Abstract <p>Viruses appear a major agent for marine phytoplankton mortality. As global warming is associated with a reduction in nutrients and an increase in sunlight intensity on the surface seawater, understanding the effect of algal viruses on phytoplankton populations under these conditions is challenging. A new algal viral strain, TvV-SS3, which lacks a supercapsid, was isolated from the Black Sea waters in the spring of 2025. We studied host-virus interaction with the Black Sea green microalgae <i>Tetraselmis viridis</i> over a period of 168 h at three different light intensities. After the latent period, host cell lysis was observed within 6 to 30–54 h. For the first time, it was demonstrated that under the condition of nitrogen-deficiency and high light intensity, inhibiting algal growth, the rate of cell lysis was the lowest (0.51 day⁻¹), while under optimal conditions this rate was 3 times higher. After the lysis period, algal abundance recovered to the initial experimental values or even higher, while the number of viruses generally decreased by an order of magnitude compared to the starting concentration. The results suggest that the inactivation rate of viral particles in the water was higher than their replication rate during the host cell lysis. A 100% lysis of cells of another experimental algal host, the green microalgae <i>Tetraselmis striata,</i> was also reported.</p>