Constraints on Aquatic Photosynthesis for Terrestrial Planets Around Other Stars

TL;DR

This paper examines the potential for aquatic photosynthesis on Earth-like exoplanets around different star types, analyzing light attenuation, depth limits, and biosignature robustness, especially around M-dwarfs.

Contribution

It provides a detailed analysis of how stellar spectral type affects aquatic photosynthesis limits and proposes the red edge as a biosignature for M-dwarf planets.

Findings

Compensation depth declines significantly from Sun-like to M-dwarf stars.

Critical depth decreases modestly across star types.

Red edge may be a more reliable biosignature than atmospheric oxygen on M-dwarf planets.

Abstract

Aquatic photosynthesis plays a major role in carbon fixation and O$_2$ production on Earth. In this Letter, we analyze the prospects for oxygenic photosynthesis in aquatic environments on modern Earth-analogs around F-, G-, K- and M-type stars. Our analysis takes into account the spectral type of the host star, attenuation of light by aquatic organisms, and rates of respiration and photosynthesis. We study the compensation depth ($\mathcal{Z}_\mathrm{CO}$) and the critical depth ($\mathcal{Z}_\mathrm{CR}$), defined respectively as the locations where the net growth rates and vertically integrated net growth rates of photoautotrophs become zero. Our analysis suggests that $\mathcal{Z}_\mathrm{CO}$ declines by more than an order of magnitude as one moves from the habitable zones around Sun-like stars to late-type M-dwarfs, but $\mathcal{Z}_\mathrm{CR}$ decreases by only a modest amount ($\sim 40\%$). For M-dwarf exoplanets, we propose that the photosynthetic red edge may constitute a more robust biosignature of aquatic photosynthesis compared to atmospheric O$_2$.