Final answer:
The sequence in question is most likely from a duckweed Spirodela polyrhiza based on the principle of sequence homology indicated by a significant match in a BLASTN search. Further support for this conclusion comes from the close match's implication of evolutionary relatedness.
Step-by-step explanation:
When conducting a BLASTN search and the only significant sequence match is to the duckweed Spirodela polyrhiza, the most reasonable conclusion is: The sequence is most likely from a duckweed Spirodela polyrhiza. This conclusion is based on the principle of sequence homology, where a high degree of similarity typically indicates a close evolutionary relationship between the sequences in question. The Bacillus sequences listed for alignment in a lab notebook are an example of how multiple sequences can be compared to determine conserved regions across related bacterial species.
In the same vein, if the query sequence submitted to BLAST closely resembles the sequence from Spirodela polyrhiza, it's logical to attribute the origin of the sequence to this species of duckweed. This conclusion is based on the assumption that the database is comprehensive; however, if the database lacks sequences from certain organisms, there might still be a possibility that the sequence originates from an under-represented species.