\section*{APPENDIX A: Materials and methods}

\textbf{Cell cultivation.} \emph{Chlamydomonas reinhardtii}, strain SAG 77.81, were grown axenically in BG-11 medium at $20^{\circ} \mathrm{C}$ at a light intensity of $\approx18\,\upmu\mathrm{mol}/ \mathrm{m^{2}}\mathrm{s}$. Samples from the culture were taken during its logarithmic growing stage and mixed with BG-11 medium to achieve a dilute suspension.\\

\textbf{Substrate and sample preparation.} As a substrate we used a partial-wetting glass slide obtained with a surface silanization technique. Prior to surface silanization, the glass slide was cleaned in a Zepto Electronic Diener plasma cleaner for $60 \mathrm{s}$. Then the glass slide was immersed in a mixture of 1 mL trichloro (octyl)-silane in 99 mL toluene. This procedure introduces a moderate degree of hydrophobicity, which we found to prevent algae cells from sticking to the substrate. The glass slide was cleaned with ethanol and deionized water prior use.\\

\textbf{Experimental settings.} The drying process of the microalgae drops was monitored using an inverted phase contrast microscope at 5x and 10x magnification. A 650 nm longpass filter was added for the experiments involving red light illumination. For the side light measurements, we positioned a collimated white light source at an incident angle of $\approx 45^{\circ}$.\\

\textbf{Tracking.} For the tracking employed in this work, we used the python-based package trackpy \cite{Crocker1996,DanAllan2019}. In the experiments involving the analysis of the whole drops we tracked particles within consecutive frames at 1 frames per second. The position vector is noted \textbf{r}=(x,y) with (x,y) being the in-plane coordinates of the algae. The vorticity \textbf{$\omega$} was calculated from the in-plane velocity \textbf{u}=($u_{x}$,$u_{y}$) as  
\begin{equation}
\omega = \frac{\partial u_{y}}{\partial x} - \frac{\partial u_{x}}{\partial y}
\end{equation}
For the in-depth analysis of the cell-surface interaction we extended the tracking time to 10 seconds recording at 10 frames per second. Within the tracking procedure we always imposed a threshold to filter out non-moving particles.
We extracted the mean square displacement (MSD):
\begin{equation}
\mathrm{MSD}(\tau) = \langle {\lvert \textbf{r}(t+\tau)-\textbf{r}(t) \rvert}^{2} \rangle
\end{equation}
where $\langle\ldots\rangle$ indicates the time average.

\section*{APPENDIX B: Supplementary Information}

\subsection*{Drop shape analysis}
To measure the time-dependent contact angle of a drop containing \textit{Chlamydomonas reinhardtii} (\textit{C.~R.}) deposited on a glass slide, we used a Drop Shape Analyzer from Krüss. For the measurements, we deposited $0.3\,\upmu\mathrm{l}$ of a drop containing algae on a silanized glass slide and monitored the evaporation every 5 seconds. This allows to extract information on both contact angle and contact radius during the evaporation (Sup.~Fig.~\ref{fig:figS1}).
We observe that the contact angle decreases, while the contact radius remains constant. The drop is pinned to the glass slide during the whole drying process.

\begin{figure}[t!]
\centering 
\includegraphics[scale=1]{SI/FigS1.pdf}
\caption{Drop shape analysis of a drop containing microalgae. The drop remains pinned during the evaporation process with a steadily decreasing contact angle. Scale bar is $300\,\upmu \mathrm{m}$.}
\label{fig:figS1}
\end{figure}

\begin{figure}[h!]
\centering 
\includegraphics[width=\columnwidth]{SI/FigS2.pdf}
\caption{Illustration of the experimental setup used in this work.}
\label{fig:figS2}
\end{figure}

\begin{figure}[h!]
\centering 
\includegraphics[width=\columnwidth]{SI/FigS3.pdf}
\caption{Rectangular intensity profiles of the final deposits of drops containing active \textit{C.~R.}. The grey area highlights the analysed section for the corresponding rectangular intensity profiles on the right. Scale bar is $250\,\upmu \mathrm{m}$ }
\label{fig:figS3}
\end{figure}

\subsection*{Experimental setup}
The experimental setup we used is schematically illustrated in Sup.~Fig.~\ref{fig:figS2}. Small drops of motile \textit{C.~R} were deposited on silanized glass slides and left to dry. The evaporation process was monitored using an inverted phase contrast microscope. During evaporation the algae drops were illuminated with either white or red light from above, or white light from the side.


\subsection*{Rectangular intensity profiles}
Rectangular intensity profiles of $\approx1600\times400\,\upmu\mathrm{m}$ were calculated using the image analysis software ImageJ \cite{schneider2012} for all final deposits to emphasize the presence or absence of algal rings (Sup.~Fig.~\ref{fig:figS3}). 


\begin{figure}[h]
\centering 
\includegraphics[width=\columnwidth]{SI/FigS4.pdf}
\caption{Density maps obtained from the PTV measurements show the spatial distribution of \textit{C.~R.} for 3 time steps under different light conditions. Evidently, the algae accumulate at the contact line when illuminated with red light, but remain homogeneously distributed in the white light case. Scale bar is $300\,\upmu \mathrm{m}$. }
\label{fig:figS4}
\end{figure}

\subsection*{Density profiles}
From the PTV measurements we obtained density maps showing the distribution of algae within the drying drops at different time steps under different light conditions (Sup.~Fig.~\ref{fig:figS4}). The data show that active algae accumulate at the contact line during the late stages of evaporation when illuminated with red light from above. This effect is not observed when illuminated with white light instead. 

\subsection*{Ring formation in the red light case}
In the red light case we observed that algae aggregate at the contact line in the later stages of evaporation (Sup.~Fig.~\ref{fig:figS4}, Fig.~5), which eventually led to the ring-like deposit shown in Fig.~1(c).
To verify our hypothesis, that the algae get trapped at the contact line at a critical contact angle $\alpha_{\mathrm{c}}$, we carried out a separate drop shape analysis and bright-field microscopy experiment on a large $5\,\upmu\mathrm{l}$ drop deposited on an untreated glass slide, which exhibits a lower initial contact angle of $\approx 15^{\circ}$ (Fig.~\ref{fig:figS5}) instead of $\approx 40^{\circ}$ (Fig.~\ref{fig:figS1}).
We observed that, although the contact angle remains constant within the time frame of the experiment, the algae aggregate at the contact line. This confirms our assumption that the algae get trapped if the contact angle reaches below a critical contact angle $\alpha_{\mathrm{c}}$.
\vfill\eject
\begin{figure}[t]
\centering 
\includegraphics[width=\columnwidth]{SI/FigS5.pdf}
\caption{Drop shape analysis (top) and bright-field microscopy images (bottom) of the first minute of a large drying drop ($5\,\upmu\mathrm{l}$) on an untreated surface. The small contact angle of $\approx 15^{\circ}$ causes the algae to get trapped at the contact line. Scale bars are $50\,\upmu \mathrm{m}$.}
\label{fig:figS5}
\end{figure}





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