Abstract EANA2025-137

Climate and biology are deeply linked in shaping planetary habitability and observability properties. Microorganisms, especially photosynthetic ones, can profoundly influence planetary environments by altering atmospheric composition, surface reflectivity, and overall habitability. We combined experimental biological research with astrophysical and climate modeling to investigate how photosynthetic microorganisms may affect planetary habitability and the potential observability of atmospheric biosignatures. This work is part of the Agenzia Spaziale Italiana – ASTERIA project (ASI N. 2023-5-U.0), which aims to explore the adaptability and productivity of photosynthetic microorganisms to ultraviolet radiation and a variety of simulated planetary environments. On the biological side, we selected the cyanobacterium Synechocystis sp. PCC 6803 exposed to simulated Solar (G2 V) light as a model microorganism capable of surviving under low-light conditions (Battistuzzi+2023). On the astrophysical and climatological side, and to evaluate the impact of microbial activity on planetary habitability, we evaluated the potential impact of microbial activity on planetary climate and habitability using our climate and atmospheric model, pRT-ESTM (Murante+2020; Bisesi+2024), which we interfaced with radiative transfer calculations from petitRADTRANS (Mollière+2019). To assess the impact of cyanobacterial reflectivity on surface albedo and temperature, we implemented laboratory-measured reflectance properties into pRT-ESTM and explored scenarios with varying surface coverage by cyanobacteria, oceans, and granite, across a range of atmospheric compositions and solar luminosities suitable for the Archean Earth. Different surface configurations are also expected to influence the potential for atmospheric oxygenation, by altering the extent of photosynthetically active areas. We considered cyanobacterial surface coverage fractions ranging from 0.05 to 0.3 and found that even within this range, the climate can be strongly affected. It is currently believed that a Snowball Earth event, the Huronian glaciation, occurred after the Great Oxidation Event (GOE) in the Late Archean (~2.5 Gyr ago). Multiple factors likely contributed to this glaciation. In our preliminary experiments, we adopted high atmospheric CO2 concentrations (40,000 ppm, compared to the present-day ~400 ppm), which were the minimum required in our model to maintain a warm climate in the absence of the albedo effect induced by cyanobacteria. However, we find that for sufficiently high oceanic coverage by cyanobacteria (≥ 25%), the planet transitions into a snowball state – without any further reduction in CO2-purely due to the increase in surface albedo. Though preliminary, these results show that cyanobacteria may exert two cooling influences: not only they produce O2, which contributes to CH4 removal, but also, they directly increase oceanic albedo. These combined effects could have contributed to global cooling. In fact, in other scenarios we examined, significant oceanic coverage by cyanobacteria consistently lowers the global mean annual temperature by ~1–4 °C. This feedback mechanism should therefore be considered alongside other well-known climate feedbacks. Such an effect may be less important in the Mesoarchean (~3 Gyr ago), when the shortage of continents likely limited widespread colonization and the associated albedo change.