Pretty good odds that you are looking for Ken McLeod's 2005 short-story "A Case of Consilience". (After a James Blish story of very similar title, with which is shares some parallels.)
The story follows Rev. Donald MacIntyre's attempts to reconcile his missionary zeal with the constraints of the purposefully multi-cultural nature of the Extra-Terrestrial Contact Station.
In particular he is interested in a kind of
'underground fungoid a hundred meters across that communicates by chemical gradients'.
Scientists from the station have a tentative, partial translation of the signaling system, and one recorded signal seems to Donald to be a confession that these creatures know sin in the Christian sense of the word.
Station intelligence is concerned that Donald might attempt to violate rules on preferential cultural access. We pick up the action when he is next on the surface of the planet in question and approaches the mud patch between two of the fungiod colonies, in which the chemical gradients could be measured in transit.
'Donald stepped to the edge of the mud and set up the apparatus the team had devised for non-intrusive examination of the mycoid's messages [...].
'There as a small experiment he had been given to perform. It had been done many times before, to no effect. Perhaps this variant would be different. He reached into his pocket and pulled out the plastic covered gel-disc [...]. The concentric circles of molecular concentration that covered it spelled out—the team had hoped—the message "We wish to communicate. Please respond."
'Donald peeled off the cover [...]. He stuffed the crumpled wrappings in his pocket and reached in deeper for a second disc: one he'd covertly prepared with a different message.*
At this point security appears and talks him down, but he falls into the mud—which turns out to be quicksand. In the end he is not rescued due to circumstance beyond human control and dies there.
The rest is a rather major spoiler:
The mycoids find the layering of chemical information that is the late Donald MacIntyre much more interesting than the boring low-bandwidth messages the science team had been sending. They record all that he knew and read the GOOD NEWS therein.