I am doing my first downstream analysis on metagenomic WGS data. I was classifying the taxonomy kraken2 (custom database - all refseqs) and calculating species abundances with bracken.

My Idea was to use qiime2 to calculate unifrac distances, since it provides nice plotting features. I obtained the pyhlogenetic tree in newick format from the kraken database using this script.

I tried to import this tree to qiime2 using the following commands:

qiime tools import --input-path ncbi_taxonomy.newick --type 'Phylogeny[Unrooted]' --output-path tax_tree.qza
qiime phylogeny midpoint-root --i-tree tax_tree.qza --o-rooted-tree tax_tree_rooted.qza

..and I get the following error:

    Traceback (most recent call last):
  File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/skbio/tree/_tree.py", line 2418, in get_max_distance
    self._set_max_distance()
  File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/skbio/tree/_tree.py", line 2359, in _set_max_distance
    raise TreeError("No support for single descedent nodes")
skbio.tree._exception.TreeError: No support for single descedent nodes

During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/q2cli/commands.py", line 274, in __call__
        results = action(**arguments)
      File "<decorator-gen-206>", line 2, in midpoint_root
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/qiime2/sdk/action.py", line 231, in bound_callable
        output_types, provenance)
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/qiime2/sdk/action.py", line 362, in _callable_executor_
        output_views = self._callable(**view_args)
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/q2_phylogeny/_util.py", line 13, in midpoint_root
        return tree.root_at_midpoint()
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/skbio/tree/_tree.py", line 862, in root_at_midpoint
        max_dist, tips = tree.get_max_distance()
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/skbio/tree/_tree.py", line 2420, in get_max_distance
        return self._get_max_distance_singledesc()
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/skbio/tree/_tree.py", line 2376, in _get_max_distance_singledesc
        distmtx = self.tip_tip_distances()
      File "/opt/conda/envs/qiime2-2018.11/lib/python3.5/site-packages/skbio/tree/_tree.py", line 2501, in tip_tip_distances
        result = np.zeros((num_tips, num_tips), float)  # tip by tip matrix
    MemoryError

I verified that the input tree is in valid Newick format. I was running the command on a +300GB RAM machine, so memory shouldn't be an issue. I don't know if I am getting something wrong? Does qiime require pyhlogentic trees to show some certain features. I would be glad if somone can give me Ideas or guidelines on how to get a my data into qiime.