I'm interested in automated algae identification using neural networks. I need compose substantial micro photography dataset of algae generas (most significant of Cyanobacteria, Chlorophyta and Bacillariophyta).
One of the largest collections of microphotographs with bibliographic data is in Windermare UK and in Poland in Krakow, Department of Phycology, W. Szafer Institute of Botany PAS https://www.botany.pl/index.php/pl/.
Black and white microscopy photos from the Imaging FlowCytobot might also help, particularly for diatoms - https://github.com/hsosik/WHOI-Plankton. Dataset download is available through a citable DOI at this link.
In addition to those mentioned above, www.boldsystems.org may also be worth trying. For example, there are 29,348 public specimen records of rhodophytes, 6,657 public records of chlorophytes, 3,072 public records of diatoms etc. Some portion of them includes pictures for sure.
I would not recommend the dataset suggested by Ihab Alfadhel without being carefuly curated (I looked at part of 2014 dataset and substantial number of micrographs is not determined well or are highly unsuitable for machine learning.
Please check the unique Fritsch Collection contains floristic information and illustrates the diverse views that have been or are still, held about the identification and taxonomy of worldwide freshwater & terrestrial algae.