Gradients of bacteria in the oceanic water column reveal finely-resolved vertical distributions

Bacterial communities directly influence ecological processes in the ocean, and depth has a major influence due to the changeover in primary energy sources between the sunlit photic zone and dark ocean. Here, we examine the abundance and diversity of bacteria in Monterey Bay depth profiles collected from the surface to just above the sediments (e.g., 2000 m). Bacterial abundance in these Pacific Ocean samples decreased by >1 order of magnitude, from 1.22 ±0.69 ×106 cells ml-1 in the variable photic zone to 1.44 ± 0.25 ×105 and 6.71 ± 1.23 ×104 cells ml-1 in the mesopelagic and bathypelagic, respectively. V1-V2 16S rRNA gene profiling showed diversity increased sharply between the photic and mesopelagic zones. Weighted Gene Correlation Network Analysis clustered co-occurring bacterial amplicon sequence variants (ASVs) into seven subnetwork modules, of which five strongly correlated with depth-related factors. Within surface-associated modules there was a clear distinction between a 'copiotrophic' module, correlating with chlorophyll and dominated by e.g., Flavobacteriales and Rhodobacteraceae, and an 'oligotrophic' module dominated by diverse Oceanospirillales (such as uncultured JL-ETNP-Y6, SAR86) and Pelagibacterales. Phylogenetic reconstructions of Pelagibacterales and SAR324 using full-length 16S rRNA gene data revealed several additional subclades, expanding known microdiversity within these abundant lineages, including new Pelagibacterales subclades Ia.B, Id, and IIc, which comprised 4-10% of amplicons depending on the subclade and depth zone. SAR324 and Oceanospirillales dominated in the mesopelagic, with SAR324 clade II exhibiting its highest relative abundances (17±4%) in the lower mesopelagic (300-750 m). The two newly-identified SAR324 clades showed highest relative abundances in the photic zone (clade III), while clade IV was extremely low in relative abundance, but present across dark ocean depths. Hierarchical clustering placed microbial communities from 900 m samples with those from the bathypelagic, where Marinimicrobia was distinctively relatively abundant. The patterns resolved herein, through high resolution and statistical replication, establish baselines for marine bacterial abundance and taxonomic distributions across the Monterey Bay water column, against which future change can be assessed.

Choanoflagellates alongside diverse uncultured predatory protists consume the abundant open-ocean cyanobacterium Prochlorococcus

Prochlorococcus is a key member of open-ocean primary producer communities. Despite its importance, little is known about the predators that consume this cyanobacterium and make its biomass available to higher trophic levels. We identify potential predators along a gradient wherein Prochlorococcus abundance increased from near detection limits (coastal California) to >200,000 cells mL^-1 (subtropical North Pacific Gyre). A replicated RNA-Stable Isotope Probing experiment involving the in situ community, and labeled Prochlorococcus as prey, revealed choanoflagellates as the most active predators of Prochlorococcus, alongside a radiolarian, chrysophytes, dictyochophytes, and specific MAST lineages. These predators were not appropriately highlighted in multiyear conventional 18S rRNA gene amplicon surveys where dinoflagellates and other taxa had highest relative amplicon abundances across the gradient. In identifying direct consumers of Prochlorococcus, we reveal food-web linkages of individual protistan taxa and resolve routes of carbon transfer from the base of marine food webs.

The need to account for cell biology in characterizing predatory mixotrophs in aquatic environments

Photosynthesis in eukaryotes first arose through phagocytotic processes wherein an engulfed cyanobacterium was not digested, but instead became a permanent organelle. Other photosynthetic lineages then arose when eukaryotic cells engulfed other already photosynthetic eukaryotic cells. Some of the resulting lineages subsequently lost their ability for phagocytosis, while many others maintained the ability to do both processes. These mixotrophic taxa have more complicated ecological roles, in that they are both primary producers and consumers that can shift more towards producing the organic matter that forms the base of aquatic food chains, or towards respiring and releasing CO₂. We still have much to learn about which taxa are predatory mixotrophs as well as about the physiological consequences of this lifestyle, in part, because much of the diversity of unicellular eukaryotes in aquatic ecosystems remains uncultured. Here, we discuss existing methods for studying predatory mixotrophs, their individual biases, and how single-cell approaches can enhance knowledge of these important taxa. The question remains what the gold standard should be for assigning a mixotrophic status to ill-characterized or uncultured taxa-a status that dictates how organisms are incorporated into carbon cycle models and how their ecosystem roles may shift in future lakes and oceans. This article is part of a discussion meeting issue 'Single cell ecology'.

Characterization of chasmoendolithic community in Miers Valley, McMurdo Dry Valleys, Antarctica

The Antarctic Dry Valleys are unable to support higher plant and animal life and so microbial communities dominate biotic ecosystem processes. Soil communities are well characterized, but rocky surfaces have also emerged as a significant microbial habitat. Here, we identify extensive colonization of weathered granite on a landscape scale by chasmoendolithic microbial communities. A transect across north-facing and south-facing slopes plus valley floor moraines revealed 30-100 % of available substrate was colonized up to an altitude of 800 m. Communities were assessed at a multidomain level and were clearly distinct from those in surrounding soils and other rock-inhabiting cryptoendolithic and hypolithic communities. All colonized rocks were dominated by the cyanobacterial genus Leptolyngbya (Oscillatoriales), with heterotrophic bacteria, archaea, algae, and fungi also identified. Striking patterns in community distribution were evident with regard to microclimate as determined by aspect. Notably, a shift in cyanobacterial assemblages from Chroococcidiopsis-like phylotypes (Pleurocapsales) on colder-drier slopes, to Synechococcus-like phylotypes (Chroococcales) on warmer-wetter slopes. Greater relative abundance of known desiccation-tolerant bacterial taxa occurred on colder-drier slopes. Archaeal phylotypes indicated halotolerant taxa and also taxa possibly derived from nearby volcanic sources. Among the eukaryotes, the lichen photobiont Trebouxia (Chlorophyta) was ubiquitous, but known lichen-forming fungi were not recovered. Instead, fungal assemblages were dominated by ascomycetous yeasts. We conclude that chasmoendoliths likely constitute a significant geobiological phenomenon at lower elevations in granite-dominated Antarctic Dry Valley systems.