Sunday, 6 April 2014

Hard life for bacteria in the ocean, some more bacterial grazer just identified.

Molecular survey in the ocean have recently decoded new diversity of microorganisms.  Novel lineages within the three domains of life were unveiled opening new evolutionary and ecological avenues. For example in the eukaryotes were found recently (last year) two new lineages: Picozoa and Rappemonads, both at high taxonomic rank. Such new diversity was detected from ribogroups (i.e. 18S rDNA environmental sequences forming monophyletic clades). People working in this field use also additional tool as FISH, single-cell genomics and (when possible) isolation in pure culture.  

In marine planktonic picoeukaryotes molecular survey had highlighted novel lineage within the supergroup of alveolates and stramenopiles, respectively named MALV and MAST (Marine ALVeolates and STramenopeles). This two group encompassed on the average of 32% and 13% of sequences in picoeukaryotes surveys, respectively. In the MAST group were defined 12 group and 7 more were proposed. Most of these group are uncultured and at the moment are available few data on their ecology and physiology. However in some group, using specific oligonucleotide probe with FISH method was possible discover that MAST cell measures 2-5 μm, are heterotrophic and flagellate. And most important, they are active bacterial grazers.

In this study the main aim was reevaluate the phylogeny, diversity and ecology of MAST ribogroups using three indepent 18S rDNA surveys. The authors (1) search in the GenBank database for stramenopile sequences, (2) analyzed data set of 454 pyrotags from European coastal site (including oxygenated seawater, sediment and anoxic plankton) and (3) used a collection of single amplified genome (SAGs) from single-cell protists.

The authors develop a phylogenetic tree that allow them to identify three additional MAST clades. These clades were rare in environmental sequences so that could explain why these were not identified before. Novel diversity was unveiled in Ochrophyta ribogroups (in this sub-phylum are encompassed diatoms and many other macroalgal group as for example brown algae) and one of these five new group (named MOCH-2) was quantitatively important (around 0.8% of pyrotags that means “a lot”). This was a member of the oxic pico- and nanoplankton. From SAGs was possible to identify some of these MOCH as new algal lineage (plastidic cell) while some other heterotrophic (aplastidic). In these work were also presented the most recent stramenopiles phylogenetic tree based on the available 18S rDNA gene sequencing. From this was clear the main division of stramenopiles phylum in two mayor sub-groups: Ochrophyta (all photosynthetic) and basal heterotrophic taxa (comprehensive also of MAST ribogroups). Combining this data with GenBank sequences authors showed that most of this picoeukaryotes are diffused in all the oceans all over the world suggesting the absence of marked geographic barriers. This confirm the supposed capacity of these minute picoeukariotes to exhibit global dispersion. In this survey also was presented the role of the oxygen status as strong barrier for protists colonization and diversification since 7 MAST group were identified as typical of anoxic systems.

In conclusion and in my opinion, the most important result presented in this work is that 11 MAST groups populate heterogeneous assemblage in surface ocean water all around the world. These group are heterotrophic and for some of them was demonstrated active bacterial grazing capacity. These means that a huge number of phylogenetic diverse picoeukariotes every day in the ocean eat bacteria having a great role in the microbial food webs of one of largest biomes on earth. These results open the way on further studies deeper in the ecology and physiology of these large groups of uncultured planktonic characters. Hopefully using other molecular tools such as FISH probes will be possible to visualize directly these great diversity. An open question is what drive and maintains the diversity of these phylogenetically various but apparently functionally redundant  group of bacterial grazers. Each picoeukariote eat a different bacteria?

Massana, R., del Campo, J., Sieracki, M. E., Audic, S., & Logares, R. (2013). Exploring the uncultured microeukaryote majority in the oceans: reevaluation of ribogroups within stramenopiles. The ISME journal.

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