Thanks for sharing.
I haven't had the luck to meet native hornworts in Hong Kong yet, but recently I'm studying the evolution of biochemical pathways and land plant features in early land plants. Thought it'll be interesting to share a bit of the material I'm reading.
Apparently many hornworts have a biophysical carbon concentration mechanism (CCM) not found in any other groups of land plants. Hornworts that possess this mechanism, such as the Notothylas javanica
photographed above, have only one single chloroplast per photosynthesizing cell. Within the single plastid is a structure called the pyrenoid, which contain most (90%) of the RuBisCo in the cell. Bicarbonate is actively pumped to the pyrenoid, where a carbonic anhydrase converts bicarbonate back to carbon dioxide for RuBisCo to fix. This mechanism greatly increases the concentration of carbon dioxide in pyrenoids and minimizes photorespiration.
Evolutionarily, the presence of of pyrenoid-based biophysical CCM is advantageous for quite a few reasons. Firstly, current atmospheric CO2 levels (400 ppmv) are much lower than that when early land plants evolved (up to 3000 ppmv). It might well be the case that plants struggle to obtain sufficient CO2 to feed the RuBisCo after the Devonian drop of atmospheric CO2. Secondly, CO2 diffuses slowly in water, and much of the inorganic carbon is in its dissolved state. A biophysical CCM could help concentrate CO2 in wet and submerging habitats. In fact, many algal lineages, including land plant ancestors, also possess a pyrenoid and biophysical CCM.
There is still an ongoing debate of whether hornwort biophysical CCM is an evolutionary relic of the algal system. This paper shows the distribution of biophysical CCM in hornworts. It argues that biophysical CCM is not ancestral in the hornworts and has evolved independently for at least 6 times:
One of the earliest studies demonstrating the presence of a biophysical CCM in hornworts:
A paper this year reviewing biochemical CCM:
[ 本帖最後由 Aland 於 2018-5-22 19:00 編輯