Description: Spring phytoplankton blooms in temperate environments contribute disproportionately to global marine productivity. Bloom-derived organic matter, much of it occurring as polysaccharides, fuels biogeochemical cycles driven by interacting autotrophic and heterotrophic communities. We tracked changes in the mode of polysaccharide utilization by heterotrophic bacteria during the course of a diatom-dominated bloom in the German Bight, North Sea. Polysaccharides can be taken up in a ‘selfish’ mode, where initial hydrolysis is coupled to transport into the periplasm, such that little to no low-molecular weight (LMW) products are externally released to the environment. Alternatively, polysaccharides hydrolyzed by cell-surface attached or free extracellular enzymes (external hydrolysis) yield LMW products available to the wider bacterioplankton community. In the early bloom phase, selfish activity was accompanied by low extracellular hydrolysis rates of a few polysaccharides. As the bloom progressed, selfish uptake increased markedly, and external hydrolysis rates increased, but only for a limited range of substrates. The late bloom phase was characterized by high external hydrolysis rates of a broad range of polysaccharides and reduced selfish uptake of polysaccharides, except for laminarin. Substrate utilization mode is related both to substrate structural complexity and to the bloom-stage dependent composition of the heterotrophic bacterial community.

Description: The South Pacific Gyre (SPG) covers 10% of the ocean’s surface and is often regarded as a marine biological desert. To gain an on-site overview of the remote, ultraoligotrophic microbial community of the SPG, we developed a novel onboard analysis pipeline, which combines next-generation sequencing with fluorescence in situ hybridization and automated cell enumeration. We tested the pipeline during the SO-245 “UltraPac” cruise from Chile to New Zealand and found that the overall microbial community of the SPG was highly similar to those of other oceanic gyres. The SPG was dominated by 20 major bacterial clades, including SAR11, SAR116, the AEGEAN-169 marine group, SAR86, Prochlorococcus, SAR324, SAR406, and SAR202. Most of the bacterial clades showed a strong vertical (20 m to 5,000 m), but only a weak longitudinal (80°W to 160°W), distribution pattern. Surprisingly, in the central gyre, Prochlorococcus, the dominant photosynthetic organism, had only low cellular abundances in the upper waters (20 to 80 m) and was more frequent around the 1% irradiance zone (100 to 150 m). Instead, the surface waters of the central gyre were dominated by the SAR11, SAR86, and SAR116 clades known to harbor light-driven proton pumps. The alphaproteobacterial AEGEAN-169 marine group was particularly abundant in the surface waters of the central gyre, indicating a potentially interesting adaptation to ultraoligotrophic waters and high solar irradiance. In the future, the newly developed community analysis pipeline will allow for on-site insights into a microbial community within 35 h of sampling, which will permit more targeted sampling efforts and hypothesis-driven research.

Description: A process of global importance in carbon cycling is the remineralization of algae biomass by heterotrophic bacteria, most notably during massive marine algae blooms. Such blooms can trigger secondary blooms of planktonic bacteria that consist of swift successions of distinct bacterial clades, most prominently members of the Flavobacteriia, Gammaproteobacteria and the alphaproteobacterial Roseobacter clade. We investigated such successions during spring phytoplankton blooms in the southern North Sea (German Bight) for four consecutive years. Dense sampling and high-resolution taxonomic analyses allowed the detection of recurring patterns down to the genus level. Metagenome analyses also revealed recurrent patterns at the functional level, in particular with respect to algal polysaccharide degradation genes. We demonstrate that even though there is substantial inter-annual variation between spring phytoplankton blooms, the accompanying succession of bacterial clades is not a purely stochastic process, but also governed by deterministic principles such as substrate-induced forcing.

Description: Temperate coastal marine habitats are replete with complex biotic and abiotic interactions that mediate relationships and niche space between resident microbial community members. The dynamics of these interactions are amplified during spring and summer when phytoplankton form massive blooms and heterotrophic bacterioplankton respond to the successional release of high molecular weight dissolved organic matter (DOM, e.g., proteins, polysaccharides, lipids) as algal cells lyse. Dedicated clades of copiotrophic bacterioplankton adapted to high nutrient loads employ a suite of specialized enzymes and transporters for the degradation and uptake of large complex molecules like polysaccharides. Functional profiles of these carbohydrate active enzymes (CAZymes) reveal successions of niche spaces determined through substrate availability during bloom periods. As bacterioplankton process these substrates, algal-derived DOM is effectively incorporated into bacterioplankton biomass where it can be re-assimilated by higher protozoans and thus recycled within the food web or rapidly exported to sediments below. The net result is an enhancement of system-level efficiency and carbon export, respectively. In order to parse potential niche space in such a dynamic system, we first present diversity data derived from high-resolution temporal sampling (weekly to bi-weekly intervals) using the high-resolution oligotyping method Minimum Entropy Decomposition (MED) to cluster OTUs from 3 replicated sampling years 2010-2012. Differential abundance of MED OTUs 〉99% similar reveals a granularity of potential niche resolution that would not have been retrieved by traditional OTU clustering. Implementation of MED effectively extracts only those polymorphisms most relevant to in situ environmental selection pressure, essentially providing an ecological fingerprint of resident North Sea bacterioplankton. Despite inter-annual variation in phytoplankton blooms, we see a population of recurrent heterotrophic bacterioplankton during both baseline and bloom periods that is remarkably constrained in composition, relative community abundance and time of appearance each year for both abundant and rare OTUs. These robustly recurrent patterns reflect the selective power of seasonal forcing in shaping temperate microbial communities with low-frequency temperature-driven seasonal shifts. Superimposing effects of temperature are higher frequency shifts in OTUs during dynamic bloom events when substrate-induced forcing drives copiotrophic bacterioplankton communities, particularly within the Gammaproteobacteria and Flavobacteriia. Metagenome data from monthly sampling during spring blooms from these same years supports 16S rRNA diversity analyses revealing recurrent groups of specialized bloom taxa with highly constrained repertories of CAZymes. We demonstrate that even though there is substantial inter-annual variation of phytoplankton bloom intensity and taxonomic composition, the accompanying succession of bacterial clades is not a purely stochastic process, but also governed by deterministic principles such as temperature and substrate-induced forcing. The result is a resident bacterioplankton community containing as few as 6-14 dominant taxa each spring responsible for a vast majority of the diversity and thus potential function in a system previously thought to be considerably more stochastic and complex.