Heterotrophic Bacteria Enhance the Aggregation of the Marine Picocyanobacteria Prochlorococcus and Synechococcus

Abstract

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.

Keywords: Prochlorococcus; Synechococcus; aggregation; bacteria; transparent exopolymeric particles.

FIGURE 1

(A) Single cell abundance of Prochlorococcus (axenic – black circles; xenic – black triangles) as well as (B) Synechococcus (axenic – black circles; xenic – black triangles) and bacteria in corresponding xenic cultures (white triangles) throughout 17–19 day incubations. Error bars represent the standard error for duplicate cultures. Note error bars are smaller than the symbol sizes in some cases.

FIGURE 2

Epifluorescence (A,C,E,G) and corresponding brightfield (B,D,F,H) photomicrographs of Alcian Blue stained cultures of Prochlorococcus in axenic (A,B), and xenic (C,D) conditions, as well as Synechococcus in axenic (E,F), and xenic (G,H) conditions. Scale bars are 10 μm.

FIGURE 3

(A) TEP concentration throughout 17–19 day incubations of Synechococcus (axenic – black circles; xenic – black triangles), and Prochlorococcus (axenic – white circles; xenic – white triangles). (B) Volume concentration of aggregates present in the culture medium throughout 17–19 day incubations of Synechococcus (axenic – black circles; xenic – black triangles), and Prochlorococcus (axenic – white circles; xenic – white triangles). Error bars represent the standard error of duplicate cultures. Note error bars are smaller than the symbol sizes in some cases.