BOL

The abundance of a species in a dataset is considered as the fraction of reads that belong to that species. For example, if there is a dataset with 10,000,000 reads and 1,000,000 of them belong to E. coli, then the abundance of E. coli will be 0.1.

/home/emilyw/.conda/envs/shotgun_134/bin/bowtie2-align-s: error while loading shared libraries: libtbb.so.2: cannot open shared object file: No such file or directory (ERR): Description of arguments failed! Exiting now ...

How to resolve this ? 

For Linux 64, Open MPI is built with CUDA awareness but this support is disabled by default.
To enable it, please set the environment variable OMPI_MCA_opal_cuda_support=true before
launching your MPI processes. Equivalently, you can set the MCA parameter in the command line:
mpiexec --mca opal_cuda_support 1 ...

In addition, the UCX support is also built but disabled by default.
To enable it, first install UCX (conda install -c conda-forge ucx). Then, set the environment
variables OMPI_MCA_pml="ucx" OMPI_MCA_osc="ucx" before launching your MPI processes.
Equivalently, you can set the MCA parameters in the command line:
mpiexec --mca pml ucx --mca osc ucx ...
Note that you might also need to set UCX_MEMTYPE_CACHE=n for CUDA awareness via UCX.
Please consult UCX's documentation for detail.