# Physiological and molecular evidence for phycobilisome degradation in maintaining carbon and nitrogen balance of cyanobacteria

## TL;DR

This study shows how cyanobacteria break down light-harvesting structures to balance carbon and nitrogen, especially under nutrient-poor conditions.

## Contribution

The study reveals that carbon availability is essential for phycobilisome degradation under nitrogen deficiency in cyanobacteria.

## Key findings

- Phycobilisome degradation requires sufficient carbon supply under nitrogen starvation.
- The nblAB and ccmLMNK pathways are crucial for phycobilisome degradation and cyanobacterial adaptation to high C/N conditions.
- Mutant strains lacking PBS degradation show impaired growth and cellular damage under high C/N conditions.

## Abstract

Phycobilisomes (PBS), the primary light-harvesting complexes in cyanobacteria, are degraded under nitrogen starvation to provide nitrogen for cell growth. This study reveals that carbon supply is a critical prerequisite for PBS degradation under nitrogen deficiency in Synechococcus sp. PCC 7002. Even under nitrogen-deficient conditions, PBS degradation is inhibited in the absence of sufficient carbon. We demonstrate that both the nblAB-mediated PBS-degradation pathway and the ccmLMNK operon-mediated CO2-concentrating mechanism are essential for PBS degradation. Furthermore, our findings highlight the critical role of PBS degradation in cyanobacterial adaptation to high C/N conditions. Mutant strains (Mut-nblA and Mut-nblB) deficient in PBS degradation exhibited impaired adaptation to high C/N conditions, as evidenced by their inability to thrive in high NaHCO3 (nitrogen-free) or CO2 (low-nitrogen) environments. While these mutants displayed a greener phenotype under high C/N conditions compared to the wild type, they exhibited extensive cellular damage, and significant downregulation of photosynthesis-related genes. These results provide novel insights into the carbon-dependent regulation of PBS degradation and its essential role in cyanobacterial C/N balance, highlighting its significance for their adaptation to fluctuating environmental conditions.

The online version contains supplementary material available at 10.1007/s42995-025-00290-0.