Foraminifera are ubiquitously distributed in marine habitats, playing a major role in marine sediment carbon sequestration and the nitrogen cycle. They exhibit a wide diversity of feeding and behavioural strategies (heterotrophy, autotrophy and mixotrophy), including species with the ability of sequestering intact functional chloroplasts from their microalgal food source (kleptoplastidy), resulting in a mixotrophic lifestyle. The mechanisms by which kleptoplasts are integrated and kept functional inside foraminiferal cytosol are poorly known. In our study, we investigated relationships between feeding strategies, kleptoplast spatial distribution and photosynthetic functionality in two shallow-water benthic foraminifera (Haynesina germanica and Elphidium williamsoni), both species feeding on benthic diatoms. We used a combination of observations of foraminiferal feeding behaviour, test morphology, cytological TEM-based observations and HPLC pigment analysis, with non-destructive, single-cell level imaging of kleptoplast spatial distribution and PSII quantum efficiency. The two species showed different feeding strategies, with H. germanica removing diatom content at the foraminifer’s apertural region and E. williamsoni on the dorsal site. All E. williamsoni parameters showed that this species has higher autotrophic capacity albeit both feeding on benthic diatoms. This might represent two different stages in the evolutionary process of establishing a permanent symbiotic relationship, or may reflect different trophic strategies.

有孔虫广泛分布于海洋生境中，在海洋沉积物固碳和氮循环中发挥着重要作用。它们表现出广泛多样的进食和行为策略（异养、自养和混合营养），包括能够从其微藻食物来源（kleptoplastidy）中隔离完整功能性叶绿体的物种，从而导致混合营养的生活方式。kleptoplasts 在有孔虫胞质溶胶中整合并保持功能的机制知之甚少。在我们的研究中，我们研究了两种浅水底栖有孔虫（Haynesina germanica和Elphidium williamsoni ）的摄食策略、kleptoplast 空间分布和光合功能之间的关系), 这两个物种都以底栖硅藻为食。我们结合了对有孔虫摄食行为、测试形态学、基于 TEM 的细胞学观察和 HPLC 色素分析的观察，以及 kleptoplast 空间分布和 PSII 量子效率的非破坏性单细胞水平成像。这两个物种表现出不同的摄食策略，H. germanica在有孔虫的孔隙区域去除硅藻含量，而E. williamsoni在背部部位。所有E. williamsoni参数显示该物种具有更高的自养能力，尽管它们都以底栖硅藻为食。这可能代表了建立永久共生关系的进化过程中的两个不同阶段，或者可能反映了不同的营养策略。