agent of sinking organic matter.

Marine picocyanobacteria are ubiquitous primary producers across the world’s oceans, and play a key role in the global carbon cycle. Recent evidence stemming from in situ investigations have shown that picocyanobacteria are able to sink out of the euphotic zone to depth, which has traditionally been associated with larger, mineral ballasted cells. The mechanisms behind the sinking of picocyanobacteria remain a point of contention, given that they are too small to sink on their own. To gain a mechanistic understanding of the potential role of picocyanobacteria in carbon export, we tested their ability to form “suspended” (5–60 μm) and “visible” (ca. > 0.1 mm) aggregates, as well as their production of transparent exopolymer particles (TEP)—which are a key component in the formation of marine aggregates. Additionally, we investigated if interactions with heterotrophic bacteria play a role in TEP production and aggregation in Prochlorococcus and Synechococcus by comparing xenic and axenic cultures. We observed TEP production and aggregation in batch cultures of axenic Synechococcus, but not in axenic Prochlorococcus. Heterotrophic bacteria enhanced TEP production as well as suspended and visible aggregate formation in Prochlorococcus, while in Synechococcus, aggregation was enhanced with no changes in TEP. Aggregation experiments using a natural plankton community dominated by picocyanobacteria resulted in aggregation only in the presence of the ballasting mineral kaolinite, and only when Synechococcus were in their highest seasonal abundance. Our results point to a different export potential between the two picocyanobacteria, which may be mediated by interactions with heterotrophic bacteria and presence of ballasting minerals. Further studies are needed to clarify the mechanistic role of bacteria in TEP production and aggregation of these picocyanobacteria.

Cyanobacteria are considered to play an important role in the oceanic biological carbon pump, especially in oligotrophic regions. But as single cells are too small to sink, their carbon export has to be mediated by aggregate formation and possible consumption by zooplankton producing sinking fecal pellets. Here, we study the aggregation and subsequent settling of the ubiquitous marine picocyanobacterium Synechococcus as a model organism in roller tanks and investigate the effects of the clays kaolinite and bentonite in a series of concentrations. We found that (1) Synechococcus cells formed aggregates as large as 1.4 mm in diameter that sank at velocities of up to 440 m d⁻¹ and 660 m d⁻¹ with and without ballasting clays, respectively; (2) clay minerals increased the number and reduced the size of aggregates, and their ballasting effects increased the excess density and sinking velocity of aggregates; (3) kaolinite, with its lower cation exchange capacity, resulted in smaller and slower sinking aggregates compared to bentonite; and (4) based on our experimental conditions, Synechococcus could potentially export carbon 2–3 times of that contained in their cells via aggregation, likely due to the scavenging of transparent exopolymer particles and dissolved organic matter. The presented results have relevance for our understanding of the role of cyanobacteria in the ecology and biogeochemistry in today's ocean and also elucidate mechanisms of carbon export in the early Proterozoic ocean.