Colonial green algae in the Cambrian plankton by: Thomas H.P. Harvey

Summarized by Jacob Hood is a senior at Binghamton University studying biology. He plans on attending graduate school for ecology and joining the field of environmental conservation. In his free time Jacob enjoys hiking and spending time with his adorable cat.

What data were used? The data used in this paper are microfossils of green algae fossilized in colonies from millions of years ago. These fossils were found in the Fortean Formation of Newfoundland and Labrador, Canada, dating from the Cambrian, around 506–514 million years ago. Scientists created thin sections (slides for a microscope); from the collected data, 59 colonies were identified and analyzed using transmitted light micrographs–think of a standard microscope with a lens on top and light shining up through a slide. Then images were then created using two wavelengths of light together to create a clear visual contrast to human eyes, known as differential interference.

What was the hypothesis being tested?  This paper is a report on findings of microfossils from the early Cambrian. These well-preserved phytoplankton give us insight into a pivotal moment in planktonic (organisms that float in the water column) development, when colonies began to take on new structures to deal with new predation and ecological changes. This allows us to see when and how ecosystems were drastically changing during the transition between the Proterozoic roughly 2.5 billion years to 541 million years old and Paleozoic from 541–252 million years ago.

Methodology: Targeted sampling locations included fine-grained, unaltered mudstones from the Middle Shale Member of the Fortean Formation, which were observed at three different sites: Mount St. Margaret Quarry, the ‘Ten Mile Lake Quarry’ located in western Newfoundland, and the L’Anse-Au-Loup Quarry in southern Labrador. Samples were collected based on knowledge of ancient geographic zones. Collections were taken at what were once moderate ocean water depths on an exposed shelf area beneath the storm wave impact zone. Occasionally, there were thin layers of organic matter made of many fragments of solidified algal organisms deposited during storms, and the samples were layered near the interpreted peak flooding surface, where the water level was interpreted to have reached its maximum depth in a particular area during this time in the early Cambrian, exactly where these algae would have lived or been dropped during their lives. Many other fossils found in the area are typical of the Cambrian, such as early arthropods (the group that contains lobsters) like trilobites, small cone-shelled creatures called hyolithids, early echinoderms (the group that includes sea stars), and sponges,. Up to 50 g samples were analyzed using conventional microfossil methods consisting of placing microbe fossils in a water solution and placing them on a slide and waiting until they’ve been dried enough for a clear image.This is known as a strew slide. Other samples were prepared through a modified hydrofluoric acid extraction method specifically designed for small carbonaceous fossils (SCFs). Out of the 59 colonies that were found, 46 were located on palynological strew slides, surrounded by thousands of unidentified specimens. 13 specimens were then carefully selected from leftover materials containing SCFs, such as bits of sponge skeleton, sections of early worm bodies, and armor-like plates called sclerites from a slug resembling creature known as Wiwaxia. These 13 specimens were placed one by one on glass slides with a pipette then left to dry and looked at under a microscope These samples were then digitized using images from light microscopes created with differential interference contrast, a type of microscopy where electrons are sent to meet at specific planes of the slide and are merged to create a clear and strong signal to be measured electronically for observation. Images were then illustrated, and traits were measured and categorized based on morphological characteristics such as size, shape, and number of cells per colony.

Results: Throughout the samples, cell size, colonial structures, cell attachment, and number of cells per colony varies significantly when measured. There is no straightforward connection found between the size of a colony, the number of cells, and the size of each individual cell. Generally, colonies with a greater size tend to have larger cells, and algae with struts, which connect cells to each other in a colony, are only present in bigger specimens. On the other hand, ring-shaped colonies that consist of a circular colony of algae were found with cells that are smaller in size. However, similar cell configurations may be seen at various sizes: for example, 12-cell colonies arranged in a hexagonal pattern can exhibit either spacious struts creating a small space between cells or denser cell arrangements with closely packed cells, while star-shaped plates composed of seven or eight cells varied in size from 40 to 95 µm in diameter. Overall, there was a wide range of sizes and considerable diversity in these ancient microbe fossils, which continued to increase during the Cambrian (Figure 1).

It is challenging to differentiate distinct subgroups of these algae due to the similarities in the shape and placement of hairs and spines which poke out from cell walls and give cells texture, the range of cell and colony sizes, and the overlapping colony patterns and cell connections present. The collection instead appears to indicate an individual species that exhibits a high degree of variability, or a small group of interconnected species sharing a similar basic biology. It is believed these algae grew as entire colonies from the moment they split form the mother colony, a strategy not common before or after the Cambrian. This is a stark difference from the single celled growing algae seen before this period, which likely evolved to deal with an increased number of large predators in the environment by growing in larger cells sizes and larger cell clumps.

Why is this study important? This study provides an insightful look into the appearance and relationships between algae in one of the most profoundly changing environments in the history of life. The Cambrian Period is time of rapid evolutionary change in organisms, and the green algae lineage has existed long before and long after the Cambrian. This time period offers a glimpse into the major changes that took place during this time, giving us a look into the very base of the food chain during a time that was incredibly formative for life as we know it. This organism shows potential response to predators by evolving larger and more complex forms. 

Broader implications beyond this study:  The specimens found in this study imply unique physical characteristics and colonial structure in green algae during the Cambrian Period. With these organisms serving as the base of many early food webs, understanding their evolution and the pressures they were under to form these shapes, as well as the relationships they formed with other early organisms, provides a crucial line of questions as to how life first evolved into complex forms.

Citation: Harvey, T. H. (2023). Colonial green algae in the Cambrian Plankton. Proceedings of the Royal Society B: Biological Sciences, 290. https://doi.org/10.1098/rspb.2023.1882